The Heart: Anatomy and Physiology
The Heart Rate
The heart rate is the number of double-sounds auscultated for one minute. The first part of the double-sound (1st heart sound, S-1) is the rebound of blood against the heart wall after contraction of the ventricles (systole) and closure of the atrioventricular valves (AV valves—mitral and tricuspid). The second part of the double-sound (2nd heart sound, S-2) is the back-recoil of blood against the closed semilunar valves—pulmonary and aortic—so-called because they are half-moon shaped). The two sounds are magnified by the stethoscope as “lub-dup”. In the adult, the average heart rate is about 70 beats per minute. The range is 60 to 100, with exceptions. Below 60 is bradycardia; above 100 is tachycardia. The heart rate or pulse is measured by counting the number of beats for 15 seconds and multiplying by four. If an arrhythmia is suspected, the number of beats is counted for one minute.
Electrical Activity of the Heart
Nerve and muscle cells are specialized for the conduction of electrochemical impulses down the length of the cell. At rest, there is an abundance of sodium on the outside of the cell and an abundance of potassium on the inside. When the cell is stimulated, the impulse proceeds down the cell in a fuse-like fashion. The cell becomes permeable to sodium and sodium flows into the cell. This is depolarization. Potassium then flows out of the cell, restoring electrochemical balance. This is repolarization (later the sodium potassium pump restores the proper ions to the correct sides of the membrane).
During repolarization calcium ions enter the cell by way of channels called “slow channels”, or “calcium channels”. The conduction system of the heart is activated and calcium initiates contraction of the heart. This electrical activity precedes the mechanical, or pumping action, by milliseconds, and is recorded as the electrocardiogram (EKG, ECG).
Calcium-channel blocking agents such as verapamil (Isoptin, Calan), nifedipine (Procardia) and diltiazem (Cardizem), used in the treatment of coronary artery disease and hypertension, block the influx of calcium during repolarization and slow the heart-rate and force of contraction.
The Conducting System of the Heart
Certain heart muscle fibers depolarize faster than others and constitute the conducting system of the heart. The sino-atrial node (SA node) in the right atrium is the first area to depolarize and sets the heart-rate at about 70 beats per minute. This is the pacemaker. Depolarization spreads throughout the atria (atrial depolarization) and creates the first wave of the EKG, the P-wave. The atrioventricular node (AV node), lying at the interatrial septum, depolarizes and the wave spreads down the interventricular septum to
the ventricles. The ventricles depolarize and create the QRS-wave on the EKG. Contraction of the ventricles then takes place (systole). The T-wave is repolarization of the ventricles (the wave for atrial repolarization is masked by the QRS-complex).
The Heart as a Pump
The ensuing sequence of mechanical events follows the electrical activity of one heart-beat:
1. Diastole: relaxation as the ventricles fill.
2. Atrial systole: contraction of the atria. Blood moves through the AV valves into the ventricles.
3. Systole: contraction of the ventricles. High pressure closes the AV valves (1st heart sound).
4. Ventricular ejection: blood is forced out the aorta and pulmonary artery.
5. Early diastole: as the pressure lessens after ejection, the pulmonary and aortic valves close (2nd heart sound).
Central Regulation of the Heart
The autonomic nervous system regulates the heart (and other internal organs such as the eye, vessels, lungs, GI tract, bladder and kidney). The system is divided into two branches, the main functions of which are essentially opposites: the sympathetic division (“fight or flight” response) secretes norepinephrine at the synapse (adrenergic); the parasympathetic, or “vegetative” division, maintains normal body functions and secretes acetylcholine at the synapse (cholinergic). The central nervous system (brain and spinal
cord) controls autonomic responses.
The medulla and part of the pons control the heart rate and blood pressure. The vasomotor center is a part of a primitive inner core, the reticular formation, running through the brainstem and upper spinal cord. Stimulation of one part of the vasomotor center (sympathetic) causes an increase in heart rate and vasoconstriction, raising the blood pressure. Stimulation of another part causes inhibition of vasoconstriction, resulting in vasodilation and a fall in blood pressure. Stimulation of a third part (parasympathetic) causes a decrease in heart rate by way of the vagus nerve.
The heart has a built-in rhythmicity. Nervous stimulation is not needed for contraction. If removed from the body, it will depolarize and contract for a long time. The autonomic nervous system modifies the rate and force of contraction. Receptors for sympathetic nerves are divided into two types, alpha and beta. Beta is subdivided into beta-1 and beta-2 (actually, this is a simplification for clinical purposes—many receptor subtypes exist). Beta-1 and beta-2 receptors are found in many organs and one type predominates. Alpha receptor stimulation causes contraction of vascular smooth muscle and vasoconstriction. Beta-1 receptors predominate in the heart. Beta-2 receptors predominate in bronchial smooth muscle. Stimulation of beta-2 receptors causes bronchodilation. Sympathetic stimulation of the heart releases norepinephrine which stimulates beta-1 receptors and causes an increase in heart rate (chronotropic action) and force of contraction (inotropic action); stimulation of parasympathetic fibers from the vagus nerve decreases the
Beta-blocking agents such as propranolol (Inderal), nadolol (Corgard), metaprolol (Lopressor) and atenolol (Tenormin) slow the heart rate and force of contraction by blocking beta-1 receptors. Some beta-2 blocking also occurs (bronchoconstriction) with older drugs. Most of the newer agents are more beta-1 selective.