Acid-base - Acidbasehomeostasis I.DavidWeiner,M.D. 3746102

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Acid base homeostasis I. David Weiner, M.D. 374 6102 David.Weiner@medicine.ufl.edu www.RenalLectures.com Acid basehomeostasisiscriticaltonormalhealthanddevelopment. Protonsareextremely reactive molecules that damage cellular function, and can lead to death, despite existing at very low concentrations. Normal proton concentration is 40 nM (40 x 10 9 M). Because protons are present at such low concentrations, we typically refer to their concentration as pH. Equation 1 shows this formula. The normal metabolism of amino acids leads to ~0.8 mEq/kg/d of hydrochloric, sulfuric and phosphoric acid production. For the average 70 kg individual this leads to ~56 mEq of strong acid production per day (equal to almost 1½ gallons of pH 2 hydrochloric acid). The daily production of acids does not lead to overwhelming acidosis and death because the protons are buffered rapidly by the body. Both intracellular and extracellular buffers are involved in this process. For simplicity sake, we will consider only the bicarbonate buffer system. Equation 2 shows how this system buffers proton production. Thissystemiseffectivebecausebicarbonate(HCO 3 ! )isastrong base and is present at a concentration of ~24 mM, an almost 10 6 fold greater concentration than protons. The net result of proton addition to the system is consumption of bicarbonate, and production of H 2 O and CO 2 . The CO 2 is then excreted through normal respiration. Because of the interrelationship between H + , HCO 3 and CO 2 shown in Equation 2, it is possible to determine pH from the blood HCO 3 and pCO 2 . This is shown in Equation 3. Clinical acid base disturbances Allacid basedisturbancesoccurasaresultofeitherinappropriate concentrations of CO 2 or HCO 3 in the blood (see Equation 3). Thus, there are four “simple” types of acid base disturbances. They are shown in Table 1. These are termed “simple” because most acid base disturbances are more complicated. In particular, the body uses the duality of HCO 3 and pCO 2 effects on pH to try to maintain acid base homeostasis. IfpHisabnormalduetoeither metabolic acidosis or alkalosis, HH C O H C O C OH O + 32 3 2 2  Equation 2 . Proton buffering by the bicarbonate buffer system. CO 2 is rapidly excreted through the lungs. pH HCO pCO  61 003 3 2 . log( [] . ) Equation 3 . Henderson Hasselbach equation. Blood pH is determined by the relative concentrations of HCO 3 and CO 2 . Blood HCO 3 concentrations are determined by acid and/or alkali intake, cellular acid production rates and renal HCO 3 transport. Blood pCO 2 is determined by pulmonary respiration. Arterial pH Abnormality Name of disorder Low ( # 7.35) Low HCO 3 Metabolic acidosis High pCO 2 Respiratory acidosis High ( $ 7.45) High HCO 3 Metabolic alkalosis Low pCO 2 Respiratory alkalosis Table 1. The four simple acid base disturbances.
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Acid-base - Acidbasehomeostasis I.DavidWeiner,M.D. 3746102

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