The pulmonary artery carries blood away from the right ventricle to the lungs. That blood is low in oxygen because it has returned from the body and has yet to pass through the lungs. The pulmonary vein carries oxygen-rich blood from the lungs back to the heart.
The 2027 SEC G3 Biology K325 syllabus includes the main blood vessels between the heart and lungs. This guide uses the direction of flow and the site of gas exchange to explain oxygen content.
- Use flow direction to distinguish arteries and veins.
- Start the pulmonary route at the right ventricle.
- Identify where blood gains oxygen.
- Follow the returning blood into the left atrium.
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Name the vessel from the direction of flow
An artery carries blood away from the heart. A vein carries blood towards it. The words describe direction, so check which way the arrow points before deciding the vessel name.
The right side of the heart receives blood returning from the body and sends it to the lungs. There, oxygen diffuses from the air in the alveoli into the blood.
After gas exchange, the blood returns to the left side of the heart through the pulmonary veins.
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Worked example | Read four vessels on a circulation diagram
A diagram shows one route between the heart and the lungs and another between the heart and the rest of the body. Match each major vessel to its direction and typical oxygen content in that diagram.
| Vessel | Blood flows | Typical oxygen content |
|---|---|---|
| Pulmonary artery | Right ventricle to lungs | Lower |
| Pulmonary vein | Lungs to left atrium | Higher |
| Aorta | Left ventricle to body tissues | Higher |
| Vena cava | Body tissues to right atrium | Lower |
The pulmonary artery is an artery because it leaves the heart. The blood is relatively low in oxygen because it is on the way to the lungs for gas exchange.
For the return route, both clues change. The vessel leads towards the heart and the blood has gained oxygen in the lungs.
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Practise giving the complete explanation
A pupil writes, ‘The pulmonary vein has more oxygen because it is a vein.’ Improve the answer by naming the process. Oxygen diffuses into the blood in the lungs before the blood enters the pulmonary vein.
If a question asks which chamber receives the blood next, follow the arrow from the pulmonary vein to the left atrium. The left ventricle pumps blood out later, after it has passed from the atrium.
When asked for a sequence, write the path with arrows first and add the change in oxygen content beside the lungs.
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Frequently asked questions
Does low-oxygen blood have no oxygen at all?
It has less oxygen than the oxygen-rich blood in the comparison.
Does an artery always carry oxygen-rich blood?
The pulmonary artery carries oxygen-poor blood away from the heart to the lungs.
Where does the blood gain oxygen?
It gains oxygen as it passes through the capillaries around the alveoli.
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