Cardiac cycle

Where it happens

The cardiac cycle takes place inside the heart, in the two atria and two ventricles, and is controlled by valves between the chambers and at the exits of the ventricles. Systole refers to contraction of a chamber, and diastole refers to relaxation.

The 4 chambers are arranged as 2 pumps working side by side. The right atrium and right ventricle receive deoxygenated blood from the vena cava and send it to the lungs through the pulmonary artery.

The left atrium and left ventricle receive oxygenated blood from the pulmonary vein and send it around the body through the aorta. The left ventricle has the thickest muscular wall because it pumps blood the greatest distance.

The whole cycle is set going by the pacemaker, a patch of specialised muscle in the wall of the right atrium that sends out electrical impulses at regular intervals without any signal from the brain.

Inputs and outputs

• Input to the right atrium: deoxygenated blood returning from the body through the superior and inferior vena cava.
• Input to the left atrium: oxygenated blood returning from the lungs through the pulmonary veins.
• Input of control: electrical impulses from the pacemaker, which spread across the atria and then down to the ventricles.
• Output from the right ventricle: deoxygenated blood pushed into the pulmonary artery toward the lungs.
• Output from the left ventricle: oxygenated blood pushed into the aorta toward the body.
• Output that can be measured: the pulse felt in an artery, the 2 heart sounds heard as the valves close, and blood pressure readings.

The steps

1. Diastole begins: all 4 chambers are relaxed, the atrioventricular valves are open and the semilunar valves are closed.
2. Blood flows passively from the vena cava into the right atrium and from the pulmonary veins into the left atrium, and much of it passes straight through into the relaxed ventricles.
3. Atrial systole: the pacemaker fires, both atria contract together and push the remaining blood through the open atrioventricular valves into the ventricles.
4. Ventricular systole starts: the impulse reaches the ventricle walls, which contract from the bottom upward; pressure in the ventricles rises above pressure in the atria, so the atrioventricular valves snap shut and produce the first heart sound.
5. Pressure in the ventricles rises above pressure in the pulmonary artery and aorta, so the semilunar valves open and blood is forced out into the arteries.
6. Ventricular diastole: the ventricles relax and pressure inside them falls below pressure in the arteries, so blood in the arteries pushes the semilunar valves shut and produces the second heart sound.
7. Pressure in the ventricles falls below pressure in the atria, the atrioventricular valves open again, and the cycle returns to step 1.

Why it matters and how it is controlled

The coordinated contraction and relaxation of the cardiac cycle, together with the one-way action of the valves, keeps blood moving continuously in one direction through the heart and around the body. The closing of the valves produces the heart sounds that can be heard through a stethoscope.

The sequence is controlled by pressure, not by any conscious effort. Valves have no muscle of their own; each one opens when pressure behind it is higher than pressure in front of it, and closes when that gradient reverses.

This is why a pressure-time graph is the standard way the cycle is tested: every valve event can be read off the point where two pressure lines cross.

The rate of the cycle is set by the pacemaker but adjusted by the nervous system and hormones. During exercise, impulses from the brain and the hormone adrenaline speed up the pacemaker, so more cycles occur each minute and more oxygenated blood reaches the muscles.

At rest, the rate falls again. The heart muscle itself receives its oxygen and glucose through the coronary arteries, so a blockage in these arteries starves the muscle and disrupts the cycle.

How it is examined

You may be asked to describe the sequence of atrial systole, ventricular systole and diastole, to explain the role of the atrioventricular and semilunar valves, or to interpret a graph or diagram showing pressure changes in the atria, ventricles and major blood vessels during one cardiac cycle.

Graph items give pressure curves for the left atrium, left ventricle and aorta plotted against time and ask when the atrioventricular valve closes, when the semilunar valve opens, or how long ventricular systole lasts. Read the crossing points: the atrioventricular valve closes when the ventricle line rises above the atrium line, and the semilunar valve opens when the ventricle line rises above the aorta line.

Structured items ask why the left ventricle wall is thicker than the right, or why the atria have thinner walls than the ventricles; the answer always links wall thickness to the distance and pressure the blood must be pumped.

Common misconceptions

Worked exam-style question

Question. Graph Z shows the pressure in the left atrium, the left ventricle and the aorta during one cardiac cycle of a resting adult. At time T1 the ventricle pressure line rises above the atrium pressure line.

At time T2 the ventricle pressure line rises above the aorta pressure line. At time T3 the ventricle pressure line falls below the aorta pressure line.

(a) State what happens to the atrioventricular valve at T1 and explain why. (b) Name the event at T2 and state which blood vessel receives blood immediately after it.

(c) Between T2 and T3, describe the state of the ventricle muscle. (d) Explain why the same graph drawn for the right ventricle would show a lower peak pressure.

Model answer. (a) At T1 the atrioventricular valve closes. The ventricle is contracting, so pressure in the ventricle exceeds pressure in the atrium, and the higher pressure pushes the valve flaps shut to prevent backflow of blood into the atrium.

(b) At T2 the semilunar valve opens because ventricle pressure now exceeds aorta pressure; blood enters the aorta. (c) Between T2 and T3 the ventricle is in systole: the muscle is contracted and blood is being ejected into the aorta.

(d) The right ventricle pumps blood only to the lungs, a short distance, so it has a thinner muscular wall and generates lower pressure; high pressure would also damage the delicate capillaries of the alveoli.

Source:SRC-DSKP-EN

Frequently asked questions