Answer this
B. Thoracic volume increases
During inspiration, the diaphragm and external intercostal muscles contract, expanding the thoracic cavity. This increased thoracic volume causes the lungs to expand, which increases the total gas volume inside the lungs. By Boyle's Law (P₁V₁ = P₂V₂), as lung volume increases, intrapulmonary (alveolar) pressure drops below atmospheric pressure - this pressure gradient is what drives air into the lungs.
Why the other options are wrong:
- Airway resistance decreases - this affects airflow rate, not the pressure drop mechanism
- Pulmonary capillary blood volume decreases - this is a consequence of inspiration, not the cause of pressure change
- Alveolar surface tension increases - surfactant actually reduces surface tension; increased surface tension would oppose lung expansion
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