Acid-base balance I

Question 1

what is the most important physiological buffer system

Answer 1

the “CO2 - HCO3” buffer system

Question 2

describe the role of a buffer system

Answer 2

regulates the pH of a solution by either binding to H+ and reducing pH or dissociating from H+ and increasing pH

Question 3

what is [HCO3] controlled by and what is pCO2 controlled by

Answer 3

[HCO3] = kidneys
pCO2 = lungs

Question 4

describe the 2 roles of the kidney in control of [HCO3]

Answer 4

variable reabsorption of filtered HCO3 and thee kidneys can add HCO3 to the blood

Question 5

what are the roles of the kidney in controlling [HCO3] dependant on

Answer 5

H+ secretion into the tubules

Question 6

what is the function of carbonic anhydrase

Answer 6

is catalyses the interconversion of H2CO3 from CO2 and H2O

ie can work both ways

Question 7

is HCO3 able to be reabsorbed at the apical membrane

Answer 7

no, not directly

Question 8

describe how HCO3 is able to be moved across the apical membrane, from the filtrate into the cell

Answer 8

H+ is secreted into the tubule, which binds HCO3 to make H2CO3, this can then form H2 and CO2 which are able to move across the apical membrane

Question 9

describe how HCO3 is formed in the tubular cells

Answer 9

CO2 and H2O are reabsorbed into the cell, these then form H2CO3(catalysed by carbonic anhydrase), which forms HCO3 and H+

Question 10

describe what happens to the HCO3 and H+ formed in the tubular cells

Answer 10

HCO3 = reabsorbed into interstitial fluid(before blood) via Na+ co-transport
H+ = recycled by secretion into the filtrate to form more H2CO3

Question 11

describe the normal ratio between the number of HCO3 ions being filtered and the number of H+ secreted

Answer 11

normally more H+ secreted than HCO3 filtered to make sure all HCO3 reabsorbed

Question 12

describe what happens when the [HCO3] in the tubular fluid is low

Answer 12

H+ binds the next most plentiful buffer in the filtrate, which is phosphate

Question 13

describe what happens when H+ binds phosphate in the tubular filtrate

Answer 13

forms H2PO4(an acid), which is then excreted

Question 14

describe the net loss/gain when H+ binds to phosphate in the tubular fluid

Answer 14

acid(H+) excreted and net gain of HCO3 into the blood

Question 15

describe what titratable acid is

Answer 15

this is when the amount of H+ excreted(largely as H2PO4) can be measured, done by measuring the amount of strong base needed to return urine to pH 7.4

Question 16

what is the maximum amount of titratable acid that can be excreted a day

Answer 16

40mmol/day

NB - an equal amount of HCO3 would have been reabsorbed into the blood

Question 17

what is used as a buffer when the maximum amount of titratable acid is reached
(ie in acidosis)

Answer 17

ammonia(NH3), combines with H+ to form NH4(ammonium) and then excreted
(NH4 is NOT measured by titratable acid)

Question 18

describe the net gain/loss when ammonia is used as a tubular buffer

Answer 18

net loss of acid(H+ binding NH3) and net gain of HCO3

Question 19

for every H+ secreted how many HCO3 are reabsorbed

Answer 19

for every one H+, one HCO3 is reabsorbed