Blood makes one complete trip in two loops: from the heart to the lungs and back, then from the heart to the body and back. This is called double circulation. The skill is to say the route in the right order and to state whether the blood is oxygenated at each stop.
This lesson is part of transport in humans and animals. The next lesson, comparing blood vessels, explains why the vessels look different.
What is the route, step by step?
Start from the body and follow the blood. Learn the names as one sentence.
- Deoxygenated blood returns from the body in the vena cava.
- It enters the right atrium.
- It passes through the right valve into the right ventricle.
- The right ventricle pumps it into the pulmonary artery.
- It reaches the lungs and gains oxygen.
- Oxygenated blood returns in the pulmonary vein.
- It enters the left atrium.
- It passes through the left valve into the left ventricle.
- The left ventricle pumps it into the aorta.
- The aorta delivers it to the body.
Why does the diagram look reversed?
Diagrams show the heart as if you face the person. The heart’s right side appears on the left of the page.
Label left and right from the body’s point of view, not yours. A quick check: the thick-walled chamber, the left ventricle, sits on the right side of the page.
Worked example: state the route from a label
Question: a diagram shows a chamber labelled X with a thick muscular wall, and a vessel leaving it that carries blood to the whole body. Name X and the vessel, and state where the blood came from.
- The thick wall pumps against high resistance, so X is the left ventricle.
- The vessel to the whole body is the aorta.
- The blood came from the left atrium, through a valve, and before that from the lungs in the pulmonary vein.
Full answer: “X is the left ventricle. The blood came from the left atrium after collecting oxygen in the lungs, and it leaves in the aorta.”
The scrambled route
A common answer writes “vena cava, left atrium, left ventricle” because the student remembers the word left. The route skips the lungs entirely.
| Wrong route | Why it fails | Correct |
|---|---|---|
| Vena cava to left atrium | Deoxygenated blood would go to the body | Vena cava to right atrium |
| Right ventricle to aorta | Bypasses the lungs | Right ventricle to pulmonary artery |
| Pulmonary vein to right atrium | Oxygenated blood returns to the wrong side | Pulmonary vein to left atrium |
Check yourself
Start at the pulmonary vein and list the next three structures the blood passes through. State whether the blood is oxygenated.
Answer
Left atrium, then the valve between the left chambers, then the left ventricle. The blood is oxygenated throughout, because it has just come from the lungs.
The next stop after the left ventricle is the aorta.
What to study next
Continue with comparing blood vessels, then check your route on the transport in humans and animals practice set. The biological data interpretation trainer adds data questions.
For a teacher who listens to your route and corrects it on the spot, see online one-to-one Biology tuition.