The Gas Exchanger: Matching Ventilation and Perfusion

Learning Objectives

• To define the concepts of dead space and shunt.
• To describe the implications of changes in breathing pattern (e.g., rapid–shallow versus slow–deep breathing) on gas exchange.
• To outline the implications for gas exchange of the differences between the ways that oxygen and carbon dioxide bind with hemoglobin (specifically, the implications of a sigmoid-shaped versus a linear relationship between partial pressure of oxygen or carbon dioxide and hemoglobin binding).
• To describe the physiological implications for gas exchange of the anatomy of the pulmonary circulation (e.g., the relationship between pulmonary pressure and perfusion of alveoli, and the effect of hypoxemia on pulmonary blood flow).
• To define the physiologic causes of hypoxemia.
• To describe the principles that are used to distinguish the physiological causes of hypoxemia.
• To define the physiologic causes of hypercapnia.

Having examined in detail the physiological function of the ventilatory pump, we now turn our attention to what some might describe as the "business end" of the respiratory system, the gas exchanger. How does the system actually get oxygen into the blood and carbon dioxide into the alveoli? What are the effects of breathing pattern and body position on oxygen and carbon dioxide levels? Why do some physiologic derangements lead to low oxygen levels alone while others produce both low oxygen and high carbon dioxide levels in the blood? These are just some of the questions we examine in this chapter.

We start our study of the gas exchanger by looking at the distribution of air that is inhaled into the lungs and the impact of pleural pressure, body position, and breathing pattern on that distribution. Then we turn our attention to blood flow in the lungs and the factors that affect flow and resistance in the pulmonary circulation. To fully understand gas exchange, we must also investigate the different ways that carbon dioxide and oxygen bind to hemoglobin. These concepts have extremely important implications for the physiological causes of low oxygen and high carbon dioxide levels in the blood.

Ultimately, gas exchange requires that we move air into and out of the alveoli (ventilation) and bring that air into contact with blood circulating through the lungs (perfusion). The matching of these two functions, or failure to do this efficiently, is one of the keys to our success or failure as aerobic beings.