The transport of oxygen (O2) and carbon dioxide (CO2) in the body is facilitated by the respiratory and circulatory systems working together. Oxygen is carried from the lungs to the tissues, while carbon dioxide is transported from the tissues back to the lungs for expiration. Various mechanisms have evolved to enhance the uptake and transport of these gases, including the binding of O2 to hemoglobin and the transport of CO2 as bicarbonate ions (HCO3 -).

To understand how these gases are transported, we must consider their diffusion properties and the mechanisms involved in their transport and delivery. Gas movement in the respiratory system mainly occurs through diffusion, which follows basic gas diffusion laws. The respiratory and circulatory systems have specific anatomical and physiological features that facilitate gas diffusion, such as large surface areas for gas exchange, short distances for diffusion, and favorable diffusion properties of the gases.

Gas diffusion occurs from regions of higher to lower partial pressure, both in gaseous and liquid states. The rate of diffusion is determined by Graham's law, which states that it is directly proportional to the solubility coefficient of the gas and inversely proportional to the square root of its molecular weight. The diffusion properties of O2 and CO2 can be compared by calculating their diffusion rates. CO2 diffuses approximately 20 times faster than O2.

Fick's law of gas diffusion predicts the rates of O2 diffusion from the lungs into the blood, from the blood into the tissues, and vice versa for CO2. The conductance of a gas from the alveolus to the blood is represented by the ratio AD:T. The overall conductance of the lung for gas diffusion, known as the diffusing capacity of the lung (DL), can be calculated using Fick's equation.

Fick's law could be used to assess the diffusion properties of O2 in the lungs, but the capillary partial pressure of oxygen cannot be measured directly. However, this limitation can be overcome by using carbon monoxide (CO) instead of O2. CO has low solubility in the capillary membrane, resulting in slow equilibrium across the capillary and a close-to-zero partial pressure of CO in capillary blood. The high solubility of CO in blood makes it a useful gas for calculating DL, as the only limitation for CO diffusion is the alveolar-capillary membrane.

DL can be measured by observing the diffusion of carbon monoxide (V̇ CO) and the average partial pressure of CO in the alveolus.
Compare and contrast the chloride shift and the Haldane effect on CO2 transport.
he respiratory and circulatory systems function together to transport 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 metabolism, is transported from the tissues via systemic veins to the lungs, where it is expired (Fig. 24.1 ). To enhance uptake and transport of these gases between the lungs and tissues, specialized mechanisms (e.g., binding of O2 and hemoglobin 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 understand the mechanisms involved in the transport of these gases, gas diffusion properties, as well as transport and delivery mechanisms, must be considered.
Gas movement throughout the respiratory system occurs predominantly via diffusion. The respiratory and circulatory systems contain several unique anatomical and physiological features to facilitate gas diffusion: (1) large surface areas for gas exchange (alveolar to capillary and capillary to tissue membrane) 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.
Diffusion of Gases From Regions of Higher to Lower Partial Pressure in the Lungs
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. 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. 24.2). The ratio AD:T represents the con ductance of a gas from the alveolus to blood. The diffusing capacity of the lung (DL) is its conductance (A•D/T) when considered for the entire lung; thus, with Fick’s equation, DL can be calculated as follows:
Equation 24.1 where V̇ gas = gas diffusion.
Fick’s law of diffusion could be used to assess the diffusion properties of O2 in the lungs, except that the capillary partial pressure of oxygen cannot be measured. This limitation can be overcome with the use of 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 0. In contrast, the solubility of CO in blood is high. Thus the only limitation for diffusion of CO is the alveolar-capillary membrane, and thus CO is a •Fig. 24.1 Oxygen (O2) and Carbon Dioxide (CO2) Transport in Arterial and Venous Blood. Oxygeninarterialbloodistransferredfromarterialcapillariestotissues.TheflowratesforO2andCO2areshownfor1Lofblood.
•Fig. 24.2 AccordingtoFick’slaw,diffusionofagasacrossasheetoftissue(Vgas)isdirectlyrelatedtothesurfacearea(A)ofthetissue, thediffusionconstant(D)ofthespecificgas,andthepartialpressuredifference(P1− P2)ofthegasoneachsideofthetissue,anditisinverselyrelatedtotissuethickness(T).
useful gas for calculating DL. The capillary partial pressure (P2 in
Eq. 24.1 ) is essentially 0 for CO, and therefore DL can be measured from the diffusion of carbon monoxide (V̇ CO) and the average partial pressure of CO in the alveolus; that is,