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Figure shown schematic plan of blood circulation in humans with labels A to D. Identify the label and give its function.

1. C : arterial capillary – passes oxygen to tissues
2. A : alveolar cavity – main site of exchange of respiratory gases
3. D : capillary wall – exchange of O2 and CO2 takes place here.
4. B : red blood cell – transport of CO2 mainly

Correct answer: A : alveolar cavity – main site of exchange of respiratory gases

Solution

The alveolar cavity is the air-filled space of an alveolus, and it is the primary site where oxygen and carbon dioxide are exchanged between air and blood. The other labels refer to structures involved in transport or exchange, but not the main gas-exchange cavity itself.

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