Defenses Against Changes in Hydrogen Ion Concentration: Buffers, Lungs, and Kidneys
There are three primary systems that regulate the H+ concentration in the body fluids to prevent acidosis or alkalosis: (1) the chemical acid-base buffer systems ofthe body fluids, which immediately combine with acidor base to prevent excessive changes in H+ concentra-tion; (2) the respiratory center, which regulates the removal of CO2 (and, therefore, H2CO3) from the extracellular fluid; and (3) the kidneys, which can excrete either acid or alkaline urine, thereby readjust-ing the extracellular fluid H+ concentration toward normal during acidosis or alkalosis.
When there is a change in H+ concentration, the buffer systems of the bodyfluids react within a fractionof a second to minimize these changes. Buffer systems do not eliminate H+ from or add them to the body but only keep them tied up until balance can be re-established.
The second line of defense, the respiratory system, also acts within a few minutes to eliminate CO2 and, therefore, H2CO3 from the body.
These first two lines of defense keep the H+ con-centration from changing too much until the more slowly responding third line of defense, the kidneys, can eliminate the excess acid or base from the body. Although the kidneys are relatively slow to respond compared with the other defenses, over a period of hours to several days, they are by far the most power-ful of the acid-base regulatory systems.
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