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Acid Base Balance II (Kidney)

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Jenny Janke

Acid Base Balance II—Renal I. Intro a. Typical American Diet—produces 15,000 mmol CO2 per day i. Renal excretion of acid! b. Vegetarian Diet—produces alkaline urine b/c of excess “ash” in diet c. Definitions i. Acid—proton donor ii. Base—proton acceptor iii. Strong acid—fully dissociates into H+ & A- at physiological pH iv. Weak acid—doesn’t fully dissociate => kidney can recombine into HA v. Titratable Acid—conjugate base of weak acid that acts as buffer in urine vi. Titratable acidity—amount of NaOH that must be added to a 24 hr. collection of urine to titrate pH from 7.4 to 4.5; measure of the amount of acid excreted by kidney in form of weak acid d. Kidney Function i. HCO3- reabsorption (~90% of 4300 mEq/day HCO3 reabsorbed) ii. Removes excess acid via net H+ excretion in the form of titratable acid & ammonium => generation of more HCO3- (~100mEq/day new HCO3-) iii. Net HCO3- excretion following consumption of excess base (base load) II. Acid Producing Diet a. Body buffers minimize ↓pH due to ↑ H+; kidneys recycle HCO3 & excrete H+ b. Tubular Acidification i. 90% of HCO3 (4300mEq/day) absorbed in PCT => slight ↓ pH ii. lowest pH occurs in collecting ducts, where H+ is actively secreted iii. min pH = 4.5-5.0 iv. excess H+ secreted as weak acids (titrated) and ammonium c. Bicarbonate Reabsorption i. Early Proximal Tubule—high capacity, reabsorbs most filtered HCO3 1. HCO3 from lumen moves into cells; IC carbonic anhydrase converts CO2 ↔ H+ + HCO3 a. H+ secreted to tubule lumen b. HCO3 reabsorbed w/ Na+ w/o changing luminal pH ii. Late Proximal Tubule—high capacity, generates new bicarbonate 1. CO2 from blood moves into cells, where carbonic anhydrase breaks it down => NEW HCO3!! a. H+ secreted to tubule lumen, where it combines w/ weak acids (AW-) => HAW b. HCO3 reabsorbed w/ Na+ 2. small drop in luminal pH: 7.4 → 6.8 iii. Collecting Duct—low capacity, but can generate large pH gradients 1. proton pumps secrete H+ into lumen 2. large drop in luminal pH: 6.8 → 4.5 3. CO2 from blood continues to move into cells, where carbonic anhydrase breaks it down => NEW HCO3!! to blood stream d. Ammonia (NH4+) 1 i. Strong acids are broken down and excreted as ammonia (NH4+) and A- NH3 + H+
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