78 Steps Health Journal

Respiratory Gases Cross the AlveolarCapillary Membrane by Diffusion

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The movement of gases in the alveoli and across the alveolar-capillary membrane is by diffusion in response to partial pressure gradients (see Chapter 2). Recall that partial pressure or gas tension can be determined by measuring barometric pressure and the fractional concentration (F) of the gas (Dalton's law,- see Chapter 19). At sea level, Po2 is 160 mm Hg (760 mm Hg X 0.21). Fo2 does not change with altitude, which means that the percentage of O2 in the atmosphere is essentially the same at 30,000 feet (about 9,000 m) as it is as sea level. Therefore, the decreased Po2 at an altitude that makes it difficult to breathe is due to a decrease in the Pb, not to a decrease in Fo2 (Fig. 21.1).

Oxygen is taken up by blood in the lungs and is transported to the tissues. Oxygen uptake is the transfer of oxygen from the alveolar spaces to the blood in the pulmonary capillaries. Gas uptake is determined by three factors: the diffusion properties of the alveolar-capillary membrane, the partial pressure gradient, and pulmonary capillary blood flow.

The diffusion of gases is a function of the partial pressure difference of the individual gases. For example, oxygen diffuses across the alveolar-capillary membrane because of the difference in Po2 between the alveoli and pulmonary capillaries (Fig. 21.2). The partial pressure difference for oxygen is referred to as the oxygen diffusion gradient; in the normal lung, the initial oxygen diffusion gradient, Pao2 (102 mm Hg) minus Pvo2 (40 mm Hg), is 62 mm Hg. The initial diffusion gradient across the alveolar-capillary membrane for carbon dioxide (Pvco2 — Paco2) is about 6 mm Hg, which is much smaller than that of oxygen.

When gases are exposed to a liquid such as blood plasma, gas molecules move into the liquid and exist in a dissolved state. The dissolved gases also exert a partial pressure. A gas will continue to dissolve in the liquid until the partial pressure of the dissolved gas equals the partial pressure above the liquid. Henry's law states that at equilibrium, the amount of gas dissolved in a liquid at a given tempera-

Atmospheric pressure

Vacuum -7SG mm Hg

Atmospheric pressure

Atmospheric pressure

Atmospheric pressure

Mercury

Sea level

Sea level

Mercury

Changes in oxygen tension with altitude. The height of the column of mercury that is supported by air pressure decreases with increasing altitude and is a result of a fall in barometric pressure (Pb). Because the fractional concentration of inspired O 2 (F1O2) does not change with altitude, the decrease in Po2 with altitude is caused entirely by a de crease in

ture is directly proportional to the partial pressure and the solubility of the gas. Henry's law only accounts for the gas that is physically dissolved and not for chemically combined gases (e.g., oxygen bound to hemoglobin).

Gas diffusion in the lungs can be described by Fick's law, which states that the volume of gas diffusing per minute (gas) across a membrane is directly proportional to the membrane surface area (As), the diffusion coefficient of the gas (D), and the partial pressure difference (AP) of the gas and inversely proportional to membrane thickness (T) (Fig. 21.3):

Vgas

Inspired air

Expired gas

Inspired air

Expired gas

Partial pressures of oxygen (Po2) and carbon dioxide (Pco2) in the lungs and sys-

temic circulation.

Partial pressures of oxygen (Po2) and carbon dioxide (Pco2) in the lungs and sys-

The diffusion coefficient of a gas is directly proportional to its solubility and inversely related to the square root of its molecular weight (MW):

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