Acid-base regulation

Question 1

Kidneys and bicarbonate

Answer 1

Reabsorb filtered load of bicarbonate

Regenrate plamsa bicarbonate to buffer NVA

Question 2

Production of acids

Answer 2

CO2 = volatile acid, excreted by the lungs
NVA = non volatile acids, produced in metabolism and need to be buffered in the blood, buffered by HCO3- from other reactions

Question 3

Buffering of NVA

Answer 3

React with bicarbonate in blood
Bicarbonate + H+ -> carbonic acid which dissociates
Depletion of bicarbonate from the extracellular fluid
Kidneys need to regenerate bicarbonate
CO2 exits at lungs

Question 4

Acid secretion by kidney

Answer 4

H+ secreted into lumen, equivalent to NVA production
and also recover filtered bicarbonate (70 mmol/L)
Used in PCT to reabsorb bicarbonate
Some H+ is excreted, mostly through reaction with urinary buffers
both acid excretion and recovering HCO3- require H+ secretion

Question 5

Process within epithelial cells (PCT)

Answer 5

Hydration of CO2 by CA and then dissociation
H+ across apical into lumen, react with bicarbonate
HCO3- from dissociation can go across basolateral back into blood
Allows for reabsorption and regeneration of bicarbonate

Question 6

HCO3 - recovery in CD

Answer 6

Regeneration of bicarbonate and excretion of NVA
Not sodium coupled
H+ move across apical and HCO3- across basolateral
H+ doesn’t combine with bicarbonate in lumen, combines with another urinary buffer
HCO3- across basolateral with Cl- exchanger

Question 7

Other urinary buffers

Answer 7

H+ can be buffered and secreted with other buffers
Phosphate: already been reabsorbed by kidney
Ammonia: synthesised in PCT
HCO3- is regenerated if H+ reacts int he lumen with these
HCO3- is reabsorbed if H+ reacts with filtered HCO3-

Question 8

Synthesis of NH3

Answer 8

From glutamine in PCT
Glutamine -> glutamic acid and then -> alpha ketoglutarate
Both steps produce NH3
- ammonia then combines with H+ in lumen
- alpha KG uses in anion secretion

Question 9

Regulation of H+ secretion

Answer 9

Decrease in plasma pH, can also up regulate through insertion of transporters

• increase in blood CO2
• increase NHE in PT
• increase H+ ATPase in collecting duct

Question 10

Regulation of HCO3- secretion

Answer 10

Can use type B intercalated cells of collecting duct, secrete HCO3- directly into lumen, used in alkalosis

• few in number
• loss of bicarb

Question 11

Regulation with K+

Answer 11

Plasma pH affected by K+
Hyperkalaemia -> acidosis
Hyperkalaemia = inhibit ammonia production
Acidosis = less K+ excretion in CD, more remains in body/fluids

Question 12

NVA titration

Answer 12

titration of NVA (70 mmol/L) by HCO3- produces the sodium salts of strong acids and depletes 70 mmol/L of bicarbonate from extracelluialr fluid
needs to be regenerated!

Question 13

roles of different nephron segments

Answer 13

glomerulus = passive filtration
PT = 80% bicarobonate recovery, minimal acid elimination, ammoniogenesis
loop = 15% bicarbonate recovery
DT, CD = residual bicarb recovery, acid elimination

Question 14

acid secretion process

Answer 14

CO2 hydration produces bicarb and H+
H+ diffuses into lumen across apical
bicarbonate diffuses across basolateral into intersititum
used to both reabsorb and regenerate bicarbonate

Question 15

reabsorb filtered bicarbonate

Answer 15

in PCT
H+ secreted into lumen across apical, membrane has H ATPase and Na-H exchanger
H+ combines with bicarb, reacts and dissociates in lumen, diffuse into cell, react together again, bicarb can exit on basolateral

Question 16

replenish lost HCO3-

Answer 16

occurs in later regions, mostly CD where filtered bicarb has been absorbed
same CA reaction occurs, H+ into lumen and HCO3- across basolateral
H+ combines with buffer and is stuck in lumen
not sodium coupled
H+ secreted by type A intercalated cells

Question 17

H+ secretion importance

Answer 17

H+ secreted is equivalent to NVA buffered in tissues
excreted with urinary buffers
- phosphate or ammonia