The heart and circulatory systemSpec 8.8
In short
Humans have a double circulatory system: blood goes from the heart to the lungs and back, then from the heart around the body and back. The right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery, and the left ventricle pumps oxygenated blood to the body through the aorta. Valves prevent backflow.
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.
- Deoxygenated blood from the body enters the right atrium through the vena cava.
- The right atrium contracts and pushes blood into the right ventricle.
- The right ventricle contracts and pumps blood to the lungs through the pulmonary artery.
- Oxygenated blood returns from the lungs through the pulmonary vein into the left atrium.
- The left atrium contracts and pushes blood into the left ventricle.
- The left ventricle contracts and pumps blood around the body through the aorta.
| Blood vessel | Carries blood | Between |
|---|---|---|
| Vena cava | Deoxygenated | Body → right atrium |
| Pulmonary artery | Deoxygenated | Right ventricle → lungs |
| Pulmonary vein | Oxygenated | Lungs → left atrium |
| Aorta | Oxygenated | Left 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.
The diagram of the heart is drawn as if you are facing the person, so the left side of the heart is on the right of the page. Check which side is which before you answer.
(opens full size in a new tab)Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026