In stable acid-base balance, net acid excretion by the kidneys equals the net rate of H+ addition to the body by metabolism or other processes, assuming that other routes of loss of acid or base (e.g., gastrointestinal losses) are small and can be neglected, which normally is the case. The net loss of H+ in the urine can be calculated from the following equation, which shows typical values in the parentheses:
Renal net acid excretion (70 mEq/day) =
urinary titratable acid (24 mEq/day) + urinary ammonia (48 mEq/day) — urinary HCO3— (2 mEq/day) (22)
Urinary ammonia (as NH4+) ordinarily accounts for about two thirds of the excreted H + , and titratable acid for about one third. Excretion of HCO3 — in the urine represents a loss of base from the body. Therefore, it must be subtracted in the calculation of net acid excretion. If the urine contains significant amounts of organic anions, such as citrate, that potentially could have yielded HCO3— in the body, these should also be subtracted. Since the amount of free H+ excreted is negligible, this is omitted from the equation.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.