When the inspiratory muscles relax, the rib cage returns to its original position as a result of:. a) gravity.
b) exhalation.
c) elastic rebound.
d) partial pressure difference.
e) accessory muscle contraction.

The correct answer is c) elastic rebound.

Explanation:

When the muscles are relaxed (end of expiration) the lung and the chest wall act in opposite directions, the lung tries to reduce its volume due to the inward retraction of the alveolar walls and the chest wall tends to increase in volume due to elastic expansion, these opposing pressures are responsible for keeping the alveolus partially open (functional residual capacity). The muscles of inspiration act by increasing the volume of the thoracic cavity causing the intrapleural pressure to become lower (Boyle's law), the intraalveolar volume increases and then the intralveolar pressure decreases below atmospheric (negative) and flow is generated towards the lung expanding it, that is, during inspiration, the diaphragm contracts moving downward, allowing the rib cage to widen and air into the lungs.

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