Respiratory Cycle / Lung volume, pressure inside the lungs (alveolar pressure) and air flow-1
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When the lungs increase in volume, the pressure inside the lungs (alveolar pressure) becomes negative compared to the atmospheric pressure. Due to the negative pressure, gas (air) flows into the lungs. When the lungs remain increased at a constant volume, the negative pressure disappears and the flow ceases.

The yellow pressure gauge indicates the pressure inside the lungs. The pressure inside the lungs is also called alveolar pressure, to specify anatomic location. When the lungs increase in volume, due to increase in the thoracic cavity volume from inspiratory muscle contraction, the pressure inside the lungs (alveolar pressure) is negative compared to the atmospheric pressure. Note that the yellow pressure gauge, with its tip inside the lung, is elevated on the lung side, compared to the outer-side, which is open and thus indicates the atmospheric pressure. When the lungs remain increased at a constant volume, the negative pressure disappears and the flow ceases.

Achievement Quiz
When the lungs increase in volume, the pressure inside the lungs (alveolar pressure) is 1 [ A negative compared / B equal / C positive compared ] to the atmospheric pressure.
When the lungs are kept at an increased volume, the pressure inside the lungs (alveolar pressure) is 2 [ A negative compared / B equal / C positive compared ] to the atmospheric pressure.
When the lungs are increasing in volume, 3 [ A gas (air) flows in / B there is no gas flow / C gas flows out ].
When the lungs are kept at an increased volume, 4 [ A gas (air) flows in / B there is no gas flow / C gas flows out ].
Answer
1 [ A negative compared ]
2 [ B equal ]
3 [ A gas (air) flows in ]
4 [ B there is no gas flow ]
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