The cardiac cycle and cardiac output

One beat

Put two fingers on your wrist and count the pulse. It is about 72 beats in a minute. Each beat is one round of events in the heart. These events are repeated in the same order again and again. The round is the cardiac cycle.

In each cycle, the atria and the ventricles each have a systole, a time of contraction, and a diastole, a time of relaxation. One cycle lasts about 0.8 s.

The four stages

1. Joint diastole. All four chambers are relaxed. The valves between the atria and the ventricles are open, and the semilunar valves are shut. Blood from the vena cavae and the pulmonary veins flows through the atria into the ventricles.
2. Atrial systole. The SAN makes an action potential. Both atria contract together. They push more blood into the ventricles. This adds about 20 to 30 per cent to the filling. NCERT prints 30 per cent.
3. Ventricular systole. The signal goes by the AVN, the AV bundle and the bundle of His to the ventricles, and they contract. The atria relax at the same time. The pressure in the ventricles rises. It first shuts the valves between the atria and the ventricles, because the blood tries to flow back. Then, as the pressure rises further, it opens the semilunar valves, and the blood goes into the pulmonary artery and the aorta.
4. Ventricular diastole. The ventricles relax, and the pressure in them falls. The semilunar valves shut, so the blood cannot flow back into the ventricles. When the pressure falls below that of the atria, the valves between the atria and the ventricles open. The heart is again in joint diastole.

A valve is moved only by the pressure of the blood. No muscle opens or shuts it.

How long does each part last?

NCERT gives the time of the whole cycle, 0.8 s. The times of the parts are the usual values in physiology books, for a heart at about 75 beats a minute. They change with the heart rate.

The heart sounds

A doctor listens to the heart with a stethoscope. There are two sounds in each cycle.

• The first sound, lub, comes from the closing of the tricuspid and the bicuspid valves, at the start of the ventricular systole.
• The second sound, dub, comes from the closing of the semilunar valves, at the start of the ventricular diastole.

These sounds are useful for diagnosis.

You should find that the valves between the atria and the ventricles are shut only during the ventricular systole, and that the semilunar valves are open only then. This is a simple model. The very short moments when all four valves are shut, while the pressure changes, are left out.

Stroke volume and cardiac output

Each ventricle pushes out almost the same volume of blood in each beat. This is the stroke volume, about 70 mL. The volume that each ventricle pumps in one minute is the cardiac output.

For a healthy person, it is about $70 \times 72 = 5040$ mL, which is about 5 L in a minute. The body can change the stroke volume and the heart rate, and so it can change the output. The cardiac output of an athlete in hard exercise is much greater than that of an ordinary person.

You should find that raising either the heart rate or the stroke volume raises the output. A trained heart can reach a high output by a larger stroke volume, without a very high rate.

Try it in Python

The program lists the phases of one cycle with their times and shares.

Practise with new numbers

Common mistakes

• Saying that the atria and the ventricles contract together. The atria contract first. The ventricles follow.
• Mixing up the two heart sounds. Lub is the closing of the valves between the atria and the ventricles. Dub is the closing of the semilunar valves.
• Saying that the semilunar valves are open in ventricular diastole. They are shut. They open only in the ventricular systole.
• Forgetting the unit. The cardiac output is in mL or L in a minute, not in a beat.
• Thinking that a muscle works the valves. The pressure of the blood does.

Remember these

• Cardiac cycle: about 0.8 s, 72 times in a minute. Joint diastole, atrial systole, ventricular systole, ventricular diastole.
• The atrial systole adds about 20 to 30 per cent to the filling of the ventricles (NCERT prints 30 per cent).
• Lub: closing of the tricuspid and bicuspid valves. Dub: closing of the semilunar valves.
• Stroke volume about 70 mL. Cardiac output equals stroke volume times heart rate, about 5 L in a minute. It is much more in an athlete.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What happens to the valves between the atria and the ventricles when the ventricles contract? Why?

2. When does the atrial systole take place, and what does it add?

3. Which valves close to make the second heart sound?

4. What is the stroke volume, and what is its usual value?

5. How is the cardiac output of an athlete in exercise different from that of an ordinary person?

References

• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 9, Cardiac Muscle; The Heart as a Pump and Function of the Heart Valves.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 19, section 19.3, Cardiac Cycle.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.