Circulatory system

Biology · Transport in humans. 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 circulatory system is a system of blood vessels with a pump (the heart) and valves. The valves make sure the blood flows in one direction only.

Mammals have a double circulation: blood passes through the heart twice for each complete circuit of the body. The right side of the heart pumps blood to the lungs, where it picks up oxygen. The blood returns to the left side of the heart, which pumps it to the rest of the body.

This gives two pressures. Blood goes to the lungs at low pressure, which protects the thin lung capillaries. It goes to the body at high pressure, so it reaches every tissue quickly.

Rules to remember

Common mistake

Students say blood passes through the heart once per circuit, or that the two sides of the heart pump at the same pressure. It passes through twice, and the left side pumps at a much higher pressure.

Worked example

In one person the highest pressure in the left ventricle is 16 kPa and in the right ventricle is 3.2 kPa. How many times greater is the pressure on the left, and why is this difference useful?

  1. 16 ÷ 3.2 = 5.
  2. The right ventricle pumps only to the lungs, which are close by and have thin capillaries that high pressure would damage.
  3. The left ventricle pumps to the whole body, so high pressure is needed to move blood quickly over long distances.

Answer: 5 times greater: low pressure for the lungs, high pressure for the body tissues.

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