BIOL 1104 Lecture Notes - Carbonic Anhydrase, Base Load Power Plant, Titration

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11 Feb 2013
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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+)
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