Gas Exchange

Gas Exchange

The Composition of a Gas Mixture Can Be Described by the Fractional Composition or Partial Pressure

Understanding gas exchange requires an understanding of the measurement of gas composition and the forces causing gas movement within the lungs, blood, and tissues. For convenience, physiologists use many abbreviations when describing gas exchange (Table 47-1). Air contains 21% oxygen (the fraction of oxygen in inspired air, FiO2, is 0.21). High in the Andes Mountains, the air still contains 21% oxygen, but visitors to those altitudes notice the lack of oxygen. Clearly, therefore, it is not only the fraction of oxygen that is important for gas exchange; the hypoxia at high altitude is a result of the low oxygen partial pressure that is a consequence of the low barometric pressure. At this lower barometric pressure, the oxygen molecules are less densely packed, and therefore the partial pressure of oxygen (PO2) in the air is decreased. It is this partial pressure (also called tension) that is important in gas transfer.

The partial pressure of oxygen (PO2) in a dry gas mixture is determined by barometric pressure (PB) and the fraction of oxygen (FO2) in the gas mixture, as follows:


In the atmosphere, FiO2 is 0.21, so PO2 in dry air at sea level when PB = 760 mm Hg is approximately 160 mm Hg:


PO2 decreases at higher altitudes because barometric pressure decreases.

During inhalation, air is warmed to body temperature and humidified in the larger air passages. The concentration of oxygen and other gases is reduced by the presence of water vapor molecules; therefore, PO2 is less in humidified than dry air. The PO2 of humidified gas is calculated as follows:


where PH2O is the partial pressure of water vapor at body temperature. The PH2O is determined by the temperature and percentage saturation of the air with water. In a mammal with a body temperature of 38.2° C, PH2O in saturated air equals 50 mm Hg; therefore, at sea level (PB = 760 mm Hg), the PO2 of warmed, completely humidified gas in the conducting airways is approximately 149 mm Hg:


Alveolar Gas Composition Is Determined by Alveolar Ventilation and the Exchange of Oxygen and Carbon Dioxide

Because there is only a negligible amount in the inspired air, the main source of carbon dioxide comes to the lungs in the blood returning from the tissues. For this reason, the PACO2 (alveolar partial pressure of carbon dioxide) is determined by the rate of carbon dioxide production (image) in relation to the amount of alveolar ventilation (image):


where K = PB − PH2O.

It is obvious from this equation that if image increases, as occurs during exercise, image must also increase if PACO2 is to remain constant. If image does not increase sufficiently, PACO2 rises. Similarly, if image remains constant and image halves, PACO2 doubles.

Alveolar oxygen tension (PAO2) is lower than that in inspired air because oxygen and carbon dioxide exchange occurs continuously. During breathing, PAO2 fluctuates around an average value, increasing during inhalation and decreasing during exhalation. The average oxygen tension in the alveoli of the lung can be calculated from the alveolar gas equation, a simplified version of which follows:


where R, the respiratory exchange ratio, is the ratio of the rate of carbon dioxide production to that of oxygen consumption. The respiratory exchange ratio is determined by the substrates being metabolized by the animal. This equation demonstrates that alveolar oxygen tension is determined by the inspired oxygen tension and the exchange of oxygen for carbon dioxide. Assuming an average R of 0.8 and a PACO2 of 40 mm Hg, PAO2 averages approximately 100 mm Hg at sea level, where PB is 760 mm Hg. The alveolar gas equation also shows that whenever PACO2 increases, PAO2 decreases, and vice versa.

Alveolar hypoventilation, a decrease in alveolar ventilation in relation to carbon dioxide production, elevates PACO2 and decreases PAO2. Figure 47-1 shows the causes of alveolar hypoventilation. It occurs when (1) the central nervous system is depressed by drugs or injury, (2) there is injury to the phrenic nerve that supplies the diaphragm, (3) there is damage to the thorax and respiratory muscles, (4) there is severe airway obstruction (e.g., in exercising horses with laryngeal hemiplegia), or (5) there is lung disease that severely decreases lung compliance.

The converse of alveolar hypoventilation, alveolar hyperventilation, causes a decrease in PACO2 because ventilation is increased in relation to carbon dioxide production. Therefore, according to the alveolar gas equation, as PACO2 decreases, PAO2 increases. Hyperventilation occurs when the need to ventilate is increased by stimuli such as hypoxia, acidosis, or an increase in body temperature.

A modified form of the alveolar gas equation can be used to determine PAO2 for clinical purposes, as follows:


In this equation, arterial carbon dioxide tension (PaCO2) is substituted for alveolar carbon dioxide tension (PACO2).

Exchange of Oxygen and Carbon Dioxide Between the Alveolus and Pulmonary Capillary Blood Occurs by Diffusion

Diffusion is the passive movement of gases down a concentration (partial pressure) gradient. The rate of gas movement between the alveolus and the blood (image) is determined by the physical properties of the gas (D), the surface area available for diffusion (A), the thickness of the air-blood barrier (x), and the driving pressure gradient of the gas between the alveolus and capillary blood (PAO2 – PcapO2), as follows:


D is determined by several factors, including the molecular weight and solubility of the gas. The alveolar surface area (A) available for diffusion is that occupied by perfused pulmonary capillaries. During exercise, more capillaries become perfused by blood, and thus the surface area available for diffusion increases.

In the lung the barrier separating air and blood (x) is less than 1 µm thick (Figure 47-2). However, although thin, this barrier includes a layer of liquid and surfactant lining the alveolar surface; an epithelial layer, usually formed by type I epithelial cells; a basement membrane; variable-thickness interstitium; and a layer of endothelium. In addition to moving gases through this air-blood barrier, diffusion also moves gases within the plasma, allowing oxygen to gain access to erythrocytes and hemoglobin.

Blood entering the alveolar capillary from the small pulmonary arteries is known as mixed venous blood because it has returned to the right side of the heart in veins from all parts of the systemic circulation. The driving pressure for gas diffusion is the difference in oxygen tension between the alveolus (PAO2) and the capillary blood. PAO2 averages 100 mm Hg; in a resting animal, blood entering the alveolar capillary—that is, mixed venous blood—has an oxygen tension (image) of approximately 40 mm Hg. The driving pressure gradient of 60 mm Hg (100−40) causes rapid diffusion of oxygen into the capillary, where it combines with hemoglobin. Hemoglobin takes up oxygen from the plasma and helps maintain the gradient for oxygen diffusion.

Normally, in the resting animal, equilibration between alveolar and capillary oxygen tensions occurs within 0.25 second, approximately one third of the time the blood is in the capillary (Figure 47-3). During strenuous exercise, muscles extract a large amount of oxygen from the blood, so the mixed venous blood returning to the lung contains little oxygen. In addition, during exercise the cardiac output is high, and the velocity of blood flow through the capillaries is rapid. More oxygen must therefore be transferred in less time than in the resting animal. Under these strenuous conditions, diffusion equilibrium may not occur, and the oxygen tension of blood leaving the lung and entering the systemic arteries (PaO2) may decrease during intense exercise. This exercise-associated hypoxemia is observed in racing Thoroughbred horses.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Gas Exchange
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