The heart and circulation

Your heart beats about 100,000 times a day, pumping blood through tubes that would stretch twice round the Earth. The blood carries oxygen and food to every cell and takes away carbon dioxide and other waste. Together, the heart, the blood vessels and the blood make the circulatory system.

Key facts

  • The heart is a muscular pump with four chambers: two atria at the top and two ventricles below. Valves stop the blood flowing backwards.
  • The right side of the heart pumps blood to the lungs; the left side pumps blood to the rest of the body. The left ventricle has the thickest wall because it pumps blood all round the body.
  • Double circulation: blood goes through the heart twice on each trip round the body: heart → lungs → heart → body → heart.
  • Arteries carry blood away from the heart at high pressure (thick, muscular, elastic walls). Veins bring blood back at low pressure (thinner walls and valves). Capillaries are one cell thick, so substances pass between the blood and the cells.
  • Blood is red blood cells (carry oxygen), white blood cells (fight infection), platelets (help clotting) and plasma (carries dissolved food, carbon dioxide, urea and hormones).
  • The vena cava brings blood from the body to the right atrium; the pulmonary artery takes it to the lungs; the pulmonary vein brings it back to the left atrium; the aorta takes it to the body.
  • Heart rate is beats per minute: count your pulse for 15 seconds and multiply by 4. Cardiac output = stroke volume × heart rate.

Worked examples

Example 1

Describe the path of blood from the body, through the heart, to the lungs.

  1. Blood from the body comes in through the vena cava into the right atrium. (On a diagram the right side of the heart is drawn on the left, as if you are facing the person.)
  2. It passes through a valve into the right ventricle.
  3. The right ventricle pumps it into the pulmonary artery, which goes to the lungs.

Answer: vena cava → right atrium → right ventricle → pulmonary artery → lungs

Example 2

Why is the wall of the left ventricle much thicker than the wall of the right ventricle?

  1. The right ventricle pumps blood only to the lungs, which are close by.
  2. The left ventricle pumps blood to the whole body, from your head to your toes.
  3. That needs a much higher pressure, so it has much more muscle in its wall.

Answer: the left ventricle pumps blood all round the body, so it needs more muscle to make a higher pressure

Example 3

Ana counts 19 pulse beats in 15 seconds. What is her heart rate?

  1. There are four lots of 15 seconds in a minute. 60 ÷ 15 = 4
  2. Multiply the count by 4. 19 × 4 = 76
  3. Her heart rate is 76 beats per minute (bpm).

Answer: 76 beats per minute

Example 4

A person’s stroke volume is 70 cm³ and their heart rate is 72 beats per minute. Calculate the cardiac output.

  1. Stroke volume is the volume of blood pumped by the left ventricle in one beat.
  2. Multiply by the number of beats in a minute. 70 × 72 = 5040
  3. So the cardiac output is 5,040 cm³ per minute, which is about 5 dm³ (5 litres) per minute.

Answer: 5,040 cm³/min (about 5 dm³ per minute)

Questions about the heart and circulation