Lecture 12: Regulation of Acid-Base Balance (Bolser)

Question 1

how much HCO3 (bicarb) is reabsorbed and where is most of it reabsorbed?

Answer 1

almost all bicarb (99.9%) is reabsorbed and most of its reabsorption is in the proximal tubule

(the proximal tubule is the 500 lb gorilla or bicarb reabsorption :-P)

Question 2

there are two other locations where bicarb is reabsorbed other than the 500 lb gorilla proximal tubule. what are the other two?

Answer 2

the thick ascending limb and collecting ducts

Question 3

how does reabsorption of bicarb occur?

Answer 3

it occurs when:
1. secreted H ions combine with filtered HCO3 in luminal fluid to produce CO2 and water which diffuse into the tubular cell

2. the CO2 and water in conjunction with CA (carbonic anhydrase) combine with H to form bicarb
3. the bicarb then moves across the basolateral membrane into the peritubular fluid via a Na-HCO3 co-transporter

Question 4

there are basically 3 things to remember regarding reabsoprtion of the filtered bicarb in the PROXIMAL TUBULE. what are they?

Answer 4

1. action of CA and H in the cell produces bicarb which is a substrate for Na passage across the basolateral limb - this is how HCO3 is reabsorbed
2. H is actively transported out of cell back into tubular fluid where it recombines to form water and CO2 with the action of CA
3. CA inhibitors are diuretics: they act to reduce the availability of H in the cell for exchange with Na. (more about this is on another card)

Question 5

CA inhibitors are diuretics. what is their function in regards to the reabsorption of filtered bicarb in the PROXIMAL TUBULE?

Answer 5

they act to reduce the availability of H in the cell for exchange with Na. as a result, more Na remains in the proximal tubule and it pulls water. this can also impair bicarb reabsorption in proximal tubule and kidneys ability to respond to metabolic acidosis

Question 6

what is important to remember about the mechanism for bicarb reabsorption in the DISTAL TUBULE and COLLECTING DUCT?

Answer 6

CA is in the cell, but NOT the tubular lumen.

H is actively transported into the lumen.

H is actively transported by another channel in exchange for K.

this results in acidic urine AND more bicarb reabsorbed.

Question 7

what type of cell is located in the distal tubule and collecting duct? what is its significance?

Answer 7

Type A intercalated cell

it its the cell that really responds to acidemia because of the multiple active transport channels secreting H+.

Question 8

which locations of the nephron are most important for the formation of titratable acid?

Answer 8

the distal tubule and collecting duct

Question 9

what happens when acidosis occurs and what is it called?

Answer 9

secreted H ions combine with non-HCO3 buffers (like H2PO4 and NH3) in the luminal fluid. it is a way of eliminating excess acid that doesn’t make pH of urine lower.

this is called diffusion trapping (or non-ionic diffusion of NH3)

Question 10

where is most of water reabsorbed?

Answer 10

CORTEX

Question 11

what happens in the proximal tubule during ammonia production?

Answer 11

1. glutamine is transported into the cell with Na via a Na co-transporter.
2. glutamine gets broken down into bicarb and ammonium.
3. bicarb is co-tranported with Na back into circulation (its reabsorbed).
4. ammonium is secreted back into the proximal tubule via a Na channel.

Question 12

what happens in the thick ascending limb during ammonia production?

Answer 12

1. ammonium is in equilibrium with NH3 (ammonia) and together they are reabsorbed into the medullary interstitum.
2. NH3 is easily permeable and moves freely across membranes, NH4 (ammonium) does not.
3. once in medullary interstitum, ammonia diffuses into the collecting duct lumen.

Question 13

what happens in the collecting duct during ammonia production?

Answer 13

1. ammonia diffuses into the collecting duct lumen
2. it combines with H+ to form ammonium which is impermeable. ammonium is now TRAPPED!

the ammonium buffers the H allowing its secretion into the tubular fluid without effecting pH.
this allows H ions to be excreted in effort to raise the pH.

Question 14

why does NH4 leave the thick ascending limb and effectively bypass the distal tubule?

Answer 14

because the thick ascending limb is the diluting segment.

if NH4 were to be transported back into the cortex into the distal tubule, it would make the tubular fluid more concentrated and reduce the kidney’s ability to reabsorb water.

bypassing the distal tubule allows the kidney to correct a low pH without compromising its ability to reabsorb water. BRILLIANT!

Question 15

NH3 is used for two things in the kidney - what are they?

Answer 15

1. help relieve acid load

2. contributes to medullary osmalality

Question 16

what does the proximal tubule do to maintain H balance?

Answer 16

1. reabsorbs most of the filtered bicarb

2. produces and secretes ammonium

Question 17

what does the thick ascending limb do to maintain H balance?

Answer 17

1. reabsorbs the second largest fraction of filtered bicarb

2. reabsorbs ammonium

Question 18

what does the distal tubule and collecting duct do to maintain H balance?

Answer 18

1. reabsorbs almost all remaining bicarb via type A intercalated cells
2. produces titratable acid (type A intercalated cells)
3. secretes bicarb (type B intercalated cells)

Question 19

look at the two flow charts in the SCAVMA notes to see how acidosis and alkalosis are defended

Answer 19

:-)