Most metabolic processes of the body depend on ATP, and most ATP production requires oxygen and generates carbon dioxide as a waste product. Oxygen is supplied to the tissues and their carbon dioxide is removed by the combined action of the cardiovascular and respiratory systems. Not only do these two systems have a close spatial relationship in the thoracic cavity, they also have such a close functional relationship that they are often considered jointly under the heading cardiopulmonary. A disorder that affects the lungs has direct and pronounced effects on the heart, and vice versa. Furthermore, as discussed in the next two chapters, the respiratory system works closely with the urinary system to regulate the body's acid-base balance. Changes in the blood pH, in turn, trigger autonomic adjustments of the heart rate and blood pressure. Thus, the cardiovascular, respiratory, and urinary systems have an especially close physiological relationship. It is important that we now address the respiratory and urinary systems and their roles in the homeostatic control of blood gases, pH, blood pressure, and other variables related to the body fluids.

The term respiration has three meanings: (1) ventilation of the lungs (breathing), (2) the exchange of gases between air and blood and between blood and tissue fluid, and (3) the use of oxygen in cellular metabolism. In this chapter, we are concerned with the first two processes. Cellular respiration was introduced in chapter 3 and is considered more fully in chapter 26.

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