Question

Identify the sequence of steps involved during inspiration in rabbits.
A. Volume of thoracic cavity increases
B. Contraction of external intercostal muscles moves the rib cage forward
C. Diaphragm becomes flat
D. Thoracic cavity expands drawing air into lungs

(A) C $ \to $ B $ \to $ A $ \to $ D
(B) C $ \to $ A $ \to $ B $ \to $ D
(C) A $ \to $ B $ \to $ C $ \to $ D
(D) A $ \to $ D $ \to $ B $ \to $ C

Answer 1

Hint: Breathing involves two stages: inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out. The air moves in and out of the lungs due to a pressure gradient between the lungs and the atmosphere.

Complete answer:
Inspiration occurs when the pressure within the lungs becomes less than the atmospheric pressure. Inspiration starts with the contraction of the diaphragm. It results in an increase in the volume of the thoracic chamber in the antero-posterior axis. The external intercostal muscles contract which lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The volume of the thoracic cavity increases which causes a similar increase in pulmonary volume. This, in turn, decreases the intrapulmonary pressure to less than the atmospheric pressure which forces the air from outside to move through the nose and mouth and is pulled into the windpipe - the trachea. The trachea divides into smaller branches called bronchi and further bronchi divide into numerous bronchioles. Finally, the tiny airways deliver the air to the smallest structures in your lung, the alveoli, where gas exchange takes place.

Hence, the correct answer is option (A).

Note: Respiration involves the following steps:
(i) Breathing is a process in which atmospheric air is drawn in and $C{O_2}$ rich alveolar air is released out.
(ii) Diffusion of gases ( ${O_2}$ and $C{O_2}$ ) across the alveolar membrane takes place.
(iii) Transport of gases by the blood takes place.
(iv) Diffusion of ${O_2}$ and $C{O_2}$ between blood and tissues takes place.
(v) Utilisation of ${O_2}$ by the cells for catabolic reactions takes place which results in the release of $C{O_2}$ .