Physiology of the normal heart

The two essential characteristics of the normally functioning heart are contractility and rhythmicity. Together they provide sufficient oxygenated blood to meet moment-to-moment metabolic needs. The electrical and mechanical events are regulated by the level of venous return, (preload), outflow resistance (afterload), inotropic state of the heart muscle, (contractility), and heart rate. The venous return determines the degree of stretch of cardiac muscle fibres in diastole and the strength of the resulting subsequent contraction. The outflow resistance is the afterload against which the ventricles contract during systole. It determines the pressures the ventricles must develop to ensure appropriate forward blood flow. The prevailing level of sympathetic outflow modulates the strength of cardiac contraction. Increases in sympathetic outflow increase the strength of cardiac contraction and work output. The renin-angiotensin system and the natriuretic peptides also both contribute importantly to cardiac performance. Angiotensin II is a potent vasoconstrictor which increases blood pressure and the production of aldosterone which promotes renal reabsorption of sodium and an increase in circulating blood volume. The natriuretic peptides are produced mainly in the atria and respond to increased atrial distension by inducing diuresis, thus reducing cardiac and circulating blood volume. Coronary blood flow is largely diastolic. It is finely adjusted to metabolic needs, and the impacting factors are principally the blood pressure in the ascending aorta, myocardial metabolism and the prevailing level of neurohumeral control. Nevertheless, control resides largely in the heart itself as it responds with intrinsic rhythmicity to these mechanical and neural influences when at rest, and during exercise.