Lecture 9: Acid-base Balance

Question 1

Define pH

Answer 1

Measures free [H+]

pH = -log [H+]

Question 2

What is normal urine pH?

Answer 2

4.4 - 8

Question 3

Describe responses to pH changes

Answer 3

1) buffering =
Immediate, prevents large changes in pH

2) respiratory =
W/in minutes, cannot remove non-volatile acid or base and will not restore bicarbonate to normal

3) renal =
Max response w/in hours to days, alters bicarbonate and H+ excretion

Question 4

What are examples of major buffer systems?

Answer 4

CO2 and bicarbonate

Phosphoric acid

Ammonium

Histidine

Question 5

What is plasma pH?

Answer 5

Around 7.4

Question 6

Compare volatile and non-volatile acids

Answer 6

Volatile/respiratory acid =
Metabolism of CHO and FA’s = CO2, eliminated by lungs

Nonvolatile/fixed acids =
Metabolism of dietary amino acids results in acid that can only be excreted by kidneys and not eliminated by lungs

Question 7

How are fixed acids buffered?

What is a consequence of this?

Answer 7

Buffered by bicarbonate

HCO3 stores are depleted, so then kidneys must replenish

Question 8

What are titratable acids (TA)?

Answer 8

Filtered urinary buffers

Mostly phosphate but also some sulfate and creatinine

Question 9

List the approx. fractional reabsorption of bicarbonate in the PT, thick AL, and distal nephron

Answer 9

PT ~80-90%

Thick AL ~5-10%

DCT, CD ~5-7%

Question 10

What are limitations of phosphate as a urinary buffer?

Answer 10

Phosphate balance tightly controlled - Reabsorption inhibited by PTH

Limits the amount that can be lost in urine for acid-base balance

Urinary phosphate doesn’t increase much in acidosis

Question 11

Describe overview of renal ammonium excretion

Answer 11

Ammonium secreted by PT from glutamine metabolism

Accumulation in interstitium (medulla) creates a gradient/driving force

Collecting duct ammonia and acid secretion

Question 12

Describe the first step in ammonium excretion: PT secretion

Answer 12

Glutamine is transported into PT cells and metabolized

Forms alpha-KG (which eventually becomes bicarbonate) and ammonium

Bicarbonate (new) goes to blood

ammonium transported to lumen via NHE3 transporter

Question 13

Describe the second step of ammonium excretion - accumulation in interstitium

Answer 13

Majority of ammonium re-absorbed in thick AL = high [NH3] in medulla

Concentration gradient directed from interstitium to tubule

Transporters involved:
NKCC2

Question 14

Describe the third step in ammonium excretion - collecting duct NH3 and H+ secretion

Answer 14

ammonia moves across epithelium to lumen
Via RhCG and RhBG channels, as well as NH3 passive diffusion

H+ secretion via apical membrane H-K ATPase and