Between breaths, when the lungs are at rest, the volume of air in the lungs is called the functional residual capacity (FRC). No air is moving into or out of the lungs during this time. Air flow results from the muscles of respiration creating pressure gradients by changing the volume of the lungs.
The relationship between pressure and volume is expressed by Boyle's law. This law states that the pressure of gas in any container is inversely related to the volume of the container. In other words, when volume increases, pressure decreases and when volume decreases, pressure increases.
Air flows into the lungs by bulk flow. The rate of flow is equal to the pressure gradient created by the difference between atmospheric pressure and intra-alveolar pressure over the resistance encountered by air as it flows through the air passageways.
The changes in intra-alveolar pressure creates the pressure gradient that causes air to flow into and out of the lungs. Two factors determine intra-alveolar pressure:
1. The quantity (moles) of air molecules in the alveoli;
2. The volume of the alveoli.
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