Three gas laws are important for understanding pulmonary ventilation. Each is named after its discoverer: Boyle’s Law, Dalton’s Law, and Henry’s Law. Boyle’s Law determines the direction of gas flow in ventilation: as volume increases (inspiration), pressure decreases and gas flows into the lungs.
Dalton’s Law tells us how to express the concentration of a gas, using the same terms, in a for a mixture of gases as for gases dissolved in solution. The total pressure of a gas in a mixture of gases is the sum of the pressures of the gases if they were present alone. This applies to gases that are in the air and to gases dissolved in blood plasma. The pressure of each individual gas, written PO2 and PCO2, is called the partial pressure of the gas. It is the contribution of that gas to the mixture.
What is powerful about this way of expressing concentrations is that the direction of gas diffusion is determined by its partial pressure. Gases diffuse from high partial pressures to lower partial pressures. The partial pressure of oxygen is higher in the alveoli than in the capillaries that surround them, so oxygen diffuses from the alveoli (air) into the plasma (solution). The partial pressure of carbon dioxide is higher in the capillaries than in the alveoli, so carbon dioxide leaves the capillaries and enters the alveoli.
We are able to equate the partial pressure of a gas in a mixture of gases with its partial pressure in solution using Henry’s Law. This law states that the amount of gas dissolved in a liquid is proportional to the partial pressure in the gas above the solution. Thus, at equilibrium, the partial pressures of the gas are equal in the air of the alveoli and the plasma of the capillaries.
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