The Heart: Structure, the Cardiac Cycle and Its Electrical System

About 100 000 beats a day

At 70 beats a minute, your heart beats about 100 000 times a day (70 × 60 × 24 = 100 800), with no rest days, no instructions from you, and no breaks for maintenance. It does this with its own built-in electrical system, a set of one-way valves, and muscle that never gets tired the way your leg muscles do. This lesson takes the heart apart: its structure, the timing of each beat, and the electrical signals you can see on an ECG.

Anatomy of the heart

The heart is about the size of a fist and sits between the lungs, tilted to the left. It is wrapped in a tough sac, the pericardium. Its wall is mostly cardiac muscle (the myocardium).

Valves (each made of flaps called cusps):

• Tricuspid valve: between the right atrium and right ventricle (three flaps).
• Bicuspid (mitral) valve: between the left atrium and left ventricle (two flaps).
• Pulmonary semilunar valve: where the pulmonary trunk leaves the right ventricle.
• Aortic semilunar valve: where the aorta leaves the left ventricle.

The AV valves are anchored by cord-like chordae tendineae attached to papillary muscles, which stop the flaps from flipping backward into the atria when the ventricles squeeze, like the strings of a parachute.

The left ventricle wall is about three times thicker than the right's, because it pumps blood through the whole body at much higher pressure, while the right ventricle pumps only to the nearby lungs. Both ventricles pump the same volume of blood per beat.

The heart feeds itself: coronary circulation

The blood inside the chambers rushes past too quickly, and the walls are too thick, for the heart muscle to get its oxygen from it. Instead, the first two branches of the aorta, the left and right coronary arteries, run over the surface of the heart and dive into the muscle. Used blood returns through cardiac veins into the right atrium. If a coronary artery is blocked, the muscle beyond the blockage is starved of oxygen; this is a heart attack (see the circulatory disorders lesson).

The cardiac cycle

One complete heartbeat is a cardiac cycle. At 75 beats per minute, each cycle takes 60 ÷ 75 = 0.8 s. Systole means contraction; diastole means relaxation.

1. Diastole (about 0.4 s): all four chambers are relaxed. Blood flows into the atria and, through the open AV valves, on into the ventricles. Most ventricular filling happens now, passively.
2. Atrial systole (about 0.1 s): the atria contract, topping up the ventricles.
3. Ventricular systole (about 0.3 s): the ventricles contract. Pressure inside them rises above the atrial pressure, so the AV valves snap shut: that is the first heart sound, "lub". When ventricular pressure rises above the pressure in the pulmonary trunk and aorta, the semilunar valves open and blood is ejected.
4. The ventricles relax. Pressure in them falls below that in the arteries, so the semilunar valves snap shut: the second sound, "dub". Diastole begins again.

Arterial blood pressure is highest during ventricular systole (the systolic pressure) and lowest during diastole (the diastolic pressure). In the next lesson you'll measure both. A heart murmur is an extra swishing sound, often from blood leaking through a valve that doesn't close fully (many murmurs, especially in children, are harmless).

The heart's electrical system

Cardiac muscle cells are connected by intercalated discs with tiny channels between them, so an electrical signal spreads from cell to cell. The beat is myogenic: it starts in the heart itself, not in the brain.

1. The sinoatrial (SA) node, in the wall of the right atrium, fires on its own about 60 to 100 times a minute at rest. The signal spreads across both atria, which contract.
2. The signal reaches the atrioventricular (AV) node, which delays it by about 0.1 s. This gives the atria time to finish emptying into the ventricles.
3. The signal travels down the bundle of His in the septum, splits into left and right bundle branches, and spreads through Purkinje fibres in the ventricle walls.
4. The ventricles contract from the apex (bottom) upward, squeezing blood up toward the exits, like squeezing toothpaste from the bottom of the tube.

The brain doesn't start the beat, but it does adjust it. The cardiac centre in the medulla oblongata speeds the heart up through sympathetic nerves (and the hormone adrenaline, also called epinephrine, from the adrenal glands) and slows it down through the vagus nerve. Pressure sensors called baroreceptors in the aorta and carotid arteries report blood pressure to the cardiac centre.

Reading an ECG

An electrocardiogram (ECG or EKG) records the heart's electrical activity using electrodes on the skin.

• P wave: the atria depolarize (electrical signal spreads), just before they contract.
• QRS complex: the ventricles depolarize, just before they contract. (The atria recover at the same time, hidden in the QRS.)
• T wave: the ventricles repolarize (recover electrically) before the next beat.

Standard ECG paper moves at 25 mm/s, so each small 1 mm square is 0.04 s and each large 5 mm square is 0.20 s. To find the heart rate, measure the time between two R peaks (the R–R interval): heart rate = 60 ÷ (R–R interval in seconds).

Cardiac output

Stroke volume (SV) is the volume pumped by one ventricle in one beat (about 70 mL at rest for an adult). Cardiac output (CO) is the volume pumped per minute:

CO = heart rate × stroke volume

At rest: 70 beats/min × 70 mL/beat = 4900 mL/min, about 5 L/min, roughly your whole blood volume every minute. In hard exercise, cardiac output can rise to several times this. Endurance training makes the heart muscle stronger and the ventricles larger, so stroke volume rises and the resting heart rate falls: many trained endurance athletes have resting rates in the 40s or 50s.