OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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What occurs in the first circuit of the double circulatory system?

Deoxygenated blood is delivered to the brain
Oxygenated blood is distributed to all organs
Deoxygenated blood is sent to the lungs for oxygenation
Oxygen is absorbed from the skin

The correct answer is: Deoxygenated blood is sent to the lungs for oxygenation

In the first circuit of the double circulatory system, deoxygenated blood is sent to the lungs for oxygenation. This process begins as the right side of the heart receives blood that has returned from the body, which is low in oxygen and high in carbon dioxide. The right ventricle pumps this deoxygenated blood through the pulmonary arteries to the lungs. In the lungs, carbon dioxide is released from the blood and oxygen is absorbed, allowing the now oxygenated blood to return to the left side of the heart, where it can then be distributed to the rest of the body. This efficient separation of blood flow into two circuits—one for oxygenation and the other for distribution—ensures that the body's tissues receive the oxygen they need for cellular respiration and function properly.