Respiratory Cycle / Quiet breath: expiration
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During expiration...

The inspiratory muscles are relaxed and thus, there is no force increasing the volumes of the thoracic cavity nor of the lungs. Thus, due to the elasticity of the lungs, the lungs and thoracic cavity decrease in volume. Naturally, the negative pleural pressure is smaller than at the end of inspiration (beginning of expiration).

Because the lungs decrease in volume, the pressure inside the lungs (alveolar pressure) becomes positive compared to the atmospheric pressure. The positive pressure pushes out the gas inside the lungs (because of O2 and CO2 exchange with the blood, it is no longer 'air') outward through the trachea (expiration).

Achievement Quiz
During expiration, the diaphragm is 1 [ A contracting / B relaxing ].
During expiration, the lung volume is 2 [ A at its minimum / B increasing from its minimum to its maximum / C at its maximum / D decreasing from its maximum to its minimum ] (within a quiet breath).
During expiration, the inward elastic force of the lungs is 3 [ A at its minimum / B increasing from its minimum to its maximum / C at its maximum / D decreasing from its maximum to its minimum ] (within a quiet breath).
During expiration, 4 [ A gas (air) flows in / B there is no gas flow / C gas flows out ].
During expiration, the pressure inside the lungs (alveolar pressure) is 5 [ A negative compared / B equal / C positive compared ] to the atmospheric pressure.
During expiration, the negative pleural pressure is 6 [ A at its minimum / B increasing from its minimum to its maximum / C at its maximum / D decreasing from its maximum to its minimum ] (within a quiet breath).
Answer
1 [ B relaxing ]
2 [ D decreasing from its maximum to its minimum ]
3 [ D decreasing from its maximum to its minimum ]
4 [ C gas flows out ]
5 [ C positive compared ]
6 [ D decreasing from its maximum to its minimum ]
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