A) Mediastinum
B) Pleurae
C) Both (a) and (b)
D) Alveolar epithelium and endothelium of blood capillary
Correct Answer: D
Solution :
The respiratory bronchioles open into the alveolar ducts which terminate in expanded passages, the atria, (sing, atrium) which in turn lead into the alveolar sacs or air sacs. The latter open into the alveoli (also called acini). There are about 300 million of alveoli in two lungs. The alveoli have very thin wall consisting of squamous epithelium. The wall of the alveoli has an extensive network of blood capillaries interspersed with elastic and reticular connective tissue fibres. Due to very intimate contact of blood capillaries with the alveoli, the exchange of gases takes place easily. Thus, each alveolus may be called a miniature lung where the exchange of gases takes place. Mediastinum is the partition between two lungs and therefore, includes the pleura of both sides. Pleurae are two membranes, which enclose lung outer membrane called parietal pleura whereas inner membrane is known as visceral pleura.
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