Question at position 9 What causes air to flow into the lungs during inspiration?Intrapleural pressure becomes positive.Intrapleural pressure is higher than intra-alveolar pressure.Intra-alveolar pressure is lower than atmospheric pressure.The pressure in the lungs is higher than atmospheric pressure.Atmospheric pressure is equal to intra-alveolar pressure.

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Question at position 9 What causes air to flow into the lungs during inspiration?Intrapleural pressure becomes positive.Intrapleural pressure is higher than intra-alveolar pressure.Intra-alveolar pressure is lower than atmospheric pressure.The pressure in the lungs is higher than atmospheric pressure.Atmospheric pressure is equal to intra-alveolar pressure.

BlackTom AI · Accepted Answer

This question asks about the mechanism that drives air into the lungs during inspiration. 
Option 1: 'Intrapleural pressure becomes positive.' Intrapleural pressure is normally negative relative to atmospheric pressure to keep the lungs expanded; if it became positive, it would actually push against the chest wall and hinder inhalation, not promote it.
Option 2: 'Intrapleural pressure is higher than intra-alveolar pressure.' If intrapleural pressure were higher than the pressure inside the alveoli, the transpulmonary pressure would be reduced or reversed, making inspiration difficult; in practice, the crucial comparison is between intra-alveolar and atmospheric pressure, not the intrapleural versus alveolar pressure, for the initial flow during typical inspiration.
Option 3 (the correct one): 'Intra-alveolar pressure is lower than atmospheric pressure.' During inspiration, the diaphragm and intercostal muscles expand the thoracic cavity, increasing lung volume and lowering intra-alveolar pressure below atmospheric pressure, causing air to flow down its gradient into the lungs.
Option 4: 'The pressure in the lungs is higher than atmospheric pressure.' If alveolar pressure were higher than the atmosphere, air would be pushed out, not drawn in, during inspiration, which contradicts the normal inhalation process.
Option 5: 'Atmospheric pressure is equal to intra-alveolar pressure.' When these pressures are equal, there is no net air flow; this situation corresponds to no inspiratory drive at that moment.
In summary, the key concept is that air moves from higher to lower pressure, and during inspiration the intra-alveolar pressure drops below atmospheric pressure, drawing air into the lungs.

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