Kidney role in Acid/base bal

Question 1

How does HCO3- prevent the continuous build up of 60meq of nonvolatile acid in the body?

Answer 1

“the twist”

• You need high [ ]s of HCO3-
• it converts nonvolatile(nongaseous) acid to a gaseous volatile form that can be ELIMINATED from ECF via respiration (CO2)
  (it acts more than just a buffer)

Question 2

Where does most of the recapture of HCO3- take place?

Answer 2

Prox tubule - and is obligatory

Question 3

Describe the process of how HCO3- is continuous replaced while maintaining electoneutrality?

Answer 3

1. H+ is moved to lumen via H+/Na+ antiporter
2. H+ binds to HCO3- → H2CO3
3. H2CO3 is broken down via CA to H2O + CO2
4. CO2 is transported into the cell via highly permeable membrane
5. CO2 binds to H20 in the cell via CA→ H2CO3
6. H2CO3 is broken down to H+ and HCO3-
7. HCO3- is transferred out of the cell into the serosa via Na+/HCO3+ cotransporter

Question 4

Which cells make HCO3-?

Answer 4

Intercalated cells (secrete H+ and HCO3-) in distal tubule

Question 5

How do you not piss out acid with all of the H+ being moved to the lumen? (via Na/H+ antiporter and H+ pump?)

Answer 5

2 main buffering mechs:

1. titratable acid
2. ammonia trapping

Question 6

titratable acid buffering mech

Answer 6

complexing H+ ion to a filtered anion (HPO42-)

Question 7

Ammonia trapping buffering mech

Answer 7

1. tubular cells break ↓ glutamine → ammonia (NH3)
2. ammonia (NH3) readily diffuses through the apical membrane into tubule
3. in Lumen, NH3 has very high affinity for H+ → NH4 +

(ammonia is now “trapped” with buffered H+)
- can be up/downregulated depending on H+ secretion

Question 8

During metabolic acidosis, what happens to ammonia (NH3)?

Answer 8

Glutaminase is upregulated → more NH3 is made to buffer the increased secretion of H+

Question 9

Is the process of HCO3- reabsorption neutral in terms of acid/base terms? What about Synthesis of HCO3-?

Answer 9

R: is neutral

S: not neutral - results in the elimination of acid in the urine

(note that R is a much larger process than S)

Question 10

How is HCO3- R and secretion similar?

Answer 10

They both require apical secretion of H+ and basolateral movement of HCO3- into the ECF

Question 11

Which takes priority, HCO3- reabsorption or Secretion?

Answer 11

reabsorption: its also a much larger process

Question 12

What competitively inhibits HCO3- synthesis?

Answer 12

When the H+ in the lumen is used to convert HCO3- to CO2 + H2O.
This causes CO2 to go into the cell and ultimately result in H+ and HCO3- forming inside the cell.

This will inhibit the entry of CO2 from the serosa/blood and will compete for secretory and HCO3- extrusion transporters required for synth pathway.

Question 13

Can synthesis take place with HCO3- in the tubular fluid?

Answer 13

No, only R will take place.
HCO3- synthesis can only occur after all the filtered load has been reabsorbed

• usually only cells in the very distal parts of the tubule will end up synth HCO3-

Question 14

What is the rate limiting for HCO3- homeostasis? What does this rate depend on?

Answer 14

Apical secretion of H+ and basolateral extrusion of bicarb

- rates depends on ECF pH and CO2 levels (they determine the # of transporters in the apical and basolateral membrane)

Question 15

During metabolic imbalances, what two organ systems work together to deal with the threat?

Answer 15

Kidney and lungs

Question 16

In the short term, how do the kidneys respond to respiratory acidosis? Long term?

Answer 16

Kidney responds to respiratory acidosis by increasing their excretion of H+ ions to counteract it.

Eventually . . . compensation is limited by increased filtered load of HCO3- that results
(thus in prolonged resp acidosis, one sees a partial renal compensation with elevated HCO3-)

Question 17

Hypokalemia will cause in alkalosis or acidosis?

Answer 17

Alkalosis

Hypokalemia induced shift of H+ into tubular cells results in inappropriately increased H+ secretion and urinary excretion → alkalosis

*the opp is also true, alkalosis leads to increased K+ and H+ excretion → hypokalemia

Question 18

Hyperkalemia will induce acidosis or alkalosis?

Answer 18

acidosis:

hyperkalemia reduces the rate of H+ secretion and subsequent excretion, and H+ is inappropriately retained in ECF