Oxygen and carbon dioxide cross the blood-gas interface by way of diffusion. The factors that determine the rate of diffusion of each gas are described by Fick's law of diffusion:

Diffusion is proportional to the surface area of the blood-gas interface (A); the diffusion constant (D); and the partial pressure gradient of the gas (AP). Diffusion is inversely proportional to the thickness of the blood-gas interface (T).

The surface area of the blood-gas interface is about 70 m2 in a healthy adult at rest. Specifically, 70 m2 of the potential surface area for gas exchange in the lungs is ventilated and perfused. The amount of this surface area may be altered under various conditions. For example, during exercise, an increased number of pulmonary capillaries are perfused (due to increased cardiac output, and therefore blood flow, through the lungs). As a result, a larger percentage of the alveoli are ventilated and perfused, which increases the surface area for gas exchange. Conversely, a fall in the cardiac output reduces the number of perfused capillaries, thus reducing the surface area for gas exchange. Another pathological condition affecting surface area is emphysema. This pulmonary disease, usually associated with cigarette smoking, causes destruction of alveoli.

The diffusion constant for a gas is proportional to the solubility of the gas and inversely proportional to the square root of the molecular weight of the gas:

^ so lub ility

Oxygen and carbon dioxide are small molecules with low molecular weights; however, carbon dioxide is 20 times more soluble than oxygen. Therefore, the value of the diffusion constant for carbon dioxide is larger than that of oxygen, which facilitates the exchange of carbon dioxide across the blood-gas interface.

The thickness of the blood-gas interface is normally less than 0.5 mm. This extremely thin barrier promotes the diffusion of gases. The thickness may increase, however, under conditions of interstitial fibrosis, interstitial edema, and pneumonia. Fibrosis involves the excess production of collagen fibers by fibroblasts in the interstitial space. Edema is the movement of fluid from the capillaries into the interstitial space. Pneumonia causes inflammation and alveolar flooding. In each case, the thickness of the barrier between the air and the blood is increased and diffusion is impaired.

The diffusion of oxygen and carbon dioxide also depends on their partial pressure gradients. Oxygen diffuses from an area of high partial pressure in the alveoli to an area of low partial pressure in the pulmonary capillary blood. Conversely, carbon dioxide diffuses down its partial pressure gradient from the pulmonary capillary blood into the alveoli.

According to Daltons law, the partial pressure of a gas (Pgas) is equal to its fractional concentration (% total gas) multiplied by the total pressure (Ptot) of all gases in a mixture:

The atmosphere is a mixture of gases containing 21% oxygen and 79% nitrogen. Due to the effects of gravity, this mixture exerts a total atmospheric pressure (barometric pressure) of 760 mmHg at sea level. Using these values of fractional concentration and total pressure, the partial pressures for oxygen (PO2) and nitrogen (PN2) can be calculated:

The PO2 of the atmosphere at sea level is 160 mmHg and the PN2 is 600 mmHg. The total pressure (760 mmHg) is equal to the sum of the partial pressures.

Under normal, physiological conditions, the partial pressure gradient for oxygen between the alveoli and the pulmonary capillary blood is quite substantial. However, this gradient may be diminished under certain conditions, such as ascent to altitude and hypoventilation. Altitude has no effect on the concentration of oxygen in the atmosphere. However, the effects of gravity on barometric pressure progressively decrease as elevation increases. For example, at an elevation of 17,000 ft, which is the height of Pike's Peak, the barometric pressure is only 380 mmHg. Therefore, the PO2 of the atmosphere at this altitude is 80 mmHg (PO2 = 0.21 x 380 mmHg = 80 mmHg). This results in a marked decrease in the partial pressure gradient between the alveoli and the pulmonary capillary blood. Consequently, diffusion is impaired. Hypoventilation decreases the rate of oxygen uptake into the alveoli; once again, the partial pressure gradient and the rate of diffusion

Table 17.1 Partial Pressures of Oxygen and Carbon Dioxide


PO2 (mmHg)

PCO2 (mmHg)




Conducting airways (inspired)



Alveolar gas



Arterial blood






Mixed venous blood



are reduced. Conditions resulting in impaired diffusion lead to the development of hypoxemia, or decreased oxygen in the arterial blood.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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