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The figure given below shows a small part of human lung where exchange of gases takes place. In which one of the options given below is the one part A, B, C or D correctly identified along with its function?
A
B : Red blood cell – transport of $CO_2$ mainly
B
C : Arterial capillary – passes oxygen to tissues
C
A : Alveolar cavity – main site of exchange of respiratory gases
D
D : Capillary wall – exchange of $O_2$ and $CO_2$ takes place here
Detailed Solution
The figure is a section of an alveolus with a pulmonary capillary. A is the alveolar cavity, the air space of the alveolus.
The alveoli are the primary sites of exchange of gases: $O_2$ diffuses from the alveolar air into the blood and $CO_2$ diffuses from the blood into the alveolar air. So A is correctly identified with its function.
B is a red blood cell, but RBCs mainly transport $O_2$ (about 97%); most $CO_2$ (about 70%) is carried as bicarbonate.
C is a pulmonary capillary, in which blood picks up oxygen; it does not pass oxygen to the tissues.
The exchange takes place across the whole diffusion membrane (alveolar squamous epithelium, basement substance and capillary endothelium) between the alveolar air and the blood, so the description given for D is not the accepted one.
Hence the correct option is A : alveolar cavity – main site of exchange of respiratory gases.
The alveoli are the primary sites of exchange of gases: $O_2$ diffuses from the alveolar air into the blood and $CO_2$ diffuses from the blood into the alveolar air. So A is correctly identified with its function.
B is a red blood cell, but RBCs mainly transport $O_2$ (about 97%); most $CO_2$ (about 70%) is carried as bicarbonate.
C is a pulmonary capillary, in which blood picks up oxygen; it does not pass oxygen to the tissues.
The exchange takes place across the whole diffusion membrane (alveolar squamous epithelium, basement substance and capillary endothelium) between the alveolar air and the blood, so the description given for D is not the accepted one.
Hence the correct option is A : alveolar cavity – main site of exchange of respiratory gases.