The Heart

The Heart

Heart

The Heart

3HeartHollow, muscular organSlightly larger than the size of a fistContains 4 chambersfound in chest between lungs surrounded by membrane called PericardiumPericardial space is fluid-filled to nourish and protect the heart.

Serous membraneContinuous withblood vesselsThe Heart Wall3 layers of the heart wall are:

Pericardium outer layer surrounding heartMyocardium Middle layerEndocardium Inner layer

Between Pericardium and Myocardium

Pericardial cavity made up of 2 layers of serous membraneParietal layerVisceral layer

Pericardial space is fluid-filled tonourish and protect the heart.

Myocardial tissue

7StructureUnder the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to glue the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).Cardiac MuscleCardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells).

It is striated due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte.

They contain one nucleus but many mitochondria, which provide the energy required for contraction.

8StructureUnder the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to glue the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).Myocardial Cells Interconnected by Gap Junctions

Cardiac Muscle Cell

The intercalated discs have two important functions to:

glue the myocytes together so that they do not pull apart when the heart contracts

allow an electrical connection between the cells, a vital to the function of the heart as a whole.

The electrical connection is made via special gap junctions between adjoining myocytes.

These are pores through which small ions and therefore electrical current can pass.

As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium. 10Cardiac Muscle CellA unique feature of cardiac muscle is the presence of intercalated discs. The intercalated discs have two important functions to:

glue the myocytes together so that they do not pull apart when the heart contracts

allow an electrical connection between the cells, a vital to the function of the heart as a whole.

The electrical connection is made via special gap junctions between adjoining myocytes.

These are pores through which small ions and therefore electrical current can pass.

As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium.

11StructureUnder the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to glue the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).The Heart

Valves

The Heart contains four valves The Tricuspid valve opens from the R atrium into the R ventricle The Pulmonary (semilunar) valve opens from the R ventricle into the pulmonary arteryThe Mitral valve opens from the L atrium into the L ventricleThe Aortic (semilunar) valve opens from the L ventricle into the Aorta

The Heart: ValvesFunction of the valvesSituated at the entrance and exit of the ventriclesEnsure that blood moves only in one directionForward Blood flows though the valves as a result of pressure changes14

The Cardiac CycleRefers to the repeated pattern of contraction and relaxation of the heart

Phase of contraction is called systole, Phase of relaxation is called diastole

The heart has a two step pumping action: the atria contract simultaneously, followed approx 0.1-0.2 seconds later by the ventricles.16Cardiac CycleSystoleContractile phase of heartElectrical and mechanical changesE.g. blood pressure changesE.g. blood volume changesDiastoleRelaxation phase of heartTakes twice as long as systoleE.g. resting HR = 60Systole = 0.3 sDiastole = 0.6 sCardiac cycleblood from the bodyblood from the lungsThe heart beat begins when theheart muscles relax and bloodflows into the atria.STEP ONE18The atria then contract andthe valves open to allow bloodinto the ventricles.Cardiac cycleSTEP TWO19Cardiac cycleThe valves close to stop bloodflowing backwards.

The ventricles contract forcing the blood to leave the heart.

At the same time, the atria arerelaxing and once again filling withblood.

The cycle then repeats itself.STEP THREE20

1. Atria fill with blood 2.AV valve opens when pressure in atria exceeds ventricle

3. Blood flows from atriato ventricles through open AV valve. This contributes approx 80% of end diastolic volume

4. Atria contractContributing approx 20%To end diastolic volume5. Ventricles contract, about two thirds of the volume they contain is ejected, leaving one third called the end systolic volumeThe Cardiac cyclecycle22

Heart conduction Electrical Activity of the HeartContraction of heart depends on electrical stimulation of myocardiumImpulse is initiated on right atrium and spreads throughout the heartMay be recorded on an ECGElectrical Conduction SystemMyocardial CellsCharacteristicsautomaticity: cells can depolarize without any impulse from outside source (self-excitation)

excitability: cells can respond to an electrical stimulus

conductivity: cells can propagate the electrical impulse from cell to another

contractility: the specialized ability of the cardiac muscle cells to contract

The Hearts conduction pathway

CONDUCT26Depolarization and Impulse ConductionHeart is autorhythmicDepolarization begins in sinoatrial (SA) nodeSpread through atrial myocardiumDelay in atrioventricular (AV) node

24 Sept. 2008EKG-Lab.ppt27Depolarization and Impulse ConductionSpread from atrioventricular (AV) nodeAV bundleBundle branchesPurkinje fibers

24 Sept. 2008EKG-Lab.ppt28ElectrocardiogramRecords electrical activity of the heartP waveAtrial depolarizationQRS complexVentricular depolarizationT waveVentricular repolarizationElectrical conduction

P Wave represents atrial depolarisation

QRS Wave represents ventricular depolarisation

T Wave represents Ventricular repolarisationCardiac blood flowCardiac blood flow is different from blood flow in other organs.Blood flows around coronary vessels during diastole.Myoglobin in myocardial cells stores oxygen.This ensures that the heart muscle has a constant supply.

Cardiac OutputRefers to the amount of blood ejected from the left ventricle of the heart per minuteDetermined by stroke volume and heart RateCardiac output = stroke volume x heart rate

Factors which influence blood pressureCardiac output (CO) Total Peripheral Resistance (TPR)

Or BP =CO x TPR

Cardiac OutputRange of normal at rest is 4 6 L.minDuring aerobic activity the increase in cardiac output is roughly proportional to intensity.Max. Q is in range of 20 40 L.min, depending on size, heredity, and conditioning.Heart Rate

Range of normal at rest is 50 100 b.mIncreases in proportion to exercise intensityMax. HR is 220 ageMedications or upper body exercise may change normal responseFactors affecting Heart RateGenderAutonomic nerve activityAgeCirculating hormones e.g adrenaline, thyroxineActivity and exerciseTemperatureThe baroreceptor reflexEmotional states Waugh and Grant (2006)Stroke VolumeRange of normal at rest is 60 100 ml.bDuring exercise, SV increases quickly, reaching max. around 40% of VO2 max.Max. SV is 120 200 ml.b, depending on size, heredity, and conditioning.Increased SV during rhythmic aerobic exercise is due to complete filling of ventricles during diastole and/or complete emptying of ventricles during systole.Cardiac blood supplyThe heart receives its blood supply via the coronary arteries.These arteries supply a huge network of capillariesEnsures that myocardial cells are close to their blood supplyDiffusion of gases between the myocardial cells and capillaries occurs very quicklyThe heart is influenced by autonomic nerves originating in the cardiovascular centre in the medulla oblongata

Consisting of sympathetic and parasympathetic nerves with antagonistic

autonomic nerves

SympatheticparasympatheticCardiac control center in medulla oblongatamaintains balance between the twoParasympathetic: from medulla oblongata (vagus nerve)

Nerve branches to S-A and A-V nodes, and slows rate

Parasympathetic activity :

Can increase to slow heart rate

Can decrease to increase heart rate

Sympathetic nervous system (celiac plexus)

SA node,AV node and the myocardium of the atria and ventricles.

Sympathetic activity : increases HR

force of contraction

SummaryThe heart is a four chambered pump, which is effectively two pumps working togetherThe heart contains valves which function to ensure no backflow of bloodThe wall of the heart is made up of the following layers: pericardium, myocardium, endocardiumThe heart has autorhythmicity it will beat without outside nervous inputThe heart has a specialist conduction pathway- ensuring a smooth coordinated contraction, Cardiac muscle contains large numbers of mitochondria, and the heart has an excellent blood supply