Heart

This article is about the internal organ. For other uses,see Heart (disambiguation).

The heart is a muscular organ in humans and other an-imals, which pumps blood through the blood vessels ofthe circulatory system.[1] Blood provides the body withoxygen and nutrients, and also assists in the removal ofmetabolic wastes.[2] The heart is located in the middlecompartment of the mediastinum in the chest.[3]

In humans, other mammals and birds the heart is dividedinto four chambers: upper left and right atria; and lowerleft and right ventricles.[4][5] Commonly the right atriumand ventricle are referred together as the right heart andtheir left counterparts as the left heart.[6] Fish in contrasthave two chambers, an atrium and a ventricle, while rep-tiles have three chambers.[5] In a healthy heart blood owsone way through the heart due to heart valves, which pre-vent backow.[3] The heart is enclosed in a protective sac,the pericardium, which also contains a small amount ofuid. The wall of the heart is made up of three layers:epicardium, myocardium, and endocardium.[7]

The heart pumps blood through both circulatory systems.Blood low in oxygen from the systemic circulation en-ters the right atrium from the superior and inferior venacavae and passes to the right ventricle. From here it ispumped into the pulmonary circulation, through the lungswhere it receives oxygen and gives o carbon dioxide.Oxygenated blood then returns to the left atrium, passesthrough the left ventricle and is pumped out through theaorta to the systemic circulationwhere the oxygen isused andmetabolized to carbon dioxide.[2] In addition theblood carries nutrients from the liver and gastrointestinaltract to various organs of the body, while transportingwaste to the liver and kidneys. In the healthy organ-ism each heartbeat causes the right ventricle to pumpthe same amount of blood into the respiratory organ asthe left ventricle pumps to the body. Veins transportblood to the heart and carry deoxygenated blood - exceptfor the pulmonary and portal veins. Arteries transportblood away from the heart, and apart from the pulmonaryartery hold oxygenated blood. Their increased distancefrom the heart cause veins to have lower pressures thanarteries.[2][3] The heart contracts at a resting rate close to72 beats per minute.[2] Exercise temporarily increases therate, but lowers resting heart rate in the long term, and isgood for heart health.[8]

Cardiovascular diseases (CVD) are the most commoncause of death globally as of 2008, accounting for 30%

of deaths.[9][10] Of these more than three quarters fol-low coronary artery disease and stroke.[9] Risk factors in-clude: smoking, being overweight, little exercise, highcholesterol, high blood pressure, and poorly controlleddiabetes, among others.[11] Diagnosis of CVD is oftendone by listening to the heart-sounds with a stethoscope,ECG or by ultrasound.[3] Specialists who focus on dis-eases of the heart are called cardiologists, although manyspecialties of medicine may be involved in treatment.[10]

1 Structure

Real-time MRI of the human heart

The heart is situated in the middle of the mediastinum be-hind the breastbone in the chest, at the level of thoracicvertebrae T5-T8. The largest part of the heart is usuallyslightly oset to the left side of the chest (though occa-sionally it may be oset to the right) and is felt to be onthe left because the left heart is stronger, since it pumpsto all body parts. Because the heart is between the lungs,the left lung in turn is smaller than the right lung becauseit has to accommodate the heart.The heart is supplied by the coronary circulation and isenclosed in a double-membraned sacthe pericardium.This attaches to the mediastinum, providing anchoragefor the heart.[13] The back surface of the heart lies nearto the vertebral column, and the front surface sits deepto the sternum and costal cartilages.[7] Two of the great

1

2 1 STRUCTURE

The human heart is in the middle of the thorax, with its apexpointing to the left.[12]

veins the venae cavae, and the great arteries, the aortaand pulmonary artery, are attached to the upper part ofthe heart, called the base, which is located at the levelof the third costal cartilage.[7] The lower tip of the heart,the apex, lies just to the left of the sternum between thejunction of the fourth and fth ribs near their articulationwith the costal cartilages.[7] The right side of the heart isdeected forwards, and the left deected to the back.[7]

The heart is cone-shaped, with its base positioned up-wards and tapering down to the apex.[7] A stethoscope canbe placed directly over the apex so that the heartbeats canbe counted. An adult heart has a mass of 250350 grams(912 oz).[14] The heart is typically the size of a st: 12cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5in) in thickness.[7] Well-trained athletes can have muchlarger hearts due to the eects of exercise on the heartmuscle, similar to the response of skeletal muscle.[7]

1.1 Heart wall

Main article: Cardiac muscleThe heart wall is made up of three layers: the innerendocardium, middle myocardium and outer epicardium.These are surrounded by a double-membraned sac calledthe pericardium.The innermost layer of the heart is called the endo-cardium. It is made up of a lining of simple squamous

Layers of the heart wall, including visceral and parietal peri-cardium.

epithelium, and covers heart chambers and valves. It iscontinuous with the endothelium of the veins and arter-ies of the heart, and is joined to the myocardium with athin layer of connective tissue.[7] The endocardium, bysecreting endothelins, may also play a role in regulatingthe contraction of the myocardium.[7]

The swirling pattern of myocardium helps the heart pump eec-tively

The middle layer of the heart wall is the myocardium,which is the cardiac muscle a layer of involuntarystriated muscle tissue surrounded by a framework ofcollagen. The myocardium is also supplied with bloodvessels, and nerve bers by way of the epicardium thathelp to regulate the heart rate.[7] Cardiac muscle tis-sue has autorhythmicity, the unique ability to initiate acardiac action potential at a xed rate spreading the im-pulse rapidly from cell to cell to trigger the contraction ofthe entire heart. This autorhythmicity is still modulatedby the endocrine and nervous systems.[7]

There are two types of cardiac muscle cell:cardiomyocytes which have the ability to contract

1.2 Chambers 3

easily, and modied cardiomyocytes the pacemaker cellsof the conducting system. The cardiomyocytes makeup the bulk (99%) of cells in the atria and ventricles.These contractile cells are connected by intercalateddiscs which allow a rapid respond to impulses of actionpotential from the pacemaker cells. The intercalateddiscs allow the cells to act as a syncytium and enable thecontractions that pump blood through the heart and intothe major arteries.[7]

The pacemaker cells make up just (1% of cells) and formthe conduction system of the heart. They are gener-ally much smaller than the contractile cells and have fewmyobrils which gives them limited contractibility. Theirfunction is similar in many respects to neurons.[7]

The cardiac muscle pattern is elegant and complex, as themuscle cells swirl and spiral around the chambers of theheart.[7] They form a gure 8 pattern around the atria andaround the bases of the great vessels.[7] Deeper ventricu-lar muscles also form a gure 8 around the two ventriclesand proceed toward the apex. More supercial layers ofventricular muscle wrap around both ventricles.[7] Thiscomplex swirling pattern allows the heart to pump bloodmore eectively than a simple linear pattern would.[7]

As with skeletal muscles the heart can increase in size andeciency with exercise.[7] Thus endurance athletes suchas marathon runners may have a heart that has increasedin size by up to 40%.[15]

The pericardium surrounds the heart. It consists of twomembranes: an inner serous membrane called the epi-cardium, and an outer brous membrane.[16] These en-close the pericardial cavity which contains the pericardialuid that lubricates the surface of the heart.

1.2 Chambers

Heart being dissected showing right and left ventricles, fromabove

The heart has four chambers, two upper atria, the receiv-ing chambers, and two lower ventricles, the dischargingchambers. The atria are connected to the ventricles by

the atrioventricular valves and separated from the ventri-cles by the coronary sulcus. There is an ear-shaped struc-ture in the upper right atrium called the right atrial ap-pendage, or auricle, and another in the upper left atrium,the left atrial appendage. The right atrium and the rightventricle together are sometimes referred to as the rightheart and this sometimes includes the pulmonary artery.Similarly, the left atrium and the left ventricle togetherare sometimes referred to as the left heart. The ventriclesare separated by the anterior longitudinal sulcus and theposterior interventricular sulcus.The cardiac skeleton is made of dense connective tis-sue and this gives structure to the heart. It forms theatrioventricular septumwhich separates the atria from theventricles, and the brous rings which serve as bases forthe four heart valves.[17] The cardiac skeleton also pro-vides an important boundary in the hearts electrical con-duction system since collagen cannot conduct electricity.The interatrial septum separates the atria and the inter-ventricular septum separates the ventricles.[7] The inter-ventricular septum is much thicker than the interatrialseptum, since the ventricles need to generate greater pres-sure when they contract.[7]

1.2.1 Valves

Main article: Heart valves

With the atria and major vessels removed, all four valvesare clearly visible.[7]

Superior Vena Cava

Aorta Pulmonary Artery

Pulmonary Vein

Right Ventricle

Left Ventricle

Right Atrium

Left Atrium

Inferior Vena Cava

Mitral Valve

Aortic Valve

Tricuspid Valve

Pulmonary Valve

The heart, showing valves, arteries and veins. The whitearrows shows the normal direction of blood ow.All four heart valves lie along the same plane. The

valves ensure unidirectional blood ow through the heartand prevent backow. Between the right atrium and the

4 1 STRUCTURE

Frontal section showing papillary muscles attached to the tricus-pid valve on the right and to the mitral valve on the left via chor-dae tendineae.[7]

right ventricle is the tricuspid valve. This consists of threecusps (aps or leaets), made of endocardium reinforcedwith additional connective tissue. Each of the threevalve-cusps is attached to several strands of connectivetissue, the chordae tendineae (tendinous cords), some-times referred to as the heart strings. They are composedof approximately 80 percent collagenous bers with theremainder consisting of elastic bers and endothelium.They connect each of the cusps to a papillary muscle thatextends from the lower ventricular surface. These mus-cles control the opening and closing of the valves. Thethree papillary muscles in the right ventricle are calledthe anterior, posterior, and septal muscles, which corre-spond to the three positions of the valve cusps.Between the left atrium and left ventricle is the mitralvalve, also known as the bicuspid valve due to its havingtwo cusps, an anterior and a posterior medial cusp. Thesecusps are also attached via chordae tendinae to two pap-illary muscles projecting from the ventricular wall.The tricuspid and the mitral valves are the atrioventric-ular valves. During the relaxation phase of the cardiaccycle, the papillary muscles are also relaxed and the ten-sion on the chordae tendineae is slight. However, as theventricle contracts, so do the papillary muscles. This cre-ates tension on the chordae tendineae, helping to hold thecusps of the atrioventricular valves in place and prevent-ing them from being blown back into the atria.[7]

The semilunar pulmonary valve is located at the base ofthe pulmonary artery. This has three cusps which are notattached to any papillary muscles. When the ventricle re-laxes blood ows back into the ventricle from the arteryand this ow of blood lls the pocket-like valve, press-ing against the cusps which close to seal the valve. Thesemilunar aortic valve is at the base of the aorta and alsois not attached to papillary muscles. This too has threecusps which close with the pressure of the blood owingback from the aorta.[7]

1.2.2 Right heart

The two major systemic veins, the superior and inferiorvenae cavae, and the collection of veins that make up thecoronary sinus which drains the myocardium, empty intothe right atrium. The superior vena cava drains bloodfrom above the diaphragm and empties into the upperback part of the right atrium. The inferior vena cavadrains the blood from below the diaphragm and emptiesinto the back part of the atrium below the opening for thesuperior vena cava. Immediately above and to the middleof the opening of the inferior vena cava is the opening ofthe thin-walled coronary sinus.[7]

In the wall of the right atrium is an oval-shaped depres-sion known as the fossa ovalis, which is a remnant of anopening in the fetal heart known as the foramen ovale.The foramen ovale allowed blood in the fetal heart to passdirectly from the right atrium to the left atrium, allow-ing some blood to bypass the pulmonary circuit. Withinseconds after birth, a ap of tissue known as the septumprimum that previously acted as a valve closes the fora-men ovale and establishes the typical cardiac circulationpattern.[7] Most of the internal surface of the right atriumis smooth, the depression of the fossa ovalis is medial,and the anterior surface has prominent ridges of pectinatemuscles, which are also present in the right atrial ap-pendage.[7]

The atria receive venous blood on a nearly continuous ba-sis, preventing venous ow from stopping while the ven-tricles are contracting. While most ventricular lling oc-curs while the atria are relaxed, they do demonstrate acontractile phase when they actively pump blood into theventricles just prior to ventricular contraction. The rightatrium is connected to the right ventricle by the tricuspidvalve.[7]

When the myocardium of the ventricle contracts, pres-sure within the ventricular chamber rises. Blood, likeany uid, ows from higher pressure to lower pressureareas, in this case, toward the pulmonary artery and theatrium. To prevent any potential backow, the papillarymuscles also contract, generating tension on the chordaetendineae. This prevents the aps of the valves from be-ing forced into the atria and regurgitation of the bloodback into the atria during ventricular contraction.[7]

The walls of the right ventricle are lined with trabeculaecarneae, ridges of cardiac muscle covered by endo-cardium. In addition to these muscular ridges, a band ofcardiac muscle, also covered by endocardium, known asthe moderator band reinforces the thin walls of the rightventricle and plays a crucial role in cardiac conduction.It arises from the lower part of the interventricular sep-tum and crosses the interior space of the right ventricleto connect with the inferior papillary muscle.[7]

When the right ventricle contracts, it ejects blood intothe pulmonary artery, which branches into the left andright pulmonary arteries that carry it to each lung. The

5upper surface of the right ventricle begins to taper as itapproaches the pulmonary artery. At the base of the pul-monary artery is the pulmonary semilunar valve that pre-vents backow from the pulmonary artery.[7]

1.2.3 Left heart

After gas exchange in the pulmonary capillaries, bloodhigh in oxygen returns to the left atrium via one of the fourpulmonary veins. Only the left atrial appendage containspectinate muscles. Blood ows nearly continuously fromthe pulmonary veins back into the atrium, which acts asthe receiving chamber, and from here through an openinginto the left ventricle. Most blood ows passively into theheart while both the atria and ventricles are relaxed, buttoward the end of the ventricular relaxation period, theleft atrium will contract, pumping blood into the ventri-cle. This atrial contraction accounts for approximately 20percent of ventricular lling. The left atrium is connectedto the left ventricle by the mitral valve.[7]

Although both sides of the heart will pump the sameamount of blood, the muscular layer is much thicker inthe left ventricle compared to the right, due to the greaterforce needed here. Like the right ventricle, the left alsohas trabeculae carneae, but there is no moderator band.The left ventricle is the major pumping chamber for thesystemic circuit; it ejects blood into the aorta through theaortic semilunar valve.[7]

1.3 Coronary circulation

Right ventricle

Right atrium

Left ventricle

Aorta

Superior vena cava Left pulmonary artery

Left pulmonary veins

Left coronary artery

Left anterior descending (or interventricular) artery

Left circumflex artery

Right marginal artery

Left marginal artery

Posterior descending artery

Right coronary artery

diagonal branch

Arterial supply to the heart (red), with other areas labelled (blue).

Main article: Coronary circulation

Cardiomyocytes, like all other cells, need to be suppliedwith oxygen, nutrients and a way of removing metabolicwastes. This is achieved by the coronary circulation. Thecoronary circulation cycles in peaks and troughs relatingto the heart muscle relaxing or contracting.[7]

Coronary arteries supply oxygen-rich blood to the heartand the coronary veins remove the deoxygenated blood.There is a left and a right coronary artery supplying theleft and right hearts respectively, and the septa. Smaller

branches of these arteries anastomose, which in otherparts of the body serve to divert blood due to a blockage.In the heart these are very small and cannot form otherinterconnections with the result that a coronary arteryblockage can cause a myocardial infarction and with it,tissue damage.[7]

The great cardiac vein receives the major branches of theposterior, middle, and small cardiac veins and drains intothe coronary sinus, a large vein that empties into the rightatrium. The anterior cardiac veins drain the front of theright ventricle and drain directly into the right atrium.[7]

2 DevelopmentMain articles: Heart development and Human embryo-genesisThe heart is the rst functional organ to develop and

Development of the human heart during the rst eight weeks(top), and the formation of the heart chambers (bottom).[7]

starts to beat and pump blood at about three weeks intoembryogenesis. This early start is crucial for subsequentembryonic and prenatal development.The heart derives from splanchnopleuric mesenchyme inthe neural plate which forms the cardiogenic region. Twoendocardial tubes form here that fuse to form a primitiveheart tube known as the tubular heart.[18] Between thethird and fourth week, the heart tube lengthens, and be-gins to fold to form an S-shape within the pericardium.This places the chambers and major vessels into the cor-rect alignment for the developed heart. Further devel-opment will include the septa and valves formation andremodelling of the heart chambers. By the end of thefth week the septa are complete and the heart valves arecompleted by the ninth week.[7]

The embryonic heart begins beating at around 22 daysafter conception (5 weeks after the last normal men-strual period, LMP). It starts to beat at a rate near to themothers which is about 7580 beats per minute (bpm).

6 3 PHYSIOLOGY

The embryonic heart rate then accelerates and reaches apeak rate of 165185 bpm early in the early 7th week(early 9th week after the LMP).[19][20][21] After 9 weeks(start of the fetal stage) it starts to decelerate, slowing toaround 145 (25) bpm at birth. There is no dierence infemale and male heart rates before birth.[22]

3 PhysiologyMain article: Cardiac physiology

Blood ow through the heart from the Khan academy

3.1 Blood ow

Blood ow through the valves

The heart functions as a pump in the circulatory system toprovide a continuous circulation of blood throughout thebody. This circulation consists of the systemic circulationto and from the body and the pulmonary circulation toand from the lungs. Blood in the pulmonary circulationexchanges carbon dioxide for oxygen in the lungs throughthe process of respiration. The systemic circulation then

transports oxygen to the body and returns carbon dioxideand relatively deoxygenated blood to the heart for transferto the lungs.[7]

The right heart collects deoxygenated blood from twolarge veins, the superior and inferior venae cavae. Theblood collects in the right atrium and is pumped throughthe tricuspid valve into the right ventricle, where it ispumped into the pulmonary artery through the pulmonaryvalve. Here the blood enters the pulmonary circulationwhere carbon dioxide can be exchanged for oxygen inthe lungs. This happens through the passive process ofdiusion.In the left heart, oxygenated blood is returned to theleft atrium via the pulmonary vein. It is then pumpedinto the left ventricle through the bicuspid valve and intothe aorta through the mitral valve for systemic circula-tion. The aorta is a large artery that branches into manysmaller arteries, arterioles, and ultimately capillaries. Inthe capillaries, oxygen and nutrients from blood are sup-plied to body cells for metabolism, and exchanged for car-bon dioxide and waste products[7]

3.1.1 Cardiac cycle

Main articles: Cardiac cycle, systole and diastoleThe cardiac cycle refers to a complete heartbeat which in-

The cardiac cycle as correlated to the ECG

cludes systole and diastole and the intervening pause. Thecycle begins with contraction of the atria and ends withrelaxation of the ventricles. Systole is when the ventriclesof the heart contract to pump blood to the body. Dias-tole is when the ventricles relax and ll with blood. Theatria and ventricles work in concert, so in systole whenthe ventricles are contracting, the atria are relaxed andcollecting blood. When the ventricles are relaxed in di-

3.2 Electrical conduction 7

astole, the atria contract to pump blood to the ventricles.This coordination ensures blood is pumped eciently tothe body.[7]

At the beginning of the cardiac cycle, in early diastole,both the atria and ventricles are relaxed. Since bloodmoves from areas of high pressure to areas of low pres-sure, when the chambers are relaxed, blood will ow intothe atria (through the coronary sinus and the pulmonaryveins). As the atria begin to ll, the pressure will riseso that the blood will move from the atria into the ven-tricles. In late diastole the atria contract pumping moreblood into the ventricles. This causes a rise in pressurein the ventricles, and in ventricular systole blood will bepumped into the pulmonary artery.When the atrioventricular valves (tricuspid andmitral) areopen, during blood ow to the ventricles, the semilunarvalves are closed to prevent backow into the ventricles.When the ventricular pressure is greater than the atrialpressure the tricuspid and mitral valves will shut. Whenthe ventricles contract the pressure forces the semilunaraortic and pulmonary valves open. As the ventricles relaxthe semilunar valves will close in response to decreasedpressure.

3.1.2 Cardiac output

Main article: Cardiac outputCardiac output (CO) is a measurement of the amount of

The x-axis reects time with a recording of the heart sounds. They-axis represents pressure.[7]

blood pumped by each ventricle in one minute. To calcu-late this, multiply the amount pumped out by each ven-tricle, the stroke volume (SV), by the heart rate (HR), inbeats per minute.[7] Cardiac output can be represented bythe equation: CO = HR x SV[7] The average cardiac out-put, using an average SV of about 70mL, is 5.25 L/min,with a range of 4.08.0 L/min.[7] The stroke volume isnormally measured using an echocardiogram and can beinuenced by the size of the heart, physical and mentalcondition of the individual, sex, contractility, duration ofcontraction, preload and afterload.[7]

Preload refers to howmuch blood is in the ventricles at theend of diastole, at their most full. A main factor is howlong it takes the ventricles to llif the ventricles con-tract faster, then there is less time to ll and the preloadwill be less.[7] Preload can also be aected by a personshydration status. . It is important because of the Frank-Starling mechanism. This states that the force of contrac-tion is directly proportional to the initial length of mus-cle ber. This means that a ventricle will contract moreforcefully, the more it is stretched.[7]

Afterload, or howmuch blood is left in the ventricles aftersystole, is inuenced by the resistance of the vascular sys-tem. This tension is called afterload. It can be inuencedby narrowing of the heart valves (stenosis) or contractionor relaxation of the peripheral blood vessels.[7]

The strength of heart muscle contractions controls thestroke volume. This can be inuenced positively or neg-atively by agents termed inotropes. These can be eitherconditions or drugs. Positive inotropes that cause strongercontractions include high blood calcium and drugs suchas Digoxin, which will act to stimulate the sympatheticnerves in the ght-or-ight response. Negative inotropescausing weaker contractions include high blood potas-sium, hypoxia, acidosis, and drugs such as beta block-ers and calcium channel blockers. These act on theparasympathetic nervous system via the vagus nerve.

3.2 Electrical conductionMain article: Electrical conduction system of the heartThe normal rhythmical heart beat, called sinus rhythm, is

Transmission of a cardiac action potential through the heartsconduction system

established by the sinoatrial node, the hearts pacemaker.

8 3 PHYSIOLOGY

Here an electrical signal is created that travels through theheart, causing the heart muscle to contract.The sinoatrial node is found in the coronary sinus ofthe right atrium.[7] The electrical signal generated by thesinoatrial node travels through the right atrium in a ra-dial way that is not completely understood. It travels tothe left atrium via Bachmanns bundle, such that both leftand right atrium contract together.[23][24][25] The signalthen travels to the atrioventricular node. This is found atthe bottom of the right atrium in the atrioventricular sep-tumthe boundary between the right atrium and the leftventricle. The septum is part of the cardiac skeleton, tis-sue within the heart that the electrical signal cannot passthrough, which forces the signal to pass through the atri-oventricular node only.[7] The signal then travels along theBundle of His to left and right bundle branches throughto the ventricles of the heart. In the ventricles the signalis carried by specialized tissue called the Purkinje berswhich then transmit the electric charge to the cardiacmuscle.[26]

Sinoatrial node

Atrioventricularnode

Left posteriorbundle

Right bundle

Bachmann'sbundle

His bundle

Purkinjefibers

Conduction system of the heart

3.3 Heart rate

Main article: Heart rate

The resting heart rate of a newborn can be 120 beats perminute (bpm) and this gradually decreases until maturityand then gradually increases again with age. The adultresting heart rate ranges from 60 to 100 bpm. Exerciseand tness levels, age and basal metabolic rate can all af-fect the heart rate. An athletes heart rate can be lowerthan 60bpm. During exercise the rate can be 150bpmwith maximum rates reaching from 200 and 220 bpm.[7]

3.3.1 Creation

The sinoatrial node create and sustains its own rhythm.Cells in the sinoatrial node do this by creating an actionpotential. The cardiac action potential is created by themovement of specic electrolytes into and out of thepacemaker cells. The action potential then spreads tonearby cells.

The prepotential is due to a slow inux of sodium ions until thethreshold is reached followed by a rapid depolarization and repo-larization. The prepotential accounts for the membrane reachingthreshold and initiates the spontaneous depolarization and con-traction of the cell; there is no resting potential.[7]

When the sinoatrial cells are resting, they have a nega-tive charge on their membranes. However a rapid in-ux of sodium ions causes the membranes charge tobecome positive. This is called depolarisation and oc-curs spontaneously.[7] Once the cell has a suciently highcharge, the sodium channels close and calcium ions thenbegin to enter the cell, shortly after which potassium be-gins to leave it. All the ions travel through ion channels inthe membrane of the sinoatrial cells. The potassium andcalcium only start to move out of and into the cell once ithas a suciently high charge, and so are called voltage-gated. Shortly after this, the calcium channels close andpotassium channels open, allowing potassium to leave thecell. This causes the cell to have a negative resting chargeand is called repolarization. When the membrane poten-tial reaches approximately 60 mV, the potassium chan-nels close and the process may begin again.[7]

The ions move from areas where they are concentrated towhere they are not. For this reason sodium moves intothe cell from outside, and potassium moves from withinthe cell to outside the cell. Calcium also plays a criticalrole. Their inux through slow channels means that thesinoatrial cells have a prolonged pleateau phase whenthey have a positive charge. A part of this is called theabsolute refractory period. Calcium ions also combinewith the regulatory protein troponin C in the troponincomplex to enable contraction of the cardiac muscle, andseparate from the protein to allow relaxation.[27]

3.3.2 Inuences

The normal sinus rhythm of the heart, giving the restingheart rate, is inuenced by the autonomic nervous sys-tem through sympathetic and parasympathetic nerves.[28]These arise from two paired cardiovascular centres inthe medulla oblongata.The vagus nerve of the parasym-pathetic nervous system acts to decrease the heart rate,and nerves from the sympathetic trunk act to increasethe heart rate. These come together in the cardiac plexusnear the base of the heart. Without parasympathetic inputwhich normally predominates, the sinoatrial node would

3.4 Heart sounds 9

Autonomic innervation of the heart

generate a heart rate of about 100 bpm.[7]

The nerves from the sympathetic trunk emerge throughthe T1-T4 thoracic ganglia and travel to both the sinoa-trial and atrioventricular nodes, as well as to the atria andventricles. The ventricles are more richly innervated bysympathetic bers than parasympathetic bers. Sympa-thetic stimulation causes the release of the neurotrans-mitter norepinephrine (also known as noradrenaline) atthe neuromuscular junction of the cardiac nerves. Thisshortens the repolarization period, thus speeding the rateof depolarization and contraction, which results in an in-creased heart rate. It opens chemical or ligand-gatedsodium and calcium ion channels, allowing an inux ofpositively charged ions.[7] Norepinephrine binds to thebeta1 receptor. High blood pressure medications areused to block these receptors and so reduce the heartrate.[7]

The cardiovascular centres receive input from a series

of receptors including proprioreceptors, baroreceptors,and chemoreceptors, plus stimuli from the limbic system.Through a series of reexes these help regulate and sus-tain blood ow. For example, increased physical activityresults in increased rates of ring by various propriore-ceptors located in muscles, joint capsules, and tendons.With increased rates of ring, the parasympathetic stim-ulation may decrease or sympathetic stimulation may in-crease as needed in order to increase blood ow.[7]

Similarly, baroreceptors are stretch receptors located inthe aortic sinus, carotid bodies, the venae cavae, and otherlocations, including pulmonary vessels and the right sideof the heart itself. Rates of ring from the baroreceptorsrepresent blood pressure, level of physical activity, andthe relative distribution of blood. The cardiac centersmonitor baroreceptor ring to maintain cardiac home-ostasis, a mechanism called the baroreceptor reex. Withincreased pressure and stretch, the rate of baroreceptorring increases, and the cardiac centers decrease sym-pathetic stimulation and increase parasympathetic stim-ulation. As pressure and stretch decrease, the rate ofbaroreceptor ring decreases, and the cardiac centers in-crease sympathetic stimulation and decrease parasympa-thetic stimulation.[7]

There is a similar reex, called the atrial reex orBainbridge reex, associated with varying rates of bloodow to the atria. Increased venous return stretches thewalls of the atria where specialized baroreceptors are lo-cated. However, as the atrial baroreceptors increase theirrate of ring and as they stretch due to the increased bloodpressure, the cardiac center responds by increasing sym-pathetic stimulation and inhibiting parasympathetic stim-ulation to increase HR. The opposite is also true.[7]

In addition to the autonomic nervous system, other fac-tors can impact on this. These include epinephrine, nore-pinephrine, and thyroid hormones; levels of various ionsincluding calcium, potassium, and sodium; body temper-ature; hypoxia; and pH balance. Factors that increasethe heart rate can include release of norepinephrine,hypoxemia, low blood pressure and dehydration, a strongemotional response, a higher body temperature, andmetabolic and hormonal factors such as a low potassiumor sodium level or stimulus from thyroid hormones.[7] De-creased body temperature, relaxation, and metabolic fac-tors can also contribute to a decrease in heart rate.[7]

3.4 Heart soundsMain article: Heart sounds

One of the simplest methods of assessing the hearts con-dition is to listen to it using a stethoscope.[7] In a healthyheart, there are only two audible heart sounds, called S1and S2. The rst heart sound S1, is the sound createdby the closing of the atrioventricular valves during ven-tricular contraction and is normally described as lub.

10 4 CLINICAL SIGNIFICANCE

3D echocardiogram showing the mitral valve (right), tricuspidand mitral valves (top left) and aortic valve (top right).The closure of the heart valves causes the heart sounds.

The second heart sound, S2, is the sound of the semilu-nar valves closing during ventricular diastole and is de-scribed as dub.[7] Each sound consists of two compo-nents, reecting the slight dierence in time as the twovalves close.[29] S2 may split into two distinct sounds, ei-ther as a result of inspiration or dierent valvular or car-diac problems.[29] Additional heart sounds may also bepresent and these give rise to gallop rhythms. A thirdheart sound, S3 usually indicates an increase in ventricu-lar blood volume. A fourth heart sound S4 is referred toas an atrial gallop and is produced by the sound of bloodbeing forced into a sti ventricle. The combined pres-ence of S3 and S4 give a quadruple gallop.[7]

Heart murmurs are abnormal heart sounds which canbe either pathological or benign.[30] One example ofa murmur is Stills murmur, which presents a musicalsound in children, has no symptoms and disappears inadolescence.[31]

A dierent type of sound, a pericardial friction rub canbe heard in cases of pericarditis where the inamedmem-branes can rub together.[32]

4 Clinical signicance

The stethoscope is used for auscultation of the heart,and is one of the most iconic symbols for medicine.A number of diseases can be detected primarily by

listening for heart murmurs.

Atherosclerosis is a condition aecting the circulatorysystem. If the coronary arteries are aected anginapectoris may result or at worse a heart attack.

Being such a complex organ the heart is prone to sev-eral cardiovascular diseases some becoming more preva-lent with ageing.[33] Heart disease is a major cause ofdeath, accounting for an average of 30% of all deaths in2008, globally.[9] This rate varies from a lower 28% to ahigh 40% in high-income countries.[10] Doctors that spe-cialise in the heart are called cardiologists. Many othermedical professionals are involved in treating diseases ofthe heart, including doctors such as general practitioners,cardiothoracic surgeons and intensivists, and allied healthpractitioners including physiotherapists and dieticians.Obesity, high blood pressure, and high cholesterol canall increase the risk of developing heart disease. How-ever, half the number of heart attacks occur in peoplewith normal cholesterol levels. It is generally acceptedthat factors such as exercise or the lack of it, good orpoor diet, and overall well-being (including emotional),aect heart health.[34][35][36][37] Exercise results in theaddition of protein myolaments and this can result inhypertrophy where the size of individual cells are in-creased but not their number.[7] This is a condition knownas athletic heart syndrome. The hearts of athletes canpump more eciently at lower heart rates. However,enlarged hearts can have a pathological cause such ashypertrophic cardiomyopathy, which can result in a heartof 1000 g (2 lb) in mass.[7] The cause of an abnormallyenlarged heart muscle is unknown, but the condition isoften undiagnosed and can cause sudden death in youngathletes.[7]

Coronary artery disease is also known as ischemic heartdisease, and atherosclerotic disease and is the most com-mon form of heart disease. The underlying mechanism ofthis disease is atherosclerosisa build-up of plaque alongthe inner walls of the arteries which narrows them, re-ducing the blood ow to the heart. It is the leading causeof heart attacks and the most common cause of death,globally.[38] It is also the main cause of angina.Cardiomyopathy and most commonly dilated cardiomy-opathy, is a noticeable deterioration of the heart musclesability to contract, which can lead to heart failure.[39][40]Other common causes of heart failure (which can also becongestive), are heart attacks, valve disorders and highblood pressure. This happens when the heart is pump-

4.2 Assessment 11

ing insuciently and cannot meet the bodys blood owdemands.[41] Because the heart is a double pump, eachside can fail independently of the other, resulting in heartfailure of the right heart or the left heart, either of whichthrough causing strain in the other side can result in thefailure of the whole heart. Congestive heart failure resultsin blood backing up in the systemic circulation. Edema(swelling) of the feet, ankles and ngers is the most no-ticeable symptom. Pulmonary congestion results fromleft heart failure. The right side of the heart continuesto propel blood to the lungs, but the left side is unable topump the returning blood into the systemic circulation.As blood vessels within the lungs become swollen withblood, the pressure within them increases, and uid leaksfrom the circulation into the lung tissue. This pleural ef-fusion causes pulmonary edema. If untreated, the per-son will suocate because they are drowning in their ownblood.[42]

Heart murmurs are abnormal heart sounds which canbe either pathological or benign and there are severalkinds.[30] Murmurs are graded by volume, from 1) thequietest, to 6) the loudest, and evaluated by their rela-tionship to the heart sounds and position in the cardiaccycle.[29] Phonocardiograms can record these sounds.[7]Murmurs can result from valvular heart diseases dueto narrowing (stenosis), regurgitation or insuciency ofany of the main heart valves but they can also resultfrom a number of other disorders, including atrial andventricular septal defects.[29]

Abnormalities in the sinus rhythm can prevent the heartfrom eectively pumping blood and cause both atrial andventricular brillation.[42] Examples of cardiac dysrhyth-mias are a very rapid heart rate (tachycardia) and a veryslow heart rate (brachycardia). Tachycardia is generallydened as a heart rate faster than 100 beats per minute,and bradycardia as a heart rate slower than 60. Asystoleis the cessation of heart rhythm which results in cardiacarrest.Cardiac tamponade, also known as pericardial tampon-ade, is the condition of an abnormal build-up of uidin the pericardium which can adversely aect the func-tion of the heart. The uid can be removed from thepericardial sac using a syringe in a procedure calledpericardiocentesis.Carditis is inammation of the heart; this can be specicto regions as in pericarditis, myocarditis, and endocarditisor it can be of the whole heart known as pancarditis.

4.1 Diagnosis

4.2 Assessment4.2.1 Examination

Main article: Cardiac examination

The cardiac examination includes inspection, palpationand auscultation.

4.2.2 Electrocardiogram

Main article: ElectrocardiographyUsing surface electrodes on the body, it is possible to

Cardiac cycle shown against ECG

record the complex electrical activity of the heart. Thistracing of the electrical signal is the electrocardiogram(ECG) or (EKG). An ECG clearly shows normal and ab-normal heart function and is an indispensable diagnostictool.There are ve prominent points on the ECG: the P wave(atrial depolarisation), the QRS complex (atrial repolar-isation and ventricular depolarisation) and the T wave(ventricular repolarisation).[7]

4.2.3 Imaging

Main article: Cardiac imaging

Several imaging methods can be used to assess theanatomy and function of the heart, including ultrasound,angiography, PET, CT and MRI. Ultrasound of theheart is called echocardiography. It is used to measurethe hearts function, assess for disease of the valves ofthe heart, and look for any anatomical abnormalities.Echocardiography can be conducted by a probe on thechest (transthoracic) or by a probe in the esophagus(transoesophageal). A typical echocardiography reportwill include information about the volumes at the endof systole and diastole, how wide the valves are (check-ing for stenosis), whether there is any backow of bloodthrough the valves (regurgitation), and an ejection frac-tion, which describes how much blood is ejected fromthe left and right ventricles after systole. Ejection frac-tions range from approximately 55 to 70 percent, with amean of 58 percent.[7]

4.2.4 Stress tests

Main article: Cardiac stress test

12 5 HISTORY

A cardiac stress test uses exercise or drugs to stimulatethe heart and provoke a measurable response to the stressin order to gauge the hearts eectiveness.

4.3 TreatmentAngiogenesis represents a therapeutic target for cardio-vascular disease.[43]

Debrillation is used to treat serious arrhythmias.Articial pacemakers used to regulate the heartbeat canalso incorporate a debrillator.

4.4 SurgeryMain articles: Coronary artery bypass surgery andCoronary stent

Coronary artery bypass surgery to improve the blood sup-ply to the heart is often the only treatment option for coro-nary heart disease.Heart valve repair or valve replacement are options forvalvular heart disease.

5 History

5.1 Ancient

Heart and its blood vessels, by Leonardo da Vinci, 15th century

The valves of the heart were discovered by a physician

of the Hippocratean school around the 4th century BC,although their function was not fully understood. Ondissection, arteries are typically empty of blood becauseblood pools in the veins after death. It was subsequentlyassumed they were lled with air and served to transportair around the body.Philosophers distinguished veins from arteries, butthought the pulse was a property of arteries. Erasistratosobserved that arteries cut during life bleed. He ascribedthe fact to the phenomenon that air escaping from anartery is replaced with blood which entered by very smallvessels between veins and arteries. Thus he apparentlypostulated capillaries, but with reversed ow of blood.TheGreek physicianGalen (2nd centuryAD) knew bloodvessels carried blood and identied venous (dark red) andarterial (brighter and thinner) blood, each with distinctand separate functions. Growth and energy were derivedfrom venous blood created in the liver from chyle, whilearterial blood gave vitality by containing pneuma (air) andoriginated in the heart. Blood owed from both creatingorgans to all parts of the body, where it was consumedand there was no return of blood to the heart or liver.The heart did not pump blood around, the hearts motionsucked blood in during diastole and the blood moved bythe pulsation of the arteries themselves.Galen believed the arterial blood was created by venousblood passing from the left ventricle to the right through'pores in the interventricular septum, while air passedfrom the lungs via the pulmonary artery to the left sideof the heart. As the arterial blood was created, sootyvapors were created and passed to the lungs, also via thepulmonary artery, to be exhaled.

5.2 Pre-modern

The earliest descriptions of the coronary and pulmonarycirculation systems can be found in the Commentary onAnatomy in Avicennas Canon, published in 1242 by Ibnal-Nas.[44] In his manuscript, al-Nas wrote that bloodpasses through the pulmonary circulation instead of mov-ing from the right to the left ventricle as previously be-lieved by Galen.[45] His work was later translated intoLatin by Andrea Alpago.[46]

In Europe, the teachings of Galen continued to dominatethe academic community and his doctrines were adoptedas the ocial canon of the Church. Andreas Vesaliusquestioned some of Galens beliefs of the heart in De hu-mani corporis fabrica (1543), but his magnum opus wasinterpreted as a challenge to the authorities and he wassubjected to a number of attacks.[47] Michael Servetuswrote in Christianismi Restitutio (1553) that blood owsfrom one side of the heart to the other via the lungs.[47]

13

5.3 Modern

The breakthrough came with the publication of De MotuCordis (1628) by the English physician William Harvey.Harveys book completely describes the systemic circula-tion and the mechanical force of the heart, leading to anoverhaul of the Galenic doctrines.[48] Otto Frank (18651944) was a German physiologist; among his many pub-lished works are detailed studies of this important heartrelationship. Ernest Starling (18661927) was an im-portant English physiologist who also studied the heart.Although they worked largely independently, their com-bined eorts and similar conclusions have been recog-nized in the name "FrankStarling mechanism.[7]

Although Purkinje bers and the bundle of His were dis-covered as early as the 19th century, their specic rolein the electrical conduction system of the heart remainedunknown until Sunao Tawara published his monograph,titled Das Reizleitungssystem des Sugetierherzens, in1906. Tawaras discovery of the atrioventricular nodeprompted Arthur Keith andMartin Flack to look for sim-ilar structures in the heart, leading to their discovery ofthe sinoatrial node several months later. These struc-tures form the anatomical basis of the electrocardiogram,whose inventor, Willem Einthoven, was awarded the No-bel Prize in Medicine or Physiology in 1924.[49]

The rst successful heart transplantation was performedin 1967 by the South African surgeon Christiaan Barnardat Groote Schuur Hospital in Cape Town. This markedan important milestone in cardiac surgery, capturing theattention of both the medical profession and the worldat large. However, long-term survival rates of patientswere initially very low. LouisWashkansky, the rst recip-ient of a donated heart, died 18 days after the operationwhile other patients did not survive for more than a fewweeks.[50] The American surgeon Norman Shumway hasbeen credited for his eorts to improve transplantationtechniques, along with pioneers Richard Lower, VladimirDemikhov and Adrian Kantrowitz. As of March 2000,more than 55,000 heart transplantations have been per-formed worldwide.[51]

By the middle of the 20th century, heart disease had sur-passed infectious disease as the leading cause of death inthe United States, and it is currently the leading cause ofdeaths worldwide. Since 1948, the ongoing FraminghamHeart Study has shed light on the eects of various inu-ences on the heart, including diet, exercise, and commonmedications such as aspirin. Although the introduction ofACE inhibitors and beta blockers has improved the man-agement of chronic heart failure, the disease continuesto be an enormous medical and societal burden, with 30to 40% of patients dying within a year of receiving thediagnosis.[52]

6 Society and cultureFurther information: Sacred Heart, Heart symbol andBlood Cultural and religious beliefs

6.1 Symbolism

Letter of the Georgian script is often used as a heartsymbol.

The seal script glyph for heart (Middle Chinese sim)

As one of the vital organs, the heart was long identi-ed as the center of the entire body, the seat of life, oremotion, or reason, will, intellect, purpose or the mind.Thus, in theHebrewBible, the word for heart lebabis used in these meanings (paralleling the use of "diaphragm" in Homeric Greek).An important part of the concept of the soul in AncientEgyptian religion was thought to be the heart, or ib. Theib or metaphysical heart was believed to be formed fromone drop of blood from the childs mothers heart, takenat conception.[53] To ancient Egyptians, the heart was theseat of emotion, thought, will, and intention. This is ev-idenced by Egyptian expressions which incorporate theword ib, such as Awt-ib for happiness (literally, wide-ness of heart), Xak-ib for estranged (literally, trun-cated of heart). In Egyptian religion, the heart was thekey to the afterlife. It was conceived as surviving death inthe nether world, where it gave evidence for, or against, itspossessor. It was thought that the heart was examined byAnubis and the deities during the Weighing of the Heartceremony. If the heart weighed more than the featherof Maat, it was immediately consumed by the monsterAmmit.The Chinese character for heart, , derives from a com-paratively realistic depiction of a heart (indicating theheart chambers) in seal script. The Chinese word xn alsotakes the metaphorical meanings of mind, intelligence,soul, or center, core. In Chinese medicine, the heartis seen as the center of shn spirit, soul, conscious-ness.

14 7 OTHER ANIMALS

The Sanskrit word for heart, hRd (), dates at least asfar back as the Rigveda and is a cognate of the word forheart in Greek, Latin, and English. The same word isused to mean mind or soul depending on the context.Many classical philosophers and scientists, includingAristotle, considered the heart the seat of thought, reason,or emotion, often disregarding the brain as contributingto those functions.[54] The identication of the heart asthe seat of emotions in particular is due to the Romanphysician Galen, who also located the seat of the passionsin the liver, and the seat of reason in the brain.[55] How-ever these emotional properties of the heart were laterdiscovered to be solely centered in the brain. This tradi-tion inuenced the development of themedieval Christiandevotion to the Sacred Heart of Jesus and the ImmaculateHeart of Mary.The idiomatic expression of pierced or broken heartsultimately derive from devotional Christianity, where thehearts of Mary or Jesus are depicted as suering varioustortures (symbolizing the pain suered by Christ for thesins of the world, and the pain of Mary at the crucix-ion of her son, respectively), but from an early time themetaphor was transferred to unfullled romantic love, inlate medieval literature dealing with the ideals of courtlylove. The notion of "Cupid's arrows is ancient, due toOvid, but while Ovid describes Cupid as wounding hisvictims with his arrows, it is not made explicit that it isthe heart that is wounded. The familiar iconography ofCupid shooting little heart symbols is Baroque.

6.2 Food

Animal hearts are widely consumed as food. As they arealmost entirely muscle, they are high in protein. They areoften included in dishes with other oal, for example inthe pan-Ottoman kokoretsi.Chicken hearts are considered to be giblets, and are of-ten grilled on skewers: Japanese hto yakitori, Brazilianchurrasco de corao, Indonesian chicken heart satay.[56]They can also be pan-fried, as in Jerusalemmixed grill. InEgyptian cuisine, they can be used, nely chopped, as partof stung for chicken.[57] Many recipes combined themwith other giblets, such as theMexican pollo en menuden-cias[58] and the Russian ragu iz kurinyikh potrokhov.[59]

The hearts of beef, pork, and mutton can generally be in-terchanged in recipes. As heart is a hard-working muscle,it makes for rm and rather dry meat,[60] so is generallyslow-cooked. Another way of dealing with toughness isto julienne the meat, as in Chinese stir-fried heart.[61]

Beef heart may be grilled or braised.[60] In the Peruviananticuchos de corazn, barbecued beef hearts are grilledafter being tenderized through long marination in a spiceand vinegar mixture. An Australian recipe for mockgoose is actually braised stued beef heart.[62]

Pig heart is stewed, poached, braised,[63] or made into

sausage. The Balinese oret is a sort of blood sausagemadewith pig heart and blood. A French recipe for cur deporc l'orange is made of braised heart with an orangesauce.

7 Other animalsFurther information: Circulatory system

The structure of the heart can vary among the dierentanimal species. Cephalopods have two gill hearts alsoknown as branchial hearts and one systemic heart. Thevertebrate heart lies in the front (ventral) part of the bodycavity, dorsal to the gut. It is always surrounded by apericardium, which is usually a distinct structure, but maybe continuous with the peritoneum in jawless and carti-laginous sh.The SA node is found in all amniotes but not in moreprimitive vertebrates. In these animals, the muscles ofthe heart are relatively continuous and the sinus veno-sus coordinates the beat which passes in a wave throughthe remaining chambers. Indeed, since the sinus veno-sus is incorporated into the right atrium in amniotes, it islikely homologous with the SA node. In teleosts, withtheir vestigial sinus venosus, the main centre of coor-dination is, instead, in the atrium. The rate of heart-beat varies enormously between dierent species, rang-ing from around 20 beats per minute in codsh to around600 in hummingbirds[64] and up to 1200 bpm in the ruby-throated hummingbird.[65]

7.1 Double circulatory systemsIn the heart of lungsh, the septum extends part-way intothe ventricle. This allows for some degree of separationbetween the de-oxygenated bloodstream destined for thelungs and the oxygenated stream that is delivered to therest of the body. The absence of such a division in livingamphibian species may be partly due to the amount ofrespiration that occurs through the skin; thus, the bloodreturned to the heart through the vena cavae is alreadypartially oxygenated. As a result, there may be less needfor a ner division between the two bloodstreams than inlungsh or other tetrapods. Nonetheless, in at least somespecies of amphibian, the spongy nature of the ventricledoes seem to maintain more of a separation between thebloodstreams. Also, the original valves of the conus ar-teriosus have been replaced by a spiral valve that dividesit into two parallel parts, thereby helping to keep the twobloodstreams separate.[64]

Adult amphibians andmost reptiles have a double circula-tory system but the heart is not separated into two pumps.The development of the double system is necessitated bythe presence of lungs which deliver oxygenated blood di-rectly to the heart.

7.3 Fish 15

A cross section of a three chambered adult amphibian heart, notethe single ventricle. The purple regions represent areas wheremixing of oxygenated and de-oxygenated blood occurs.1. Pulmonary Vein2. Left Atrium3. Right Atrium4. Ventricle5. Conus arteriosus6. Sinus venosus

In amphibians, the atrium is divided into two chambersby a muscular septum but there is only one ventricle. Thesinus venosus, which remains large, connects only to theright atrium and receives blood from the venae cavae,with the pulmonary vein by-passing it to enter the leftatrium.The heart of most reptiles is similar in structure to that oflungsh but the septum is generally much larger. This di-vides the ventricle into two halves but the septum does notreach the whole length of the heart and there is a consid-erable gap near the pulmonary artery and aorta openings.In most reptilian species, there appears to be little, if any,mixing between the bloodstreams, so the aorta receives,essentially, only oxygenated blood.[64]

7.2 The fully divided heart

Archosaurs (crocodilians and birds) and mammals showcomplete separation of the heart into two pumps for atotal of four heart chambers; it is thought that the four-chambered heart of archosaurs evolved independentlyfrom that of mammals. In crocodilians, there is a smallopening, the foramen of Panizza, at the base of the ar-terial trunks and there is some degree of mixing be-tween the blood in each side of the heart, during a diveunderwater;[66][67] thus, only in birds and mammals arethe two streams of blood those to the pulmonary andsystemic circulations permanently kept entirely sepa-rate by a physical barrier.[64]

7.3 FishMain article: Fish anatomy HeartPrimitive sh have a four-chambered heart, but the

Blood ow through the sh heart: sinus venosus, atrium,ventricle, and outow tract

chambers are arranged sequentially so that this primi-tive heart is quite unlike the four-chambered hearts ofmammals and birds. The rst chamber is the sinus veno-sus, which collects deoxygenated blood, from the body,through the hepatic and cardinal veins. From here, bloodows into the atrium and then to the powerful muscu-lar ventricle where the main pumping action will takeplace. The fourth and nal chamber is the conus arte-riosus which contains several valves and sends blood tothe ventral aorta. The ventral aorta delivers blood to thegills where it is oxygenated and ows, through the dorsalaorta, into the rest of the body. (In tetrapods, the ventralaorta has divided in two; one half forms the ascendingaorta, while the other forms the pulmonary artery).[64]

In the adult sh, the four chambers are not arranged in astraight row but, instead form an S-shape with the lattertwo chambers lying above the former two. This relativelysimpler pattern is found in cartilaginous sh and in theray-nned sh. In teleosts, the conus arteriosus is verysmall and can more accurately be described as part ofthe aorta rather than of the heart proper. The conus ar-teriosus is not present in any amniotes, presumably hav-ing been absorbed into the ventricles over the course ofevolution. Similarly, while the sinus venosus is presentas a vestigial structure in some reptiles and birds, it isotherwise absorbed into the right atrium and is no longerdistinguishable.[64]

7.4 InvertebratesArthropods have an open circulatory system, and oftensome short open-ended arteries.The arthropod heart istypically a muscular tube that runs the length of the body,under the back and from the base of the head. Instead ofblood the circulatory uid is haemolymph which carriesthe most commonly used respiratory pigment, copper-based haemocyanin as the oxygen transporter; iron-basedhaemoglobin is used by only a few arthropods. The heartcontracts in ripples from the rear to the front of the ani-

16 9 REFERENCES

The tube-like heart (green) of the mosquito Anopheles gam-biae extends horizontally across the body, interlinked with thediamond-shaped wing muscles (also green) and surrounded bypericardial cells (red). Blue depicts cell nuclei.

Basic arthropod body structure heart shown in red

mal transporting water and nutrients. Pairs of valves runalongside the heart, allowing uid to enter whilst prevent-ing backow.In insects, the circulatory system is not used to transportoxygen and so is much reduced, having no veins or ar-teries and consisting of a single perforated tube runningdorsally which pumps peristaltically. The simpler unseg-mented invertebrates have no body cavity, and oxygenand nutrients pass through their bodies by diusion.

8 Additional images The human heart viewed from the front The human heart viewed from behind The coronary circulation Play media

An MRI scan showing the human heart

An anatomical specimen of the heart Heart Illustration with circulatory system.

9 ReferencesThis article incorporates text from the CC-BY book:OpenStax College, Anatomy & Physiology. OpenStaxCNX. 30 jul 2014..

[1] Taber, Clarence Wilbur; Venes, Donald (2009). Taberscyclopedic medical dictionary. F a Davis Co. pp. 101823. ISBN 0-8036-1559-0.

[2] Hall, John (2011). Guyton and Hall textbook of medi-cal physiology (12th ed. ed.). Philadelphia, Pa.: Saun-ders/Elsevier. p. 157. ISBN 978-1-4160-4574-8.

[3] Keith L. Moore; Arthur F. Dalley; Anne M. R. Agur.1. Clinically Oriented Anatomy. Wolters KluwelHealth/Lippincott Williams & Wilkins. pp. 127173.ISBN 978-1-60547-652-0.

[4] Cecie Starr; Christine Evers; Lisa Starr (2 January 2009).Biology: Today and Tomorrow With Physiology. CengageLearning. pp. 422. ISBN 978-0-495-56157-6. Re-trieved 7 June 2012.

[5] Reed, C. Roebuck; Brainerd, Lee Wherry; Lee,, Rodney;Inc, the sta of Kaplan, (2008). CSET : California SubjectExaminations for Teachers (3rd ed. ed.). New York, NY:Kaplan Pub. p. 154. ISBN 9781419552816.

[6] Phibbs, Brendan (2007). The human heart: a basicguide to heart disease (2nd ed.). Philadelphia: LippincottWilliams & Wilkins. p. 1. ISBN 9780781767774.

[7] Betts, J. Gordon (2013). Anatomy& physiology. pp. 787846. ISBN 1938168135. Retrieved 11 August 2014.

[8] Hall, John (2011). 84. Guyton and Hall textbook ofmedical physiology (12th ed. ed.). Philadelphia, Pa.:Saunders/Elsevier. pp. 10391041. ISBN 978-1-4160-4574-8.

[9] Cardiovascular diseases (CVDs) Fact sheet N317March 2013. WHO. World Health Organization. Re-trieved 20 September 2014.

[10] Longo, Dan; Fauci, Anthony; Kasper, Dennis; Hauser,Stephen; Jameson, J.; Loscalzo, Joseph (August 11,2011). Harrisons Principles of Internal Medicine (18ed.). McGraw-Hill Professional. p. 1811. ISBN9780071748896.

[11] Graham, I; Atar, D; Borch-Johnsen, K; Boysen, G; Bu-rell, G; Cifkova, R; Dallongeville, J; De Backer, G;Ebrahim, S; Gjelsvik, B; Herrmann-Lingen, C; Hoes,A; Humphries, S; Knapton, M; Perk, J; Priori, SG;Pyorala, K; Reiner, Z; Ruilope, L; Sans-Menendez, S;Scholte op Reimer, W; Weissberg, P; Wood, D; Yarnell,J; Zamorano, JL; Walma, E; Fitzgerald, T; Cooney, MT;Dudina, A; European Society of Cardiology (ESC) Com-mittee for Practice Guidelines, (CPG) (Oct 2007). Eu-ropean guidelines on cardiovascular disease prevention inclinical practice: executive summary: Fourth Joint TaskForce of the European Society of Cardiology and OtherSocieties on Cardiovascular Disease Prevention in Clini-cal Practice (Constituted by representatives of nine soci-eties and by invited experts).. European heart journal 28

17

(19): 2375414. doi:10.1093/eurheartj/ehm316. PMID17726041.

[12] Grays Anatomy of the Human Body 6. Surface Mark-ings of the Thorax. Bartleby.com. Retrieved 2010-10-18.

[13] Dorlands (2012). Dorlands Illustrated Medical Dictio-nary (32nd ed.). Elsevier. p. 1461. ISBN 978-1-4160-6257-8.

[14] http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm

[15] Hall, Arthur C. Guyton, John E. (2005). Textbook of med-ical physiology (11th ed. ed.). Philadelphia: W.B. Saun-ders. pp. 106364. ISBN 978-0-7216-0240-0.

[16] Dorlands (2012). Dorlands Illustrated Medical Dictio-nary. Elsevier Saunders. p. 1412. ISBN 978-1-4160-6257-8.

[17] Pocock, Gillian (2006). Human Physiology. Oxford Uni-versity Press. p. 264. ISBN 978-0-19-856878-0.

[18] Main Frame Heart Development. Meddean.luc.edu.Retrieved 2010-10-17.

[19] DuBose, Miller , Moutos. Embryonic Heart RatesCompared in Assisted and Non-Assisted Pregnancies.Obgyn.net. Retrieved 2010-10-18.

[20] DuBose, T. J.; Cunyus, J. A.; Johnson, L. (1990). Em-bryonic Heart Rate and Age. J Diagn Med Sonography 6(3): 151157. doi:10.1177/875647939000600306.

[21] DuBose, TJ (1996) Fetal Sonography, pp. 263274;Philadelphia: WB Saunders ISBN 0-7216-5432-0

[22] Terry J. DuBose Sex, Heart Rate and Age

[23] Antz, Matthias et al. (1998). Electrical ConductionBetween the Right Atrium and the Left Atrium via theMusculature of the Coronary Sinus. Circulation 98 (17):17901795. doi:10.1161/01.CIR.98.17.1790.

[24] De Ponti, Roberto et al. (2002). Electroanatomic Anal-ysis of Sinus Impulse Propagation in Normal HumanAtria. Journal of Cardiovascular Electrophysiology 13(1): 110. doi:10.1046/j.1540-8167.2002.00001.x.

[25] SA node denition Medical Dictionary denitionsof popular medical terms easily dened on MedTerms.Medterms.com. 27 April 2011. Retrieved 7 June 2012.

[26] Purkinje Fibers. Biology.about.com. 9 April 2012. Re-trieved 7 June 2012.

[27] Davis, J. P.; Tikunova, S. B. (2008). Ca2+ exchange withtroponin C and cardiac muscle dynamics. Cardiovascu-lar Research 77 (4): 619626. doi:10.1093/cvr/cvm098.PMID 18079104.

[28] Hall, Arthur C. Guyton, John E. (2005). Textbook of med-ical physiology (11th ed. ed.). Philadelphia: W.B. Saun-ders. pp. 116122. ISBN 978-0-7216-0240-0.

[29] Talley, Nicholas J.; O'Connor, Simon. Clinical Exam-ination. Churchill Livingstone. pp. 7682. ISBN9780729541985.

[30] Dorlands (2012). Dorlands IllustratedMedical Dictonary(32nd ed.). Elsevier Saunders. p. 1189. ISBN 978-1-4160-6257-8.

[31] Newburger, Jane (2006). Nadas Pediatric Cardiology 2ndEdition. Philadelphia: Elsevier. p. 358. ISBN 978-1-4160-2390-6.

[32] Cantarini L, Lopalco G et al. (Oct 2014). Autoimmunityand autoinammation as the yin and yang of idiopathicrecurrent acute pericarditis.. Autoimmun Rev 14: 907.doi:10.1016/j.autrev.2014.10.005. PMID 25308531.

[33] Dantas AP, Jimenez-Altayo F, Vila E (August 2012).Vascular aging: facts and factors. Frontiers in VascularPhysiology 3 (325): 12. doi:10.3389/fphys.2012.00325.PMC 3429093. PMID 22934073.

[34] Eating for a healthy heart. MedicineWeb. Retrieved2009-03-31.

[35] Division of Vital Statistics; Arialdi M. Minio, M.P.H.,Melonie P. Heron, Ph.D., Sherry L. Murphy, B.S., Ken-neth D. Kochanek, M.A. (21 August 2007). Deaths: Fi-nal data for 2001 (PDF). National Vital Statistics Reports(United States: Center for Disease Control) 55 (20): 7.Retrieved 2007-12-30.

[36] White House News. American HeartMonth, 2007. Re-trieved 16 July 2007.

[37] National Statistics Press Release 25 May 2006

[38] Heart attack/coronary artery disease. Mount Sinai Hos-pital, New York.

[39] Cardiopulmonary Pharmacology for Respiratory Care,Jahangir Moini, Ch.2; page 24

[40] http://www.nhlbi.nih.gov/health/health-topics/topics/cm/types.html

[41] Dorlands (2012). Dorlands Illustrated Medical Dictio-nary (32nd ed.). Elsevier Saunders. p. 677. ISBN 978-1-4160-6257-8.

[42] Marieb, Elaine N. (2009). Essentials of Human Anatomy& Physiology 9th Edition. San Francisco, CA: PearsonBenjamin Cummings. ISBN 978-0-321-51353-3.

[43] Stegmann TJ (2005). New vessels for the heart : angiogen-esis as new treatment for coronary heart (1st ed.). Hender-son, NV: CardioVascular Biotherapies Inc. ISBN 978-0-9765583-0-9.

[44] Michelakis, E. D. (19 June 2014). PulmonaryArterial Hypertension: Yesterday, Today, To-morrow. Circulation Research 115 (1): 109114.doi:10.1161/CIRCRESAHA.115.301132.

[45] West, John (2008). Ibn al-Nas, the pulmonarycirculation, and the Islamic Golden Age. Journalof Applied Physiology 105 (6): 18771880.doi:10.1152/japplphysiol.91171.2008. PMC 2612469.PMID 18845773. Retrieved 28 May 2014.

[46] Bondke Persson, A.; Persson, P. B. Form and functionin the vascular system. Acta Physiologica 211 (3): 468470. doi:10.1111/apha.12309.

18 10 EXTERNAL LINKS

[47] West, J. B. (30 May 2014). Galen and the be-ginnings of Western physiology. AJP: Lung Cellu-lar and Molecular Physiology 307 (2): L121L128.doi:10.1152/ajplung.00123.2014.

[48] AIRD, W. C. Discovery of the cardiovascular system:from Galen to William Harvey. Journal of Thrombo-sis and Haemostasis 9: 118129. doi:10.1111/j.1538-7836.2011.04312.x.

[49] Silverman, M. E. (13 June 2006). Why Does the HeartBeat?: The Discovery of the Electrical System of theHeart. Circulation (journal) 113 (23): 27752781.doi:10.1161/CIRCULATIONAHA.106.616771. PMID16769927.

[50] Cooley, Denton A. Recollections of the Early Years ofHeart Transplantation and the Total Articial Heart. Ar-ticial Organs 35 (4): 353357. doi:10.1111/j.1525-1594.2011.01235.x.

[51] Miniati, Douglas N.; Robbins, Robert C. Heart trans-plantation: a thirty-year perspective: A Thirty-Year Per-spective. Annual Review of Medicine 53 (1): 189205.doi:10.1146/annurev.med.53.082901.104050.

[52] Neubauer, Stefan (15 March 2007). The Fail-ing Heart An Engine Out of Fuel. New Eng-land Journal of Medicine 356 (11): 11401151.doi:10.1056/NEJMra063052. PMID 17360992.

[53] Britannica, Ib; Slider, Ab, Egyptian heart and soul concep-tion. The word was also transcribed by Wallis Budge asAb.

[54] Aristotle. On the Parts of Animals. book 3, ch. 4 (Departibus animalium)

[55] Galen, De usu partium corporis humani (The Use of theParts of the Human Body), book 6.

[56] Indonesia Magazine, 25 (1994), p. 67

[57] Samia Abdennour, Firakh mahshiya wi mihammararecipe 117, Egyptian Cooking: And Other Middle EasternRecipes, American University in Cairo Press, 2010.

[58] Diana Kennedy, My Mexico: A Culinary Odyssey withRecipes, University of Texas Press, updated edition, 2013,ISBN 029274840X, p. 100

[59] Alla Sacharow, Classic Russian Cuisine: A Magnicent Se-lection ofMore Than 400 Traditional Recipes, 1993, ISBN1559701749, page unknown

[60] Irma S. Rombauer, Marion Rombauer Becker, The Joy ofCooking, 1975, p. 508

[61] CalvinW. Schwabe,Unmentionable Cuisine, University ofVirginia Press, 1979 (reprint), ISBN 0813911621, p. 96

[62] John Torode, Beef: And Other Bovine Matters, TauntonPress, 2009, ISBN 1600851266, p. 230

[63] Jennie Milsom, The Connoisseurs Guide to Meat, 2009mISBN 1402770502, p. 171

[64] Romer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders In-ternational. pp. 437442. ISBN 0-03-910284-X.

[65] June Osborne (1998). The Ruby-Throated Hummingbird.University of Texas Press. p. 14. ISBN 0-292-76047-7.

[66] Crigg, Gordon; Johansen, Kjell (1987). CardiovascularDynamics In Crocodylus Porosus Breathing Air And Dur-ing Voluntary Aerobic Dives (PDF). Journal of Compar-ative Physiology B (Springer-Verlag) 157 (3): 381392.doi:10.1007/BF00693365. Archived from the original on3 July 2012. Retrieved 3 July 2012.

[67] Axelsson, Michael; Craig, Franklin; Lfman, Carl; Nils-son, Stefan; Crigg, Gordon (1996). Dynamic Anatomi-cal Study Of Cardiac Shunting In Crocodiles Using High-Resolution Angioscopy (PDF). The Journal of Experi-mental Biology (The Company of Biologists Limited) 199(2): 359365. PMID 9317958. Retrieved 3 July 2012.

10 External links What Is the Heart? NIH The Gross Physiology of the Cardiovascular System(2nd Ed., 2012) Robert M. Anderson, M.D. (CC-BY-NC)

Atlas of Human Cardiac Anatomy Dissection review of the anatomy of the HumanHeart including vessels, internal and external fea-tures

Prenatal human heart development Anatomy of the Human Heart Texas Heart Insti-tute

Animal hearts: sh, squid

19

11 Text and image sources, contributors, and licenses11.1 Text

Heart Source: https://en.wikipedia.org/wiki/Heart?oldid=673150513 Contributors: AxelBoldt, Magnus Manske, Kpjas, Sodium, Mav,Bryan Derksen, The Anome, Ap, Alex.tan, Andre Engels, Enchanter, William Avery, Heron, Fonzy, Bth, Montrealais, Sfdan, Patrick,PhilipMW, Michael Hardy, Mgmei, Wshun, Lexor, Gabbe, Menchi, Wapcaplet, Ixfd64, IZAK, GTBacchus, Delirium, Jonsilva, Gbleem,Tregoweth, Egil, Ihcoyc, Ahoerstemeier, Ronz, Angela, Jebba, Andrewa, Julesd, Ugen64, Glenn, Tristanb, Evercat, Samw, Raven inOrbit, Lommer, Ec5618, Mpt, Wayne~enwiki, AHands, DJ Clayworth, Tpbradbury, Ed g2s, Samsara, Renato Caniatti~enwiki, Dpb-smith, Tech77jp, Jerzy, Gurry, Owen, Shantavira, Jni, Writerite, Bearcat, Robbot, Korath, Jredmond, ZimZalaBim, Altenmann, Low-ellian, Merovingian, Academic Challenger, Hadal, Fuelbottle, Diberri, Xanzzibar, Dina, SimonMayer, Pmaguire, DavidCary, Kim Bruning,Nichalp, BenFrantzDale, Geeoharee, Herbee, Dissident, Marcika, Obli, Peruvianllama, Everyking, Bkonrad, No Guru, Moyogo, Curps,Alison, Bensaccount, Jfdwol, Erdal Ronahi, Yekrats, Edsuom, Pascal666, Macrakis, Jackol, SWAdair, OldakQuill, Gadum, Utcursch,Andycjp, Farside~enwiki, Tonymaric, LiDaobing, Sonjaaa, Antandrus, Jossi, Zantolak, Rdsmith4, Oneiros, Maximaximax, PFHLai, Bod-notbod, Kigoe, Honguy86, Marcos, Huaiwei, Anirvan, Neutrality, Joyous!, Syvanen, Adashiel, Trevor MacInnis, Lacrimosus, Mike Rosoft,Kmccoy, D6, Freakofnurture, AliveFreeHappy, Dceck, Discospinster, Solitude, Rich Farmbrough, FiP, Luqui, Ericamick, Notinasnaid,Zazou, Mani1, Night Gyr, DcoetzeeBot~enwiki, Bender235, ESkog, Kaisershatner, Violetriga, BjarteSorensen, CanisRufus, Mikcohen,Ascorbic, NickGorton~enwiki, Barfooz, Shanes, Alberto Orlandini, Adambro, PatrikR, Bobo192, Hurricane111, Smalljim, Brim, Dungo-dung, Arcadian, Giraedata, SpeedyGonsales, Man vyi, Shereth, Minghong, Haham hanuka, Kierano, Nsaa, Jumbuck, Calebe, GustavoBar-bieri, Alansohn, Arcenciel, Eric Kvaalen, Lokicarbis, Quatermass, Jeltz, Andrewpmk, Rdysn5, Iothiania, Wouterstomp, Riana, AzaToth,Kurt Shaped Box, Junk Jungle, Water Bottle, DanD, Mac Davis, Gareld226, Theodore Kloba, Eukesh, Caesura, Wtmitchell, Velella,David Henderson, AndreasPraefcke, Dalillama, Cburnett, Evil Monkey, Max Naylor, AngryParsley, RainbowOfLight, Stevegray, Ver-sageek, Talkie tim, Aryeh~enwiki, Kitch, Forderud, TShilo12, Tom.k, Stemonitis, Nuno Tavares, Jerey O. Gustafson,Woohookitty, Linas,TigerShark, MamaGeek, Rocastelo, CptUnconscious, BillC, Ilario, Dodiad, Matijap, MONGO, Madhav, Eleassar777, Damicatz, JRHorse,Silverleaftree, Kralizec!, Pinkgothic, Neostarbuck, Palica, Algormortis, Graham87, Magister Mathematicae, FreplySpang, Mendaliv, Ef-feietsanders, Phoenix-forgotten, Canderson7, Sjakkalle, Rjwilmsi, Syndicate, Ae77, Strait, JingleBells, MZMcBride, SMC, Vegaswikian,Nneonneo, Krash, Bhadani, DoubleBlue, Nigosh, Matt Deres, Platypus222, Yamamoto Ichiro, Johnrpenner, Titoxd, FlaBot, Schumin-Web, Who, Nivix, Dantecubed, Andy85719, RexNL, Gurch, Leslie Mateus, Ayla, Jrtayloriv, Atomic Cosmos, Krun, Stevenfruitsmaak,Alphachimp, Bmicomp, McDogm, King of Hearts, Chobot, DVdm, Gdrbot, 334a, Murgatroyd, Bgwhite, Hall Monitor, Wjfox2005, TheRamblingMan, YurikBot,Wavelength, ToddVierling, Hairy Dude, Deeptrivia, Brandmeister (old), Pip2andahalf, RussBot, Epolk, Hellbus,Hydrargyrum, Stephenb, Gaius Cornelius, Bisqwit, Nicke L, Wimt, NawlinWiki, Wiki alf, UCaetano, Grafen, NickBush24, Mantavani,Dureo, Apokryltaros, JDoorjam, Irishguy, Nick, Aaron Brenneman, Bobbo, Anetode, Nephron, Dhollm, InvaderJim42, RUL3R, Sem-perf, Supten, Samir, Mysid, Pablomartinez, PS2pcGAMER, Paaskynen, Caspian, Nescio, Elkman, IceCreamAntisocial, Martinwilke1980,Alpha 4615, Nlu, User27091, Raysacks, Wknight94, Tetracube, Richardcavell, K.Nevelsteen, Kilfoylea, Zzuuzz, PTSE, Mdpickle, Jack-lee, Knotnic, Closedmouth, Cheddon, Fang Aili, KGasso, Nemu, Josh3580, CharlesHBennett, GraemeL, SV Resolution, Red Jay, Pe-ter, Thelb4, ArielGold, Garion96, Johnpseudo, MagneticFlux, Allens, Bluezy, Katieh5584, Asterion, Zvika, Miniyazz, DVD R W, Luk,Vanka5, KnightRider~enwiki, SmackBot, MattieTK, YellowMonkey, Unschool, Tarret, Prodego, KnowledgeOfSelf, Hydrogen Iodide,Shoy, AndyZ, Bomac, Thunderboltz, EncycloPetey, Delldot, Kopaka649, Frymaster, Silverhand, Edgar181, HalfShadow, Gary2863, Du-Bose, Aksi great, Macintosh User, Gilliam, Dark jedi requiem, Keegan, Quinsareth, NCurse, JDCMAN, Thumperward, Moogle001,Miquonranger03, MalafayaBot, Fluri, SchftyThree, RayAYang, Akanemoto, Zinneke, Hongooi, Hallenrm, Darth Panda, A. B., Rlevse,Mikker, Htra0497,MaxSem, Suicidalhamster, Can't sleep, clownwill eat me, Vanished user llkd8wtiuawfhiuweuhncu3tr, Fairychild, Jorvik,Sephiroth BCR, Snowmanradio, Rrburke, RedHillian, Pax85, SundarBot, Jehnidiah, Soosed, Pepsidrinka, Krich, Flyguy649, BesselDekker,Arun Philip, Nakon, Jiddisch~enwiki, Dreadstar, Only, SpiderJon, Jna runn, Springnuts, J.smith, Thejerm, The undertow, SashatoBot,LtPowers, Me123456789, SuperTycoon, Kuru, AmiDaniel, Rodsan18, Kipala, Epingchris, Sir Nicholas de Mimsy-Porpington, JoshuaZ,Adam7davies, Mgiganteus1, Tlesher, IronGargoyle, Alpha Omicron, Drakebiologylaboratory, Ckatz, 16@r, Beetstra, George The Dragon,Jimmy Pitt, Kyoko, WikiSSMN, Waggers, TastyPoutine, ElBeeroMan, Dr.K., RichardF, Cd.dolata, Jose77, Squirepants101, MrDolomite,Dl2000, KJS77, JYi, Abonilla, BranStark, HelloAnnyong, Iridescent, Laurens-af, Flangiel, Computer 3, Viton~enwiki, Cbrown1023,RekishiEJ, Gnusmas, Freecat, Az1568, Courcelles, Shohag, Tawkerbot2, Jh12, Ouishoebean, George100, MightyWarrior, Sonhyangel,SkyWalker, Xcentaur, JForget, Stifynsemons, CmdrObot, NKSCF, Sir Vicious, Tevus, Iced Kola, Makeemlighter, Brainslug, NunquamDormio, Xabi17, Babyblack, Avillia, ONUnicorn, Tim1988, MrFish, Fordmadoxfraud, Oden, Bobnorwal, JVinocur, Peripitus, Quick-sand10, Besieged, Mato, Vanished user vjhsduheuiui4t5hjri, Eubanks718, Gogo Dodo, Travelbird, BlueAg09, Anonymi, Guilingkwek,Anthonyhcole, Kimyu12, Chasingsol, Craig9000, Christal1999, Eu.stefan, Jjkusaf, Danny beaudoin, Dynaow, Bernard the Varanid,DumbBOT, Rsheridan6, Eribro, Kozuch, W746ehj, Walid Ashinehgar, Ebyabe, AstroPig7, Satori Son, Thijs!bot, Epbr123, DavidHolden,O, Kablammo, Mojo Hand, Anupam, Wompa99, Marek69, SomeStranger, Raisesdead, X201, Astronaut97, Mefanch, Davidhorman, Stan-whit607, E. Ripley, Dfrg.msc, AgentPeppermint, DaveJ7, Nick Number, Dawnseeker2000, TeslaMaster, Mario Luigi, E!, RoboServien,Escarbot, Oreo Priest, Mentisto, Cyclonenim, Gossamers, AntiVandalBot, Majorly, Luna Santin, Chubbles, Seaphoto, Yomangani, Earth-Person, Masamage, Rcej, Lordmetroid, Farosdaughter, Gdo01, Myanw, Figma, Gkhan, Joehall45, Golgofrinchian, Kariteh, JackSparrowNinja, JAnDbot, Leuko, Husond, Wiki0709, MER-C, Patxi lurra, BigBadUglyBugFacedBabyEatingO'Brian, Sheitan, JensNeu, Plm209,Chizeng, Hut 8.5, PhilKnight, Kirrages, Meeples, FaerieInGrey, Tmaguild, Celithemis, Bongwarrior, Kolindigo, VoABot II, Carlwev,AuburnPilot, Hasek is the best, JamesBWatson, BillyWagner13BS, Jojojofook, Swpb, Think outside the box, Sedmic, ThoHug, Nyttend,WODUP, Aka042, Brusegadi, Littlewood~enwiki, Mapetite526, Catgut, Cyktsui, Mtd2006, Adrian J. Hunter, 28421u2232nfenfcenc,Wrad, Freddyd945, DerHexer, Edward321, Esanchez7587, Khalid Mahmood, Crampedson, WLU, Hintswen, Scottalter, S3000, Lreynol,Hdt83, MartinBot, CliC, Medrise, Naohiro19, Rettetast, Mike6271, Rob Lindsey, Anaxial, Generalkornrow, Glrx, D Dinneen, R'n'B,CommonsDelinker, AlexiusHoratius, Nono64, Nathan J. Hamilton, *crups*, PrestonH, 3dscience, Tgeairn, J.delanoy, CFCF, Trusilver,Rgoodermote, MercuryBlue, Bogey97, Numbo3, Nbauman, TyrS, Uncle Dick, Xris0, Cogorno, Ollem, OttoMkel, Droid392, Isamed-ina, Icseaturtles, Barts1a, Sam sam sam88, McSly, Wikieditor94, Laladu1982, Holybarbarian125, Jigesh, Mikael Hggstrm, Jasonasosa,Coatbutton, AntiSpamBot, Adh30, Bushcarrot, NewEnglandYankee, SJP, Endomorphic, Kraftlos, Cmichael, Jackaranga, Juliancolton,Cometstyles, The ever man, JavierMC, Dorftrottel, Vatic7, Halmstad, Xiahou, Squids and Chips, Technick29, CardinalDan, Idioma-bot, Fharper1961, VolkovBot, Preston47, Je G., Jmrowland, Nburden, MisterKS, AlnoktaBOT, Soliloquial, Macbo, Ddbruce, Do-minics Fire, Ryan032, Philip Trueman, Fluybun, DoorsAjar, TXiKiBoT, Alesnormales, Etruria, Zidonuke, Slackerboss, Abcmmmm,Vipinhari, Bobdoc, Zurishaddai, Worldchanger816, JayC, Someguy1221, Lradrama, Sintaku, Martin451, Faggyass17, Derg999, Rois-inxx, LeaveSleaves, Pbiimgp, Seb az86556, Bob f it, Qwertyu868, Cremepu222, Kaio393, Henryodell, Earthdirt, RiverStyx23, Mad-hero88, Uannis~enwiki, Wenli, Lerdthenerd, Wolfrock, Anonymousboy04, Purgatory Fubar, Burntsauce, RaseaC, Newsaholic, Nssbm117,

20 11 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

Why Not A Duck, Ageyban, Spitre8520, HiDrNick, Doc James, AlleborgoBot, Quantpole, Bakerstmd, Deconstructhis, Doctor11,SieBot, Coee, Gopher292, Tresiden, Scarian, WereSpielChequers, Weeliljimmy, BotMultichill, Zippo12341234, Caltas, Bielle, WikiSuper Guardian, Swaq, Sr. farts alot, Drypelia, Purbo T, Alexyu1, France3470, Computer66, Vegetable4, Keilana, Le feu, Happy-sailor, Flyer22, Tiptoety, Andr987, Oda Mari, Handy Pack, Localzhee, Oxymoron83, Antonio Lopez, Judicatus, Faradayplank, Nutty-coconut, Baseball Bugs, Samatarou, KPH2293, Kochipoik, KoshVorlon, Jdaloner, Peteparker, Lightmouse, Techman224, Lozzalicious,Voltron, Fratrep, Vikram.d.singh, Drewthedude, Kudret abi, Spitre19, Maelgwnbot, Memethor, LonelyMarble, Torchwoodwho, Stat-icGull, Queen Ele 111, Thelmadatter, Nidhal79, Capitalismojo, Simile, RhiannonAmelie, Nimbusania, Mygerardromance, Vanished User8902317830, Beastinwith, Dabomb87, Florentino oro, Williammande, TubularWorld, Denisarona, Stillwaterising, Wilstrup, TheCata-lyst31, ImageRemovalBot, WikipedianMarlith, Faithlessthewonderboy, RegentsPark, ClueBot, Avenged Eightfold, GorillaWarfare, Mk3,Parvazbato59, Narom, The Thing That Should Not Be, Chris bonney, Ndenison, Arakunem, Pshahmumbai, Drmies, Bobisbob, Califor-nia123123, Polyamorph, SuperHamster, Mpd1989, CounterVandalismBot, Niceguyedc, Lbeben, Joel.delima, Blanchardb, Wallaeyozaah,Socrates SLB KA, MCR.rox.my.world, Pmcaldu, Puchiko, Smitty5555555555, Grunty Thraveswain, DragonBot, Benpayne2004, Ex-cirial, Propeng, Anonymous101, Alexbot, Jusdafax, WikieditorOOX, Ejay, Jerry Zhang, SpikeToronto, NuclearWarfare, Bomberzboy,Arjayay, Peter.C, Jotterbot, JohnnyCashIsNotDead, The Red, Kakofonous, Calor, Johnakabean, Thingg, StaticSignals, Kikos, Versus22,GlacierWolf, Psymier, DumZiBoT, Skunkboy74, XLinkBot, Fastily, Spitre, Stickee, Jovianeye, Rror, Jellotine, DaL33T, Od22, Facts707,Skarebo, PL290, Alexius08, WikiDao, JinJian, ZooFari, Airplaneman, Gggh, HexaChord, Cornucopia, Addbot, Proofreader77, Cxz111,Willking1979, Oenbach, Some jerk on the Internet, Jojhutton, Guoguo12, Ronhjones, Jncraton, Fieldday-sunday, CanadianLinuxUser,SoSaysChappy, Freqsh0, LAAFan, Glane23, Chzz, Favonian, LinkFA-Bot, West.andrew.g, Bob K31416, Numbo3-bot, Tide rolls, Ver-bal, Dr Zimbu, Krano, Luckas Blade, Gail, Zorrobot, MuZemike, Arbitrarily0, Legobot, Chaldor, 2enable, Luckas-bot, TheSuave, Yobot,Tohd8BohaithuGh1, Senator Palpatine, Fraggle81, TaBOT-zerem, Ojay123, THENWHOWAS PHONE?, Sarrus, KamikazeBot, Amicon,Tempodivalse, Synchronism, Fodo96, Backslash Forwardslash, AnomieBOT, Tryptosh, Ta2Ed, Kristen Eriksen, ThaddeusB, Pyrrhus16,Ramonesnumer1fan, JackieBot, AdjustShift, Fahadsadah, Kingpin13, Flewis, Materialscientist, Estreet58, A123a, Citation bot, Felyza,Carlsotr, Im gonna mock u'z, Deadly Dozen, Neurolysis, Parthian Scribe, Xqbot, Mia2009, Bigboithecoolest, Timir2, Owned45, FrodoMuijzer, Sionus, The Banner, Addihockey10, Rhysoverton, Capricorn42, Gigemag76, CplHare992, Wperdue, TechBot, Asaunders135,Paigntonuk, Sylis9, Calco blue, Wol1i3, Bubbyreallystinks, The Evil IP address, Vette982, Maddie!, Stink weasel, GrouchoBot, Jhb-del, Omnipaedista, Jezhotwells, H8erade, SassoBot, Davidsfarm, Cardiac Morph, Doulos Christos, X0cbabii0x, PacmanMasterisback,MerlLinkBot, Shadowjams, Erik9, A.amitkumar, Dougofborg, Hersfold tool account, Ninjadalton, Alucardxt, FrescoBot, Johser001, To-bby72, Nobuya, Master1ryan, StaticVision, Michael93555, Tavakoli543, Heyholetsgoitstimeformmyshow, VI, HJ Mitchell, Vishnu2011,VFDA, Written123, Bobthesmall, AlexandKevin, GraemeHR, Finalius, Purplepalsss, DivineAlpha, HamburgerRadio, Tonoe84, Citationbot 1, Javert, Amplitude101, Intelligentsium, Demoish, Pinethicket, Haaqfun, Anjelelsy, Diomedea Exulans, LinDrug, Jackrace, CalmerWaters, Hamtechperson, Skydiver123, A8UDI, Batman n' robin, 2v11, Thisisajm, Aetylus, SpaceFlight89, ContinueWithCaution, Foot-warrior, Dac04, SkyMachine, Tgv8925, TobeBot, Fama Clamosa, Lotje, Callanecc, Dinamik-bot, Vrenator, SCOOTTR666, January,Allen4names, Manegro, Thomas.Nelson05, Manny gunz, Weedwhacker128, Jhenderson777, Adi4094, Brian the Editor, Suusion of Yel-low, Gazman 1874, Bookfairy12, Ryanenglish, Tbhotch, Stroppolo, Jerkonyourface, Lord of the Pit, Rahuloof, Angelito7, DARTH SID-IOUS 2, Andrea105, Dgonzalezz62, The Utahraptor, Pisean282, TjBot, Bento00, Accarpenter, Ripchip Bot, FetchcommsAWB, Forenti,Aircorn, Pavz01, Salvio giuliano, DASHBot, EmausBot, WikitanvirBot, Baswellbrat408, Immunize, Nadelschwein, Fitlad8, Joeywallace9,Slayeroyd, GoingBatty, RA0808, Gregwins, Xdiashley, RenamedUser01302013, Robbyjob, Sumukhaprsd, Solarra, Zeeber79, Buil-lon sexycat798, Slightsmile, Tommy2010, Thegreatamericanlagerhead, Danny32797, Wikipelli, Vedicsciences, Lamb99, Thecheesykid,Sarpasarpeti101, KzKrann, Holygirl12, Katanaofdoom, ZroBot, F, MithrandirAgain, TheCryptiiC, Bowserxxx, Lateg, Bringerofrain,Athn, Derekleungtszhei, Midas02, Aznluck, Monkeys 1234567, SporkBot, Monterey Bay, Wayne Slam, Frigotoni, OnePt618, Karthikndr,Brandmeister, L Kensington, Upinsmok, ChuispastonBot, Clementina, Torbay007, RockMagnetist, Peter Karlsen, Jack.OSull, Herk1955,DASHBotAV, Shi Hou, Tvanbr, Socialservice, Torklarlund, Linear equation, Johnbayot, Xonqnopp, ClueBot NG, Gareth Grith-Jones,Hatwan~enwiki, Superstarmcawesomeguy, Nutter453, A520, Vamos124, Stormodboyo, Cntras, Muon, MBC2011, Widr, DANIELzom-bie, Tinklem125, Reverence Still, Diyar se, Noname565062, Rxnaomi, Plaidman, Beavercake1996, Kinaro, Colour Spectrum, BG19bot,Roberticus, Sydneytriplett, Lo Ximiendo, Tgdrebtyrtytyty, Riyaspace1, ., Wiki13, David815, Tgdreptyrtytyty, AvocatoBot, Met-ricopolus, Gautehuus, Mark Arsten, TheMrSachinator, Smasher1220, Jjwilson23, Eman2129, Martin.uecker, Sacchipersempr, Jjavon12,MultiEggboy, OsamaIsALiar, DPL bot, This article has died, Snow Blizzard, NotWith, 12barney, Anatomist90, DBooty1907, Nutter454,Madmanad15, Superkonrad, Astros4477, Shaun, A1candidate, BattyBot, James Disambiguation-Smith, Barkha dhamechai, Mrt3366,12345benz, Lenitudent, Khazar2, EuroCarGT, Mathelius, Santga, Diabolical Potato, Mugwhyjackyboy, Darth Molo, Dexbot, SantoshBot,Currymuncher69er, WhistleGangHumor, Webclient101, Grapesodaandalittlechicken, Lugia2453, JakobSteenberg, Jamesx12345, What-IfWeCould, DocElisa, Bolt77, Jackstersam, Reatlas, Sydney.rossow, Dartthrower1234, Haha Destroyer, Mrbuttery03, BreakfastJr, Mel-onkelon, Iztwoz, Eyesnore, Tentinator, Ugog Nizdast, Spyglasses, LT910001, BruceBlaus, Tattrox, Josepaulopineda, Afeerah, The Cos-mos Master, Ahmedhassan199z, AlexBardoel, Spec95, Madre102, AB Blake, Coolman1999, Alimalik10, Saloofbourrow, Gokufart,Yoloswag420420420420420, Lukesmum, Wco99, Monkbot, Qwertyxp2000, HoneyBadger4, Benrusholme, Ujimix, Tilifa Ocaufa, Kas-parBot, Chand3994, Unenthusiastic and Anonymous: 1949

11.2 Images File:2004_Heart_Wall.jpg Source: https://upload.wikimedia.org/wikipedia/commons/3/35/2004_Heart_Wall.jpg License: CC BY 3.0

Contributors: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Original artist: OpenStaxCollege

File:2006_Heart_Musculature.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/0e/2006_Heart_Musculature.jpg Li-cense: CCBY 3.0Contributors: Anatomy&Physiology, ConnexionsWeb site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Originalartist: OpenStax College

File:2010_Chordae_Tendinae_Papillary_Muscles.jpg Source: https://upload.wikimedia.org/wikipedia/commons/a/a7/2010_Chordae_Tendinae_Papillary_Muscles.jpg License: CC BY 3.0 Contributors: Anatomy & Physiology, Connexions Web site.http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Original artist: OpenStax College

File:2011_Heart_Valves.jpg Source: https://upload.wikimedia.org/wikipedia/commons/b/b9/2011_Heart_Valves.jpg License: CC BY3.0 Contributors: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Original artist: Open-Stax College

11.2 Images 21

File:2020_SA_Node_Tracing.jpg Source: https://upload.wikimedia.