CHAPTER 23 Oxygen and Carbon Dioxide Transport

The respiratory and circulatory systems function together to transport sufficient oxygen (O₂) from the lungs to the tissues to sustain normal cellular activity and to transport carbon dioxide (CO₂) from the tissues to the lungs for expiration. CO₂, a product of active cellular glucose metabolism, is transported from the tissues via systemic veins to the lungs, where it is expired (Fig. 23-1). To enhance uptake and transport of these gases between the lungs and tissues, specialized mechanisms (e.g., O₂-hemoglobin binding and HCO₃⁻ transport of CO₂) have evolved that enable O₂ uptake and CO₂ expiration to occur simultaneously. Moreover, these specialized mechanisms facilitate uptake of O₂ and expiration of CO₂. To gain an understanding of the mechanisms involved in the transport of these gases, one must consider gas diffusion properties, as well as transport and delivery mechanisms.

Figure 23-1 Oxygen and CO₂ transport in arterial and venous blood. Oxygen in arterial blood is transferred from arterial capillaries to tissues. The flow rates for O₂ and CO₂ are shown for 1 L of blood. The ratio of CO₂ production to O₂ consumption is the respiratory exchange ratio, R, which at rest is approximately 0.80.

GAS DIFFUSION

Gas movement throughout the respiratory system occurs predominantly via diffusion. The respiratory and circulatory systems contain several unique anatomic and physiological features to facilitate gas diffusion: (1) large surface areas for gas exchange (alveolar to capillary and capillary to tissue membrane barriers) with short distances to travel, (2) substantial partial pressure gradient differences, and (3) gases with advantageous diffusion properties. Transport and delivery of O₂ from the lungs to the tissue and vice versa for CO₂ are dependent on basic gas diffusion laws.

Gases in the Lung Diffuse from Regions of Higher to Lower Partial Pressure

The process of gas diffusion is passive and similar whether diffusion occurs in a gaseous or liquid state. The rate of diffusion of a gas through a liquid is described by Graham’s law, which states that the rate is directly proportional to the solubility coefficient of the gas and inversely proportional to the square root of its molecular weight. Calculation of the diffusion properties for O₂ and CO₂ reveals that CO₂ diffuses approximately 20 times faster than O₂ does. Rates of O₂ diffusion from the lungs into blood and from blood into tissue, and vice versa for CO₂, are predicted by Fick’s law of gas diffusion (Fig. 23-2). Fick’s law states that the diffusion of a gas (_gas) across a sheet of tissue is directly related to the surface area (A) of the tissue, the diffusion constant (D) of the specific gas, and the partial pressure difference (P₁ − P₂) of the gas on each side of the tissue and is inversely related to tissue thickness (T). That is,

Figure 23-2 Fick’s law states that diffusion of a gas across a sheet of tissue is directly related to the surface area of the tissue, the diffusion constant of the specific gas, the partial pressure difference of the gas on each side of the tissue and is inversely related to tissue thickness.

Equation 23-1

The ratio A ⋅ D/T represents the conductance of a gas from the alveolus to blood. The diffusing capacity of the lung (D_L) is its conductance (A ⋅ D/T) when considered for the entire lung; thus, applying Fick’s equation, D_L can be calculated as follows:

Equation 23-2

Fick’s law of diffusion could be used to assess the diffusion properties of O₂ in the lung, except that ΔP (alveolar − capillary PO₂) cannot be determined because capillary PO₂ cannot be measured. This limitation can be overcome by using carbon monoxide (CO) rather than O₂. Because CO has low solubility in the capillary membrane, the rate of CO equilibrium across the capillary is slow and the partial pressure of CO in capillary blood remains close to zero. This is in contrast to the high solubility of CO in blood. Thus, the only limitation for diffusion of CO is the alveolarcapillary membrane, which makes CO a useful gas for calculating D_L. The capillary partial pressure (P₂ above) is essentially zero for CO, and therefore D_L can be measured from and the average partial pressure of CO in the alveolus. That is,

Equation 23-3

Assessment of D_LCO has become a classic measurement of the diffusion barrier of the alveolar-capillary membrane. It is useful in the differential diagnosis of certain restrictive and obstructive lung diseases, such as interstitial pulmonary fibrosis and emphysema.

IN THE CLINIC

A patient with interstitial pulmonary fibrosis (a restrictive lung disease) inhales a single breath of 0.3% CO from residual volume to total lung capacity. He holds his breath for 10 seconds and then exhales. After discarding the exhaled gas from the dead space, a representative sample of alveolar gas from late in exhalation is obtained. The average alveolar CO pressure is 0.1 mm Hg, and 0.25 mL of CO has been taken up. The diffusion capacity for CO in this patient is

The normal range for D_LCO is 20 to 30 mL/min/mm Hg. Patients with interstitial pulmonary fibrosis have an initial alveolar inflammatory response with subsequent scar formation within the interstitial space. The inflammation and scar tissue thicken the interstitial space and thus make it more difficult for gas diffusion to occur, which results in decreased D_LCO. This is a classic characteristic of certain types of restrictive lung disease; gas readily enters the alveolus but is inhibited in its ability to diffuse into blood.

Oxygen and Carbon Dioxide Exchange in the Lung Is Perfusion Limited

Different gases have different solubility factors. Gases that are insoluble in blood (i.e., anesthetic gases, nitrous oxide and ether) do not chemically combine with proteins in blood and equilibrate rapidly between alveolar gas and blood. The equilibration occurs in less time than the 0.75 second that the red blood cell spends in the capillary bed (capillary transit time). The diffusion of insoluble gases between alveolar gas and blood is considered perfusion limited because the partial pressure of gas in the blood leaving the capillary has reached equilibrium with alveolar gas and is limited only by the amount of blood perfusing the alveolus. In contrast, a diffusion limited gas, such as CO, has low solubility in the alveolar-capillary membrane but high solubility in blood because of its high affinity for hemoglobin (Hgb). These features prevent the equilibration of CO between alveolar gas and blood during the red blood cell transit time.

The high affinity of CO for Hgb enables large amounts of CO to be taken up in blood with little or no appreciable increase in its partial pressure. Gases that are chemically bound to Hgb do not exert a partial pressure in blood. Like CO, both CO₂ and O₂ have relatively low solubility in the alveolar-capillary membrane but high solubility in blood because of their ability to bind to Hgb. However, their rate of equilibration is sufficiently rapid for complete equilibration to occur during the transit time of the red blood cell within the capillary. Equilibration for O₂ and CO₂ usually occurs within 0.25 second. Thus, O₂ and CO₂ transfer is normally perfusion limited. The partial pressure of a diffusion limited gas (i.e., CO) does not reach equilibrium with the alveolar pressure over the time that it spends in the capillary (Fig. 23-3). Although CO₂ has a greater rate of diffusion in blood than O₂ does, it has a lower membrane-blood solubility ratio and consequently takes approximately the same amount of time to reach equilibration in blood.

Figure 23-3 Uptake of nitrous oxide (N₂O), CO, and O₂ in blood relative to their partial pressures and the transit time of the red blood cell in the capillary. For gases that are perfusion limited (N₂O and O₂), their partial pressures have equilibrated with alveolar pressure before exiting the capillary. In contrast, the partial pressure of CO, a gas that is diffusion limited, does not reach equilibrium with alveolar pressure. O₂ uptake in rare conditions can become diffusion limited.

Diffusion limitation for O₂ and CO₂ would occur if red blood cells spent less than 0.25 second in the capillary bed. Occasionally, this can be seen in very fit athletes during vigorous exercise and in healthy subjects who exercise at high altitude.

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