Question

Explain the mechanism of breathing with neat labelled sketches.

Answer 1

Breathing involves two stages:

1. Inspiration: Inspiration is initiated by the contraction of  diaphragm, which increases the volume of thoracic chamber in the anteroposterior axis. The contraction of external intercostal muscles lifts up the ribs and the sternum causing an increase in the volume of thoracic chamber in the dorso-ventral axis also. Such an increase in thoracic volume leads to a similar increase in pulmonary volume resulting in decreased intra- pulmonary pressure to less than atmospheric pressure. This causes the movement of external air into the lungs, i.e., inspiration.
2. Expiration: The inter-costal muscles return the diaphragm and sternum to their normal positions with the relaxation of the diaphragm. This reduces the thoracic volume and thereby the pulmonary volume. As a result, an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causes the expulsion of air from the lungs i.e., expiration.

(a)

(b)