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Lecture

Acid Base Balance II (Kidney)

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Department
Biology
Course
BIOL 2P93
Professor
Jenny Janke
Semester
Fall

Description
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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