Acid-Base II: Lungs and Kidneys

Question 1

Is NEAP regulated?

Answer 1

No, but RNAE is! The kidneys regulate RNAE to match NEAP and replenish the bicarbonate lost by nonvolatile acid neutralization.

Question 2

How much do titratable acids vs. NH4+ account for RNAE?

Answer 2

1/3 TAs and 2/3 NH4+

Question 3

Most of the H+ secreted into tubular fluid serves to…?

Answer 3

reabsorb the filtered load of bicarb. H+ combines with HCO3- in tubular fluid to form carbonic acid, which rapidly dissociates into H2O and CO2 via CA. H2O and CO2 diffuse into PT cells, CA rxn occurs in reverse to yield H+ and HCO3-, and HCO3- is then sent out of the cell and into the blood with sodium via the NBC1 transporter.

Question 4

What is the difference between CA-II and CA-IV?

Answer 4

• CA-II is localized to the cytoplasm of cells along the nephron
• CA-IV is membrane-bound (to apical membrane) and exposed to the tubular fluid

Question 5

Does acidosis or alkalosis stimulate RNAE?

Answer 5

acidosis (obviously, lol)

Question 6

What is the implication of proximal tubule cells having a high permeability to H+ and HCO3-?

Answer 6

tubular pH is not lowered very much (only to ~6.5)

Question 7

What is the implication of collecting duct cells having a low permeability to H+?

Answer 7

The tubular fluid can become very acidic because there is no way for H+ to leave; however, this is critical for the excretion of titratable acids!

Question 8

The kidneys’ response to acid-base dynamics involves ______ changes in transporter activity/numbers in the membrane and _______ changes in expression levels.

Answer 8

immediate; long-term

Question 9

Many of the changes in cellular H+ transport are regulated by hormones and other factors, including…

Answer 9

endothelin, cortisol, angiotensin II, and PTH

Question 10

True or false: There is net secretion of protons in the proximal tubule.

Answer 10

False! There is NO net secretion of H+ in the PT because the same protons are used and reused to reabsorb the filtered bicarbonate ions.

Question 11

How can the proton concentration gradient generated be 1000-fold that of plasma?

Answer 11

The proton pumps are so strong and the renal tight junctions are so impermeable. This is why urinary pH can range from 7.4-4.4 in humans.

Question 12

The 3 forms of protons in the urine are:

Answer 12

1. Free protons (which govern urinary pH)
2. Titratable acids (ex: phosphate, creatinine)
3. Diffusion-trapped ions (ex: NH4+)

Question 13

Free protons in the urine derive from what?

Answer 13

strong acids, like sulfuric acid

Question 14

Which acids are classified as titratable acids?

Answer 14

only weak acids with pKa’s within the range of urinary pH (4.4-7.4)

Question 15

What happens to ammonium if it is not excreted into the urine and enters the circulation?

Answer 15

it will be converted to urea in the liver

Question 16

How is ammonium produced?

Answer 16

via the metabolism of glutamine, which yields 2 bicarb and 2 ammonium (ammoniagenesis)

Question 17

What is the primary site of reabsorption of ammonium? Which part of the nephron is responsible for the secretion of ammonium into the tubular fluid?

Answer 17

thick ascending limb; collecting duct

Question 18

What is diffusion trapping of NH4+?

Answer 18

It refers to NH4+ remaining in the tubular lumen because it has a low permeability to the apical membrane.

Question 19

What are Rhesus glycoproteins?

Answer 19

They are membrane transporters that collecting duct cells express, and they specifically transport NH4+.

Question 20

What happens to ammonium if H+ secretion is inhibited?

Answer 20

The NH4+ reabsorbed by the TAL in the LOH will not be excreted in the urine. Instead, it will return to systemic circulation and be converted to urea by the liver, which will consume HCO3-.