NEET 2020 Botany Question 24 Solution

Inspiration is an active neuromuscular process driven by atmospheric-to-alveolar pressure gradients: - (a) Contraction of Diaphragm: The dome-shaped phrenic-innervated muscular diaphragm contracts and flattens downward, increasing the volume of the thoracic cavity along the antero-posterior axis. - (b) Contraction of External Intercostal Muscles: Intercostal-innervated muscles pull the ribs upward and outward while pushing the sternum ventrally, increasing thoracic volume along the dorso-ventral axis. - Pulmonary Volume Increase: Because the pleura adhere tightly to the thoracic wall via pleural fluid surface tension, any expansion of thoracic volume drives an obligatory increase in pulmonary volume (negating statement (c)). - Intra-Pulmonary Pressure Decrease: According to Boyle’s Law ($P \propto 1/V$), an expansion in pulmonary volume causes intra-pulmonary pressure to drop below atmospheric pressure (negative pressure of approx. $-1\text{ mm Hg}$ to $-3\text{ mm Hg}$), drawing ambient air into the bronchial tree (negating statement (d)).

Only events (a) and (b) occur during inspiration. Statements (c) and (d) describe events occurring during expiration.