How Blood Circulates

1Two circuits

The circulatory system is really two loops joined at the heart:

• a short loop from the heart to the lungs and back (the pulmonary circulation);
• a long loop from the heart to the whole body and back (the body, or systemic, circulation).

So on one full trip around your body, your blood passes through the heart twice. This is called double circulation.

2Oxygenated and deoxygenated blood

Blood full of oxygen is oxygenated; it is bright red. Blood that has given its oxygen away and carries carbon dioxide is deoxygenated; it is dark red. In diagrams, oxygenated blood is coloured red and deoxygenated blood blue.

3The journey, step by step

1. Deoxygenated blood from the body flows through the vena cava into the right atrium.
2. The right atrium squeezes it through a valve into the right ventricle.
3. The right ventricle pumps it through the pulmonary artery to the lungs.
4. In the lungs, carbon dioxide leaves the blood (to be breathed out) and oxygen enters it. The blood is now oxygenated.
5. The pulmonary veins carry it back to the heart, into the left atrium.
6. The left atrium squeezes it through a valve into the left ventricle.
7. The strong left ventricle pumps it into the aorta, which branches into arteries going all over the body.
8. In the capillaries, the cells take oxygen and food and give back carbon dioxide and wastes. The blood is deoxygenated again and returns through the veins to the vena cava, and the journey starts again.

4Heart rate changes

When you run, dance or play football, your muscles work harder and need more oxygen and food. Your heart beats faster and you breathe faster, so more oxygen reaches them. When you rest, your heart rate slowly returns to normal. Here are Gatete's results:

A fit person's heart rate goes back to normal more quickly after exercise. Checking your heart rate, and, for adults, their blood pressure, is a good habit that helps people notice when something is wrong.

Try it yourself

1. Blood leaving the lungs goes into which chamber of the heart?
2. Which blood vessel carries blood from the right ventricle?
3. Where does blood become oxygenated?
4. Starting from the right ventricle, put these in order: left ventricle, lungs, right ventricle, left atrium.
5. Why does your heart beat faster when you play football?

Answers

1. The left atrium.
2. The pulmonary artery.
3. In the lungs.
4. Right ventricle, lungs, left atrium, left ventricle.
5. Your muscles need more oxygen and food, so the heart pumps blood faster to deliver them.