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Respiratory center is stimulated

Rate and depth of breathing increase

More CO2 is eliminated through lungs

Normal CO 2 and H+ concentration is maintained

Figure

An increase in carbon dioxide elimination follows an increase in carbon dioxide production.

U What is the difference between a strong acid or base and a weak acid or base?

^9 How does a chemical buffer system help regulate pH of body fluids?

^9 List the major buffer systems of the body.

Chemical buffer systems only temporarily solve the problem of acid-base balance. Ultimately, the body must eliminate excess acid or base. The lungs (controlled by the respiratory center) and the kidneys accomplish this task.

Respiratory Excretion of Carbon Dioxide The respiratory center in the brain stem helps regulate hydrogen ion concentrations in the body fluids by controlling the rate and depth of breathing. Specifically, if body cells increase their production of carbon dioxide, as occurs during periods of physical exercise, carbonic acid production increases. As the carbonic acid dissociates, the concentration of hydrogen ions increases, and the pH of the internal environment drops (see chapter 19, p. 802). Such an increasing concentration of carbon diox ide in the central nervous system and the subsequent increase in hydrogen ion concentration in the cere-brospinal fluid stimulate chemosensitive areas within the respiratory center.

In response, the respiratory center increases the depth and rate of breathing so that the lungs excrete more carbon dioxide. As a result, hydrogen ion concentration in body fluids returns toward normal, because the released carbon dioxide is in equilibrium with carbonic acid and CO2 (fig. 21.11):

Conversely, if body cells are less active, concentrations of carbon dioxide and hydrogen ions in body fluids remain relatively low. As a result, breathing rate and depth fall. This increases the carbon dioxide level in the body fluids, returning pH to normal. If the pH drops below normal, the respiratory center is stimulated to increase the rate and depth of breathing.

Thus, the activity of the respiratory center changes in response to shifts in the pH of the body fluids, reducing these shifts to a minimum. Because most of the

Increased concentration of H+ in urine

Concentration of H+ in body fluids returns toward normal

Increased secretion of H+ into fluid of renal tubules

Increased concentration of H+ in body fluids