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Exchange and transport in animals, subtopic 4 of 5Spec 8.8, 8.12

The heart and circulation

How the structure of the heart, valves and blood vessels allows blood to circulate, and how to calculate cardiac output.

2 sections, with a quick check at the end.

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The heart and circulatory systemSpec 8.8

Humans have a double circulatory system. In one circuit the blood goes from the heart to the lungs and back. In the other the blood goes from the heart around the body and back. Blood passes through the heart twice on each complete circuit of the body.

  1. Deoxygenated blood from the body enters the right atrium through the vena cava.
  2. The right atrium contracts and pushes blood into the right ventricle.
  3. The right ventricle contracts and pumps blood to the lungs through the pulmonary artery.
  4. Oxygenated blood returns from the lungs through the pulmonary vein into the left atrium.
  5. The left atrium contracts and pushes blood into the left ventricle.
  6. The left ventricle contracts and pumps blood around the body through the aorta.
Major blood vessels of the heart
Blood vesselCarries bloodBetween
Vena cavaDeoxygenatedBody → right atrium
Pulmonary arteryDeoxygenatedRight ventricle → lungs
Pulmonary veinOxygenatedLungs → left atrium
AortaOxygenatedLeft ventricle → body

Valves

The heart has valves between the atria and ventricles, and where the pulmonary artery and aorta leave the ventricles. They open to let blood through in one direction and close to prevent backflow.

Thickness of the chamber walls

  • The ventricles have thicker, more muscular walls than the atria, because they pump blood out of the heart and further.
  • The left ventricle has a thicker wall than the right ventricle. It pumps blood around the whole body, which needs a higher pressure. The right ventricle pumps blood only to the lungs, which are close by.

The blood pressure falls as blood passes through the capillaries in the lungs. Returning the blood to the heart before it goes to the body lets the left ventricle increase the pressure again, so blood can reach the rest of the body quickly.

Diagram of the heart drawn as if facing the person, showing the right atrium and right ventricle on the left of the page and the left atrium and left ventricle on the right, the vena cava, pulmonary artery, pulmonary vein and aorta, valves between the atria and ventricles and at the bases of the pulmonary artery and aorta, the thick muscular wall of the left ventricle, and arrows showing deoxygenated blood in blue and oxygenated blood in red.Tap to enlarge
Blood flows vena cava → right atrium → right ventricle → pulmonary artery, then pulmonary vein → left atrium → left ventricle → aorta. The left side of the heart is on the right of the page.

Heart rate, stroke volume and cardiac outputSpec 8.12

Heart rate
The number of times the heart beats per minute (bpm).
Stroke volume
The volume of blood pumped out of the heart in each beat, in cm³.
Cardiac output
The volume of blood pumped out of the heart per minute, in cm³ per minute.
cardiac output = stroke volume × heart rate

Calculating cardiac output

A person has a resting heart rate of 70 beats per minute and a stroke volume of 75 cm³. Calculate the cardiac output.

  1. cardiac output = stroke volume × heart rate
  2. cardiac output = 75 × 70 = 5250

Answer: 5250 cm³ per minute

You may be asked to rearrange the equation. Stroke volume = cardiac output ÷ heart rate. Heart rate = cardiac output ÷ stroke volume.

You can measure heart rate by taking a pulse for a set time, for example counting beats in 30 seconds and doubling. A suggested practical is to investigate the short-term effect of exercise on heart rate and breathing rate, which both increase to supply more oxygen to the working muscles.

Quick check

  1. Which vessel carries oxygenated blood from the lungs to the heart?

    Show answer

    The pulmonary vein.

  2. Why is the wall of the left ventricle thicker than the wall of the right ventricle?

    Show answer

    It pumps blood around the whole body, which needs a higher pressure than pumping blood to the lungs.

  3. What is the job of the valves in the heart?

    Show answer

    To prevent backflow and keep blood flowing in one direction.

  4. State the equation for cardiac output.

    Show answer

    Cardiac output = stroke volume × heart rate.