Final answer:
The correct sequence describing the changes in alveolar pressure, starting with inspiration, is (d) a, c, b, c, which means intra-alveolar pressure is less than atmospheric pressure during inspiration, equalizes at the end of inspiration, becomes greater during expiration, and equalizes again after expiration.
Step-by-step explanation:
To answer which sequence correctly describes the changes in alveolar pressure starting with inspiration, it's important to understand how pressure changes facilitate breathing.
During inspiration, the diaphragm contracts, increasing the volume of the thoracic cavity, and hence the volume of the lungs. According to Boyle's law, an increase in volume results in a decrease in pressure. Therefore, intra-alveolar pressure (Palv) decreases below atmospheric pressure (Patm), causing air to flow into the lungs. This corresponds to Palv < Patm.
At the end of inspiration, the pressures equalize because air has entered the lungs until the pressure gradient is resolved. This is represented by Palv = Patm.
With the onset of expiration, the diaphragm relaxes, the volume of the thoracic cavity decreases, and the pressure within the alveoli increases. Hence, Palv becomes greater than Patm, and air is expelled from the lungs. This corresponds to Palv > Patm.
After expiration, pressures equalize again, represented by Palv = Patm.
Therefore, the correct sequence of changes in alveolar pressure, starting with inspiration, is:
a, b, c, b, which means the correct answer is (d) a, c, b, c.