[box type=”download”] SA node as the originator of the cycle, modulated via autonomic nerves Activation of atrial myocytes via gap junctions and desmosomes Role of annulus fibrosus in limitation of exctitation AV node Relatively slow conduction in light of its small cells and purpose of this slowness Propagatory fibres Bundle of His and Purkinje fibres as carriers of the depolarisation wave Basis of the ECG Einthoven’s triangle as a representation of voltages [leads I, II, III] [/box]
Basic anatomy
The heart consists of four chambers – two thin-walled atria and two muscular ventricles.
The atria are separated from the ventricles by a band of fibrous connective tissue (annulus fibrosus).
Annulus fibrosus provides a skeleton for the attachment of muscle and the insertion of cardiac valves.
It also prevents electrical conduction between the atria and ventricles, except at the atrioventricular node (AVN).
The walls are formed from cardiac muscle (myocardium).
The inner surface of the heart is covered by a thin layer of cells called the endocardium, similar to vascular endothelial cells (antithrombogenic surface).
The outer surface is covered by the epicardium, a layer of mesothelial cells.
The whole heart is enclosed in a thin fibrous sheath (the pericardium), containing interstitial fluid as a lubricant.
Cardiac valves
Blood flows from the right atrium into the right ventricle via the tricuspid (three cusps or leaflets) atrioventricular (AV) valve, and from the left atrium to the left ventricle via the mitral (two cusps) AV valve.
The AV valves are prevented from being everted into the atria by fine cords (chordae tendinae or trabeculae) attached between the edge of the valve cusps and papillary muscles in the ventricles. Blood is ejected from the right ventricle through the pulmonary semilunar valve into the pulmonary artery, and from the left ventricle via the aortic semilunar valve into the aorta; both semilunar valves have three cusps. When closed, the cusps form a tight seal at the commissure (line at which the edges meet). They open and close passively according to the pressure difference across them. Stenosis describes narrowed valves; stenotic AV valves impair ventricular filling, and stenotic outflow valves increase afterload and thus ventricular work. Incompetent valves do not close properly and leak (regurgitate).
Cardiac pacemaker, conduction of the impulse and electrocardiogram
The heart beat is initiated in the sinoatrial node (SAN- by Spontaneous depolarization), a region of specialized myocytes in the right atrium, close to the coronary sinus.
Its rate is modulated by autonomic nerves.
Action potentials in the SAN activate adjacent atrial myocytes via gap junctions contained within the intercalated discs; desmosomes provide a physical link.
A wave of depolarization and contraction therefore sweeps through the atrial muscle.
This is prevented from reaching the ventricles directly by the annulus fibrosus.
AVN, located between the right atrium and ventricle near the atrial septum contains small cells and conducts slowly; it therefore delays the impulse for ∼120 ms, allowing time for atrial contraction to complete ventricular filling.
The impulse is transmitted from the AVN by specialized, wide and thus fast conducting myocytes in the bundle of His and Purkinje fibres, by which it is distributed over the inner surface of ventricles.
From here, a wave of depolarization and contraction moves from myocyte to myocyte across the endocardium until the whole ventricular mass is activated.
Electrocardiogram.
The classical ECG records the voltage between the left and right arm (lead I), the right arm and left leg (lead II), and the left arm and left leg (lead III).
This is represented by Einthoven’s triangle.
The size of voltage at any time depends on the quantity of muscle depolarizing (more cells generate more current), and the direction in which the wave of depolarization is travelling (i.e. it is a vector quantity).
Thus, lead II normally shows the largest deflection during ventricular depolarization, as the muscle mass is greatest and depolarization travels from apex to base, more or less parallel to a line from the left hip to the right shoulder.
Coronary circulation
The left and right coronary arteries arise from the aortic sinus.
Most of the blood returns to the right atrium via the coronary sinus.
The large and small coronary veins run parallel to the right coronary arteries, and empty into the coronary sinus.
Small vessels, such as the thebesian veins, empty into the cardiac chambers directly.
The left ventricle is mostly supplied by the left coronary artery.
During systole, contraction of the ventricles compresses the coronary arteries and suppresses blood flow.
As a result, more than 85% of left ventricular perfusion occurs during diastole.
This is problematic in disease if the heart rate is increased (e.g. exercise), as the diastolic interval is shorter.