The pulmonary artery carries blood away from the heart to the lungs. Pulmonary veins carry blood from the lungs back to the heart. The words artery and vein describe direction relative to the heart. In this circuit, the artery carries blood lower in oxygen and the veins carry blood higher in oxygen.
The 2027 SEC G3 Biology K325 syllabus includes identifying the major vessels between the heart and lungs. This guide works through a diagram question using the arrows first, then explains the oxygen difference.
- Locate the heart and lungs.
- Follow each blood-flow arrow.
- Name the vessel from its direction.
- Explain the oxygen difference using gas exchange.
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Start at the heart
Blood leaves the right ventricle through the pulmonary trunk, which divides into pulmonary arteries leading to the lungs. Blood returns from the lungs through pulmonary veins into the left atrium. A simple diagram may represent each route with a single arrow.
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Worked example | Identify X and Y from the arrows
Vessel X has an arrow from the right ventricle towards the lungs. Vessel Y has an arrow from the lungs to the left atrium. Identify X as the pulmonary artery in the simplified diagram. Identify Y as a pulmonary vein. The direction of each arrow gives the answer even when the diagram uses no colours.
Blood in Y has a higher oxygen content than blood in X. In the lungs, oxygen diffuses from the air in the alveoli into the blood in nearby capillaries. Haemoglobin in red blood cells binds much of that oxygen. The blood then returns through the pulmonary veins.
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CHAPTER 3 OF 4 · Back to contents
Check the other circuit
The aorta carries blood away from the left ventricle towards the body. It is an artery and carries oxygen-rich blood. The pulmonary artery also carries blood away from the heart, but it leads towards the lungs and carries blood with less oxygen. Direction is the feature these arteries share.
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Frequently asked questions
Does deoxygenated blood contain zero oxygen?
It contains less oxygen than oxygenated blood. Some oxygen remains.
What should I write if asked why Y carries more oxygen?
Describe oxygen diffusing from the alveoli into lung capillaries, then link that blood to the vessel returning to the heart.