The circulatory system, part of the team of body systems, is the body's delivery route. The heart is really two pumps side by side, each sending blood around its own loop.
Key idea
Blood makes two loops. The right side of the heart pumps oxygen-poor blood to the lungs (the pulmonary loop). Oxygen-rich blood returns to the left side, which pumps it to the body (the systemic loop), and it comes back to the right side. Valves keep blood moving one way.
Why it works
Why two loops? The lungs' capillaries are delicate and need gentle pressure, while blood sent to the whole body needs strong pressure to reach the toes and back. Two separate pumps let each loop run at its own pressure. That is also why the left ventricle has the thickest wall: it does the hardest pumping.
- Arteries carry blood away from the heart; veins carry it back to the heart; capillaries, one cell thick, connect them and exchange materials with the tissues.
- The pulmonary arteries carry oxygen-poor blood to the lungs; the pulmonary veins carry oxygen-rich blood back.
- The aorta, the largest artery, carries oxygen-rich blood from the left ventricle to the body.
Worked example: One trip around
A red blood cell is in the right atrium. List, in order, where it goes until it reaches the aorta.
- Right atrium → right ventricle.
- Right ventricle → pulmonary artery → lungs, where it picks up oxygen.
- Pulmonary vein → left atrium → left ventricle → aorta.
Answer: Right ventricle, lungs, left atrium, left ventricle, aorta.
Worked example: How many times through the heart?
On one complete trip from the toes back to the toes, how many times does a blood cell pass through the heart?
- Once through the right side, on the way to the lungs.
- Once through the left side, on the way back to the body.
Answer: Twice.
Common mistake
Two errors are common. One is picturing one loop: heart → lungs → body → heart. The blood comes back to the heart between the lungs and the body. The other is thinking every artery carries oxygen-rich blood. Arteries are defined by direction, away from the heart: the pulmonary arteries carry oxygen-poor blood to the lungs.