The heart and circulation · Grade 6
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
A person’s stroke volume is 70 cm³ and their heart rate is 72 beats per minute. Calculate the cardiac output.
Answer: 5,040 cm³/min (about 5 dm³ per minute)