Cardiac Anatomy

Outline

Heart Anatomy Muscle Layers of the heart Myocardial muscle fibres Atria and Ventricles Semilunar and Atrioventricular Valves Blood flow through the Heart Cardiac Cycle Coronary Circulation Coronary Artery Blood flow Nervous supply to the heart

Heart

The heart is a four chambered muscular organ located in the chest behind the sternum in the mediastinal cavity between the lungs and rest on the diaphragm.

The heart is usually the size of a person’s fist and typically weighs 225-340gm, with size varying upon height, age, sex, athletic condition and heart disease.

The heart’s function is to pump blood throughout the body in response to the body’s metabolic requirements. Image:www.nlm.nih.gov

Muscle Layers of the Heart

The heart is composed of three distinct

muscular layers and a dense fibrous tissue

which makes up the “skeleton” of the heart. The muscular layers are made up of:

Endocardium. Myocardium Epicardium Pericardium,

Within the myocardium consists a specialized electrical conduction system consisting of the :

Sinoatrial (SA) Node, Atrioventricular (AV) Node, the Bundle of HIS Purkinje Fibres.

Endocardium

Endocardium:The endocardium is a

thin, smooth layer of epithelium and connective tissue membrane which lines the heart’s inner chambers, valves, chordae tendineae and papillary muscles.

The endocardium also extends into the lumen of the coronary arteries (tunica intima), systemic arteries, veins and capillaries; creating a continuous closed circulatory system

Image: www.stepwallpaper.com

Myocardium The myocardium is the muscular middle layer of

the heart consisting of cardiac muscle fibres which are responsible for contraction in a twisting motion

Is divided into two areas: Innermost half subendocardial Outermost half subepicardial

The muscle fibres are of the myocardium are separated by connective tissue which have a rich supply of capillaries and nerve fibres

Thickness of the myocardium varies from one chamber to another and is related to the amount of resistance that must be overcome to pump blood from each chamber i.e:

the atria encounter little resistance and hence are thinned walled

The LV is three times thicker than the RV because the pressure in the pulmonary circulation is much less than the arterial circulation

Image: classes.midlandstech.edu

Epicardium

The heart’s outer most layer is called the epicardium and is continuous with the inner lining of the pericardium at the heart’s apex.

Epicardium contains: Blood capillaries Lymph capillaries Nerve fibres Fat

The main coronary arteries lie on the epicardial surface and feed this layer before entering the myocardium and supplying the heart’s inner layers

The subendocardial area is at greatest risk of ischaemia because: This area as a high demand for O2 This area is fed by the most distal branches of the coronary circulation

Pericardium

Is a double walled sac consisting of two layers which encase the heart and attaches to the roots of the great vessels and the fascia of the diaphragm which helps protect the heart from trauma and infection.

The pericardium has two portions: The outer layer is called the parietal

pericardium and is fibrous and non distensible.

It then doubles back to form the visceral pericardium (also termed the epicardium) which directly adheres to the surface of the myocardium.

The space between the two (pericardial Space) usually contains 10-20ml of serous fluid, termed pericardial fluid

The pericardial fluid and acts as a lubrication to enable the heart to beat within without friction.

Excess pericardial fluid (pericardial effusion) can compromise the heart’s ability to function by causing a tamponade.

Image:www.nlm.nih.gov

Pericardium

If the pericardium becomes inflamed (pericarditis) more serous fluid is secreted

Pericarditis can be caused by bacterial or viral infection, rheumatoid arthritis destruction of the heart muscle post MI and many other causes

A build up of blood and fluid within this space can compress the heart and impair contraction by impairing chamber filling and the stoke volume

As a result blood returning to the heart is also decreased. These changes can result in a life threatening condition call Cardiac Tamponade

Image:www.nlm.nih.gov

Cardiac Muscle

Cardiac muscle is only found within the heart and makes up the wall of the heart Each muscle is made up of thousands of muscle cells, which is enclosed in a

membrane called sarcolemma, which:Have holes within leading to T-tubules which are extensions of the cell

membraneOther tubules the sarcoplasmic reticulum located within store calciumMuscles need calcium to contract

Within each cell are mitochondria, the energy producing parts of the and hundreds of long tube like structures called myofibrils

Myofibrils are made up of many sarcomeres, the basic protein units responsible for contraction

Channels within the cell membranes allow electrolytes such as Na, K and Ca to pass through during various phases of the cell cycle

When the myocardium is relaxed the calcium channels are closed T-tubules conduct impulses form the cells membrane down into the sarcoplasmic

reticulum, where the Ca Channels open, where the cells are stimulated to contract Thus the force of contraction depends largely on the concentration of Ca within the

blood

Cardiac Muscle Each sarcomere is composed of thin and thick filaments:

Thin filaments are made up of actin and actin binding proteins which are made up of tropomyosin, troponin-T, troponin-C and troponin-I

Thick filaments are made up of hundreds of myosin molecules

Contraction occurs when projections on the thin filaments interact with the thick myosin an form cross bridges

This allows the actin filaments to slide over the myosin filaments causing shortening of the muscle cells

The force of contraction is directly related to the amount of end-diastolic stretch, determined by the amount of blood within the heart at end-diastole. Increased stretch of the myofibrils results in increased force which can be compared to that of a rubber band.

When myocardial cells die as in MI, substances within the cells pass through the ruptured membranes and into the blood stream. These substance involve the troponins, creatinine kinase and myoglobin

Blood tests are used the measure the presence of these markers and verify MI

Atria and Ventricles The heart is effectively divided into four

chambers and functions as a two sided pump.

The left and right sides are separated by

a muscular wall called the septum.

The right side of the heart accepts blood form the venous system from 3 different vessels:

Superior vena cava Inferior vena cava Coronary sinus

Venous blood is deoxygenated and from the RA is pumped to the lungs where it becomes oxygenated.

Once oxygenated it travels through the left side of the heart where it is pumped throughout the circulatory system. .

Image:www.nlm.nih.gov

Atria

Atria The atria are thin walled, low pressured chambers, act as a reservoir to facilitate the

passage of blood through to the ventricles.

The wall thickness of both atria vary slightly: The right atrium is 2mm thick The left atrium is 3mm thick

When the valves open 70-75% of this blood flows directly into the ventricles, they then contract to push the remaining volume through.

This contraction termed the atrial kick contributes to 25-30% of cardiac output and also prevents the pooling of blood within the atria which has the potential to develop into life threatening clots.

Ventricles

The ventricles are larger and thicker walled chambers than the atria and are responsible

for pumping blood to the lungs and systemic circulation. When the LV contracts it normally produces an impulse which can be felt at the apex

of the heart (apical impulse). This occurs because when the LV contracts it rotates forward and can be felt 5th intercostal space mid clavicular and is called the point of maximal impulse (PMI)

The outside surface of the hearts has grooves called sulci, in which the coronary arteries and their branches lie in.

The coronary sulcus (groove) encircles the outside of the heart and separates the atria from the ventricles

The Right Ventricle pumps low pressured venous blood to the pulmonary circulation The Left Ventricle is responsible for pumping high pressured oxygenated blood

throughout systemic circulation, its wall thickness is triple that of the RV and is to facilitate this.

The diameter of the ventricular walls: Right ventricular wall thickness is 3-5mm Left ventricular wall thickness is 13-15mm

Ventricles Each ventricle holds approx 150ml and

normally ejects about half this volume 70-80ml with each contraction also termed stroke volume.

The percentage of blood ejected from the LV with each contraction is called ejection fraction, one of the most important piece of diagnostic information reflecting cardiac function.

A normal ejection fraction is 50-60%. A person is said to have impaired ventricular function when the ejection fraction is less than 40%

Ejection Fraction is estimated on echocardiograms and left ventricular angiograms

Image:www.nlm.nih.gov

Valves

The heart has a skeletal fibrous tissue located within which supports its structure. The

skeleton is made up of four rings of thick connective tissue which surrounds the base of

the pulmonary trunk, aorta and heart valves. The skeleton:

Helps from the partitions (septum) which separate atria and ventricles Provides secure attachments for the valves and chambers

There are 4 valves within the heart: Two sets of atrioventricular valves Two sets of semilunar valves

The flow of blood through the heart is only made possible by the valves which lie within, separating the chambers and great vessels.

The valves’ function is to isolate blood in each chamber and to prevent backflow during contraction.

They open and close in response to the changes in pressure within the chambers, thus ensuring blood is always pumped in a forwards motion preventing backflow.

Valves

Atrioventricular Valves Separate atria from ventricles, tricuspid and mitral and consist of:

• Tough, fibrous rings (annuli fibrosi)• Leaflets or cusps of endocardium• Chordae tendinae• Papillary muscles

Tricuspid valve lies between the right atrium and right ventricle

Mitral valve lies between left atrium and left ventricle

Tricuspid consists of three leaflets

Mitral consists of two leaflets (bicuspid)

Tricuspid valve is larger in diameter and thinner than the mitral

ValvesChordae Tendineae

Are thin strands of connective tissue which attach to under side of the valve and small mounds attached to the endocardium called papillary muscles

Papillary muscles project inward from the ventricles and contract and relax with the ventricles

When the ventricles contract the papillary muscles pull on the chordae tendineae stretching them thus preventing the leaflets from bulging back into the atria

Valves

Semilunar ValvesConsist of the pulmonic and aortic valves

Their role also is to prevent back flow from the pulmonary vein and aorta into the ventricles

Have three cusps shaped like half moons

Their openings are small than those of the AV valves

Have thicker leaflets than those of the AV valves

SL valves are not attached to chordae tendineae

Blood flow through the Heart

The right atrium receives venous (deoxygenated) blood from: Superior vena cave (from head and upper extremities) Inferior vena cave ( form lower body) Coronary sinus (from coronary circulation)

Blood then flows into right ventricle through the tricuspid valve When Right ventricle contracts tricuspid valve closes and blood is ejected into the

pulmonary trunk via the pulmonary artery. The pulmonary trunk divides into the right and left pulmonary artery which carries

blood to each lung where gaseous exchange occurs Once oxygenated blood flows form capillaries into pulmonary veins where it drains

into the left atrium Blood flows form the left atrium into the left ventricle through the mitral valve When the left ventricle contracts the aortic valve opens and blood is ejected into he

aorta where it is then distributed throughout the systemic circuit. The mitral valve closes during left ventricular contraction to prevent back flow into the

left atrium

Blood flow through the heart

Image: www.quizlet.com

Reference List

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o Darovic, G. Haemodynamic Monitoring, Invasive and Clinical Application; 2005. W.B. Saunders Compamy

o Hampton, J. ECG Made Easy. Sixth Edition. 2003. Churchill and Livingstone

o Huszar, R. Basic Dysrhythmias, Interpretation and Management. Second Edition. 2004. Mosby – Year Book, Incorporated.

o Mayer, B; Duksta, C; and Follin, S. ECG Interpretation Made Incredibly Easy. 2002. Springhouse Corporation.

o Purdie, R and Earnest, S. Pure Practice for 12-Lead ECG’s. 1997. A times Mirror Company

o Runge, M and Ohman, E. Netter’s Cardiology. 2004. Icon Learning Systems, MediMedia, Inc