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

Because there is only a negligible amount of carbon dioxide in the inspired air, PAco₂ is determined by the rate of carbon dioxide production (Vco₂) in relation to the amount of alveolar ventilation (Va):

where K = PB - PH₂O.

It is obvious from this equation that if Vco₂ increases, as occurs during exercise, Va must also increase if PACo₂ is to remain constant. If Va does not increase sufficiently, PAco₂ rises. Similarly, if Veo₂ remains constant and VA halves, PAco₂ doubles.

Po₂ is lower in the alveolus than in inspired air because oxygen and carbon dioxide exchange occurs continually. During breathing, alveolar oxygen tension (PAo₂) fluctuates

4. Lung resisting inflation

(e.g., airway obstruction, decreased lung compliance)

FIGURE 47-1 Diagrammatic representation of the brain, peripheral nerves, thorax, airways, and lung to show the causes of alveolar hypoventilation. CNS_f Central nervous system.

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 an alveolar carbon dioxide tension (PAco₂) of 40 mm Hg, PAo₂ averages approximately 100 mm Hg at sea level, where PB is 760 mm Hg.

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 damage to the peripheral nerves, (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 severe lung disease that 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, increased production of hydrogen ions (H^f), 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).