Acid-Base Regulation

Question 1

What is the definition of pH?

Answer 1

-log10[H+]

Question 2

What is the HCO3- buffering equation?

Answer 2

HCO3- + H+ –> H2O + CO2

Question 3

What are the normal values for pH, HCO3- and PCO2 in arterial blood?

Answer 3

pH = 7.4
HCO3- = 24mM
pCO2 = 40mmHg (5.3kPa)

Question 4

What are the normal values for pH, HCO3- and PCO2 in venous blood?

Answer 4

pH = 7.35 (tissue dependent, very variable)
HCO3- = 25mM
pCO2 = 46mmHg

Question 5

What is the henderson-hasselbach equation/

Answer 5

pH = pK + log(base/acid)
pH = pK + log(HCO3-/H2CO3

Question 6

In what situations does net H+ production occur?

Answer 6

ATP hydrolysed
During anaerobic respiration, with the production of lactate
During the production of ketones
During ingestion of acids

Question 7

How is H+ removed from the body?

Answer 7

Reaction with HCO3- producing CO2 which is exhaled. However, this results in the loss of HCO3-

Question 8

How does the kidney compensate for the loss of HCO3- in H+ excretion?

Answer 8

reabsorption of filtered HCO3- and the production of new to replace the losses occurring elsewhere in the body

Question 9

Draw HCO3- absorption in the proximal tubule

Answer 9

see lecture notes

Question 10

• Proximal tubule reabsorption of HCO3 has a …………. ………….. (Tm), but this limit depends on …. in the proximal tubule, which is turn relies on the ………… …………….. .
• This ………. limited process means that almost all of the filtered bicarbonate is reabsorbed under resting condition, but that an excess of …. will not be reabsorbed. This is a method of rapidly correcting HCO3 excess
• If the source of CO2 is from the ……. ……., rather than the filtrate, the acts a mechanism for de novo ……. production, replacing losses elsewhere in the body. In this case, luminal H+ is buffered by ……

Answer 10

tubular maximum, H+, Na+/H+ exchanger
transport, HCO3-
vasa recta, HCO3-, HPO4^2-

Question 11

Describe the process of distal tubule H+ secretion

Why is this needed?

Answer 11

In the distal tubule, primary active transport is the dominant mechanism for H+ secretion. This is through apical H+K=ATPase and H+ATPase
This process occurs in the alpha-intercalated cells in the distal tubule
This is needed if CO2 is in XS and the kidneys are trying to prevent an acidosis

Question 12

How is additional H+ buffered, why is this necessary?

Answer 12

Necessary as very acidic urine would be painful to excrete
XS H+ needs to be buffered in the filtrate keep the free urine H+ conc low. A key mechanism is through buffering by hydrogen phosphate.
H+ + HPO4^2- H2PO4-

Question 13

Which phosphate molecule predominant in the blood?

Answer 13

HPO4^2-

Question 14

Describe ammonia secretion

Answer 14

the ammonium ion (NH4+) is produce in the proximal tubule by conversion of glutamine to glutamic acid and alpha-ketoglutarate
NH4+ is in equilibrium with NH3 which, being small and uncharged, is membrane permeable
NH4+ reforms in the filtrate lumen, acting as another reservoir for H+

glutamine –> glutamic acid –> a-ketoglutarate

Question 15

How does the pH change along the nephron?

Answer 15

by the end of the proximal tubule, the pH has fallen to about 6.9, but by the end of the tubule the ph is highly variable (depending on the body’s acid load), down to about 4.5

Question 16

What is the cause of respiratory acidosis?

Answer 16

hypoventilation

Question 17

What are the causes of respiratory alkalosis?

Answer 17

hyperventilation, high altitude

Question 18

Describe the compensation that occurs in a respiratory acidosis

Answer 18

CO2 high so H+ goes up

To compensate, the kidney increases the production of HCO3-, returning the pH towards normal

Question 19

Describe the compensation that occurs in a respiratory alkalosis

Answer 19

CO2 low, so H+ does down

To compensate, the kidney decreases the production oor recovery of HCO3-, retiring the pH towards normal

Question 20

What are the causes of metabolic acidosis?

Answer 20

renal failure, lactic acidosis, ketoacidosis, aspirin poisoning

Question 21

What are the causes of metabolic alkalosis?

Answer 21

vomiting, contraction alkalosis

Question 22

Describe the compensation that occurs in a metabolic acidosis

Answer 22

H+ goes up, so HCO3- goes down
or HCO3- goes down so H+ goes up
To compensate, the CNS increases the ventilation rate, decreasing CO2, retiring the pH towards normal

Question 23

Describe the compensation that occurs in a metabolic alkalosis

Answer 23

H+ goes down, so HCO3- goes up
or HCO3- goes up, so H+ goes down
To compensate, the CNS decreases the ventilation rate, increasing the CO2, retiring the pH towards normal

Question 24

Shade on the four pH disturbances of a graph of ph against HCO3-

Answer 24

see lecture notes

Question 25

What is the anion gap?

Answer 25

The sum of positive and negative charges in our bodies has to be equal. However, f we look at the difference between the commonly measured cations and anions, we find that they don’t add up to be equal.
The anion gap is usually measured as Na+ - Cl- - HCO3-

Question 26

What is the normal anion gap and when might it change?

Answer 26

The reference range for the normal anion gap varies between labs but is typically 3-11mmol
NA increase in the anion gap suggests that there is a high concentration of anions that are not being counted. These might be:
- lactate (anaerobic respiration)
- ketones (in diabetes or alcohol toxicity)
- sulphates, phosphates, urate, hippurate (renal failure)
- aspirin overdose
So, several causes of metabolic acidosis change the anion gap