How Your Heart Keeps Beating Without Being Told To

Your heart is a fist-sized muscle that contracts and relaxes without rest, pushing blood through a network of vessels that reaches every cell of the body.

This lesson covers the structure of the heart and blood vessels, the cardiac cycle and ECG, double circulation and its regulation, and circulatory disorders.

The human heart is a four-chambered muscular organ with two thin-walled atria above two thick-walled ventricles, separated by septa and guarded by valves, while arteries carry blood away from the heart, veins return it, and capillaries allow exchange with the tissues.

External structure. The heart lies in the thoracic cavity between the lungs, tilted slightly to the left, inside a double-walled pericardium.

Internal structure:

- Septa separate the two atria and the two ventricles
- The tricuspid valve guards the right atrioventricular opening, and the bicuspid or mitral valve the left
- Semilunar valves guard the openings into the pulmonary artery and aorta
- Nodal tissue — the sino-atrial node (SAN) in the right atrium, the atrio-ventricular node (AVN), the bundle of His and Purkinje fibres

Blood vessels:

- Arteries — thick, elastic walls; carry blood away from the heart under high pressure
- Veins — thinner walls and valves that stop backflow; carry blood towards the heart
- Capillaries — walls one cell thick, allowing exchange of gases, nutrients and wastes

An everyday example. The veins on the back of your hand bulge when the arm hangs down, as blood collects behind their valves.

The substance. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — arteries and veins are named by their direction relative to the heart, not by oxygen content.

The cardiac cycle is one round of systole and diastole of the atria and ventricles, lasting about 0.8 seconds; stroke volume is the blood pumped per beat, cardiac output the blood pumped per minute, and an ECG a record of the heart's electrical activity.

The cardiac cycle:

- Joint diastole — all chambers relax, and blood fills the ventricles from the veins
- Atrial systole — the SAN fires and the atria contract, adding about 30 per cent more blood to the ventricles
- Ventricular systole — the impulse passes through the AVN, bundle of His and Purkinje fibres; the ventricles contract, the atrioventricular valves shut with the first sound, lub, and blood is forced into the aorta and pulmonary artery
- Ventricular diastole — the semilunar valves shut with the second sound, dub, and the cycle begins again

Stroke volume and cardiac output. Stroke volume is about 70 mL, and cardiac output = stroke volume × heart rate:

$$70\text{ mL}\times 72\text{ beats per minute}=5040\text{ mL per minute}$$

ECG (electrocardiogram):

- P wave — depolarisation of the atria
- QRS complex — depolarisation of the ventricles, starting ventricular contraction
- T wave — the ventricles returning to normal, marking the end of systole

An everyday example. An ECG taken at a local clinic prints these waves on graph paper, and counting the QRS complexes over a set time gives the heart rate.

The substance. Heart sounds come from valves closing, not from the muscle contracting — lub is the atrioventricular valves shutting, and dub is the semilunar valves.

In double circulation blood passes through the heart twice in each circuit — through the pulmonary circuit to the lungs and the systemic circuit to the body — and the heartbeat, started by the SAN, is adjusted by nerves and hormones.

Double circulation:

- Pulmonary circulation — from the right ventricle to the lungs through the pulmonary artery, and back to the left atrium through the pulmonary veins
- Systemic circulation — from the left ventricle to the body through the aorta, and back to the right atrium through the venae cavae

Regulation of the heart:

- The heart is myogenic — the SAN, the pacemaker, starts each beat by itself
- A centre in the medulla oblongata adjusts the heart through autonomic nerves
- Sympathetic nerves and adrenal medullary hormones raise the heart rate, force of contraction and cardiac output
- Parasympathetic nerves lower the heart rate and cardiac output

Blood pressure. Normal blood pressure is about 120/80 mm Hg — 120 during ventricular systole and 80 during diastole.

An everyday example. A racing heartbeat just before a board exam comes from sympathetic nerves and adrenaline speeding up the SAN.

The substance. A heart removed from the body can keep beating for a while — each beat starts in its own nodal tissue, so nerves only change the rate.

Circulatory disorders such as high blood pressure, coronary artery disease, angina and heart failure strain the heart or starve its muscle of blood.

- Hypertension — blood pressure repeatedly at or above 140/90 mm Hg; it can damage the heart, brain and kidneys
- Coronary artery disease (atherosclerosis) — deposits of calcium, fat, cholesterol and fibrous tissue narrow the arteries supplying the heart muscle
- Angina pectoris — sharp chest pain when too little oxygen reaches the heart muscle, often during exertion
- Heart failure — the heart cannot pump blood well enough to meet the body's needs
- Cardiac arrest — the heart stops beating; heart attack — heart muscle is suddenly damaged by an inadequate blood supply

An everyday example. Free blood pressure checks at pharmacies and health camps help catch hypertension early, since it often causes no symptoms for a long time.

The substance. Heart failure is not the same as a heart attack or cardiac arrest — in heart failure the heart still beats but pumps too weakly, while in cardiac arrest it stops.