What is Diffusion?

Diffusion is the result of high-speed random motion of gas or liquid molecules. For example, if the number of molecules present in area A is greater than in area B, mathematical probability dictates that more molecules will move from A to B than from B to A. This net movement of molecules from high to low concentrations is called diffusion. Diffusion in the above-mentioned example continues until the molecules are evenly distributed such that the number of molecules moving from A to B and from B to A is the same. This state is called equilibrium.

The high-speed random impacts of atmospheric air molecules on solid surfaces create the atmosphere’s pressure. Air is a gas mixture; the contribution that each gas makes to the atmospheric pressure is proportional to the number of its molecules present (i.e., each gas exerts its own partial pressure, as explained in Chapter 4). A gas diffuses from one point to another when there are differences in its partial pressures within the mixture; the direction of diffusion is always from high to low partial pressure. When no partial pressure difference exists for any gas throughout the mixture, equilibrium is present. Individual gas partial pressure differences are called diffusion gradients. During diffusion, each gas in a mixture moves according to its own diffusion gradient. That is, two different gases may simultaneously diffuse in opposite directions because of oppositely oriented partial pressure gradients. This occurs for oxygen (O₂) and carbon dioxide (CO₂) across the alveolar capillary membrane (Figure 7-1).

Diffusion is not the same as bulk gas flow, in which a large pressure gradient causes all molecules of all gases to move together in one direction. An example of bulk gas flow is ventilation, in which mouth and alveolar pressure differences cause gas to move in and out of the lungs. In terminal airways and alveoli, random molecular diffusion is the main mechanism whereby gas molecules reach the alveolar surface.

Diffusion Gradients of Respiratory Gases

Figure 7-2 illustrates diffusion gradients between alveolar gas and blood and between blood and body tissues. Inspired air contains about 21% O₂ and essentially no CO₂. Inspired oxygen partial pressure (P_IO₂) is about 160 mm Hg, as the following calculation shows:

Conducting airway PO₂ is lower than the air’s PO₂ because gas in the lung is 100% saturated with water vapor. At body temperature, the partial pressure of water vapor in the lung (P_H2O) is 47 mm Hg. PO₂ in conducting airways is about 150 mm Hg, as the following calculation shows:

Alveolar PO₂ (P_AO₂) is lower still because CO₂ diffuses into the alveoli, diluting incoming O₂ and lowering P_AO₂ to about 100 mm Hg. (Calculation of P_AO₂ is discussed in the next section.)

The diffusion gradient between alveolar gas and mixed venous blood is much larger for O₂ than it is for CO₂ (60 mm Hg vs. 6 mm Hg), as shown in Figure 7-2. At rest, these diffusion gradients transfer about 250 mL of O₂ into the blood and 200 mL of CO₂ into the alveoli each minute. By the time blood leaves the alveolar capillary, the PO₂ and PCO₂ of the blood have reached equilibrium with alveolar gases, even during exercise when blood flows very rapidly through the capillary. PO₂ of blood entering the left atrium is never as high as PO₂ of blood leaving the pulmonary capillaries (see Figure 7-2) because a small amount of deoxygenated bronchial venous blood mixes with capillary blood; this constitutes a normal anatomical shunt. Anatomical shunt is mostly responsible for the normal P(A-a)O₂ (alveolar-to-arterial oxygen pressure difference). Left atrial blood normally flows unaltered into the systemic arteries.

Alveolar Air Equation

The sum of all gas pressures at any point in the lung must equal 760 mm Hg at sea level. When air—which is CO₂-free—is inspired and enters the alveoli, its PCO₂ immediately increases to 40 mm Hg as it mixes with the CO₂ present in the alveoli; because the sum of all alveolar gas pressures is constant (760 mm Hg), the inspired air PO₂ decreases by about 40 mm Hg. If the amount of O₂ diffusing out of alveoli into the blood each minute were exactly equal to the amount of CO₂ diffusing

CLINICAL FOCUS 7-1   Why Alveolar Volume Shrinkage Increases Alveolar Nitrogen Partial Pressure but Decreases Alveolar Oxygen Partial Pressure

If CO₂ entering the alveoli does not replace all the volume vacated by O₂ as it diffuses into the blood, the total alveolar volume must decrease. Why does this not concentrate all alveolar gases, making all partial pressures increase?

Discussion

O₂ (but not nitrogen [N₂]) concentration decreases because (1) alveolar CO₂ and alveolar partial pressure of water vapor (P_AH₂O) are predetermined constants, (2) nitrogen is metabolically inert, and (3) the sum of all alveolar gas partial pressures must equal 760 mm Hg at sea level (P_AO₂ + P_AN₂ + P_ACO₂ + P_AH₂O = 760 mm Hg) (P_AO₂ is alveolar oxygen partial pressure, P_AN₂ is alveolar nitrogen partial pressure, and P_ACO₂ is alveolar partial pressure of carbon dioxide). Temperature and relative humidity determine P_AH₂O and are constant in the lung. The rate of CO₂ entry into alveoli (V˙CO₂) and its removal by alveolar ventilation (V˙_A) determine the P_ACO₂; for a given V˙CO₂ and V˙_A, P_ACO₂ remains constant. Because nitrogen is a metabolically inactive gas, the number of nitrogen molecules in inspired gas is the same as in expired gas. When the inspired gas volume shrinks in the alveoli, the concentration and partial pressure of nitrogen (P_AN₂) increase. The amount of the P_AO₂ reduction is exactly equal to the P_AN₂ increase.

from the blood into alveoli each minute, P_AO₂ would be calculated by simply subtracting alveolar PCO₂ (normally 40 mm Hg) from the result of equation 2. However, O₂ diffuses out of the alveolus at a greater rate than CO₂ diffuses into the alveolus. At rest, pulmonary capillary blood removes about 250 mL per minute of O₂ from the alveoli, replacing it with only 200 mL per minute of CO₂. The ratio of alveolar CO₂ excretion (V˙CO₂) to blood oxygen uptake (V˙O₂) is called the respiratory exchange ratio (R), and its value is normally about 0.8 (R=V˙CO2/V˙O2=200/250=0.8).

When R is equal to 0.8, the CO₂ diffusing into the alveolus replaces only 80% of the volume that O₂ vacated when it diffused out of the alveolus. This uneven exchange causes the alveolar gas volume to shrink slightly, but alveolar gas pressure remains constant at 760 mm Hg. The shrinkage in alveolar volume concentrates the alveolar nitrogen molecules, which the alveolar air equation takes into account. This equation is known as the ideal alveolar air equation because it assumes the ventilation-to-blood flow ratios of each alveolus in the lung are identical, as the following calculation shows:

3. PAO2=PIO2−PACO2 [FIO2+1−FIO2R]

In this equation P_IO₂ equals F_IO₂ (760 − 47). F_IO₂ represents inspired oxygen concentration expressed as a decimal fraction. The bracketed part of equation 3 is a correcting factor that considers the effect of R on P_AO₂. When R equals 1, the correction factor equals 1 and does not need to be applied.

CONCEPT QUESTION 7-2

What percentage of the volume vacated by O₂ diffusing out of the alveoli is replaced by incoming CO₂ when R equals 1.0?

The term 1 − F_IO₂ is equal to the inspired nitrogen concentration. When R is less than 1, the effect is to increase nitrogen concentration, causing the bracketed term to increase to values greater than 1. In equation 3, if R = 0.8 and F_IO₂ = 0.21 (room air), the bracketed factor is equal to 1.2.

Examination of equation 3 shows that higher F_IO₂ values require progressively smaller correction factors; at 100% inspired oxygen (F_IO₂ = 1.0), no correction is needed. A sufficiently accurate equation for clinical use is a simplified form of equation 3 for patients breathing an F_IO₂ of 0.60 or less. This is shown as follows:¹

4. PAO2=PIO2−(PaCO2)1.2

For F_IO₂ values greater than 0.60, a sufficiently accurate clinical equation is as follows:¹

5. PAO2=PIO2−PaCO2

In equations 4 and 5, PaCO₂ is substituted for P_ACO₂ because these two values are generally equal, unless the lungs have a high degree of alveolar dead space.

A normal P_AO₂ for a person breathing room air at sea level, with a PaCO₂ equal to 40 mm Hg and an R equal to 0.8, is about 100 mm Hg (using equation 4). This is shown as follows:

PAO2=0.2093(760−47)−(40×1.2)

PAO2=149.2−48

PAO2=101.2

Table 7-1 summarizes respiratory gas partial pressures at sea level in dry inspired air, humidified (tracheal) air, alveolar air, and mixed expired air. Expired gas PO₂, PCO₂, and PN₂ differ from alveolar values because expired air contains dead space gas mixed with alveolar gas.

TABLE 7-1

Partial Pressures of Gases at Sea Level

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	Dry Air		Humidified Air		Alveolar Air		Expired Air
Gases	mm Hg	%	mm Hg	%	mm Hg	%	mm Hg	%
Nitrogen	600.2	78.98	563.4