Respiratory Cycle / Quiet breath: Inspiration
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During inspiration...

Inspiratory muscles, such as the diaphragm, contract, increasing the volume of the thoracic cavity. Because of the thin liquid film, the lung volume is also increased. Since this is in the opposite direction of the lung elasticity (note that the 'rubber bands' are further stretched), the negative pleural pressure becomes larger (note the blue pressure gauge).

With lung volume increasing, the pressure inside the lungs (alveolar pressure) becomes negative, compared to the atmospheric pressure, causing gas (air) inflow into the lungs.

Achievement Quiz
During inspiration, the diaphragm is 1 [ A contracting / B relaxing ].
During inspiration, 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 inspiration, 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 inspiration, 4 [ A gas (air) flows in / B there is no gas flow / C gas flows out ].
During inspiration, the pressure inside the lungs (alveolar pressure) is 5 [ A negative compared / B equal / C positive compared ] to the atmospheric pressure.
During inspiration, 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 [ A contracting ]
2 [ B increasing from its minimum to its maximum ]
3 [ B increasing from its minimum to its maximum ]
4 [ A gas (air) flows in ]
5 [ A negative compared ]
6 [ B increasing from its minimum to its maximum ]
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