The heart

MDCAT Biology · Biology · Circulation. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The human heart has four chambers. The right side receives deoxygenated blood from the body and pumps it to the lungs. The left side receives oxygenated blood from the lungs and pumps it round the body. Blood passes through the heart twice in one full circuit: this is double circulation.

Valves keep the flow in one direction. They open and close because of pressure differences, not by their own muscle.

The heartbeat is myogenic: it starts in the heart muscle itself, at the SA node (the pacemaker) in the right atrium. Nerves and hormones only change its rate.

Rules to remember

Path: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aorta.
The pulmonary artery carries deoxygenated blood; the pulmonary veins carry oxygenated blood.
Impulse: SA node → AV node (short delay) → bundle of His → Purkinje fibres → ventricles contract from the apex upwards.
Cardiac cycle at 75 beats per minute = 60 ÷ 75 = 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole of the whole heart 0.4 s.
First sound 'lubb': the AV (tricuspid and bicuspid) valves close as the ventricles start to contract.
Second sound 'dupp': the semilunar valves close as the ventricles start to relax.
Sympathetic nerves speed up the SA node; the vagus (parasympathetic) nerve slows it.

Common mistake

Arteries are not defined by oxygenated blood. An artery carries blood away from the heart, so the pulmonary artery carries deoxygenated blood.

Worked example

At the moment the first heart sound ('lubb') is heard, what are the ventricles doing and what is the state of each set of valves?

'Lubb' is made by the AV valves snapping shut.
They shut because ventricular pressure has just risen above atrial pressure, so ventricular systole is beginning.
The semilunar valves are still closed; they open a moment later, when ventricular pressure rises above the pressure in the arteries.

Answer: The ventricles are starting to contract; the tricuspid and bicuspid valves have just closed and the semilunar valves have not yet opened.

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