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renal > Acid-base regulation L13 > Flashcards

Acid-base regulation L13 Flashcards

1
Q

Define pH.

A
  • log[H+]
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2
Q

what is normal arterial pH?

A

7.4

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3
Q

what is normal venous pH?

A

7.35

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4
Q

Do we measure blood pH from the arterial or venous circulation?

A

arterial as venous can be more variable depending on the tissue bed the blood is coming from (different substances in venous blood)

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5
Q

What is arterial HCO3- concentration?

A

24mM

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6
Q

What is venous HCO3- concentration?

A

25mM

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7
Q

why is venous HCO3- concentration slightly higher than arterial?

A

CO2 produced in tissue bed which reacts with water to form HCO3- (and H+)

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8
Q

Normal arterial Pco2 in mmHg and kPa?

A

40mmHg, 5kPa

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9
Q

Normal venous Pco2?

A

46mmHg

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10
Q

What is the Henderson-Hasselbach equation

A

pH = pK + log[base]/[acid]

where pK = rate constant

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11
Q

What is the normal value of pK (rate constant)?

A

6.1

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12
Q

What does the HH equation tell us about the relationship between HCO3- concentration and pH?

A

proportional - increase in HCO3- leads to increase in pH

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13
Q

what is the relationship between CO2 concentration and pH?

A

inversely proportional - increase in CO2 leads to decrease in pH

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14
Q

What are some of the ways that H+ can be produced in the body?

A
  • hydrolisation of ATP
  • production of ketones (high in diabetes)
  • ingestion of acids
  • production of lactate
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15
Q

How is H+ formed in anaerobic respiration?

A

glucose > lactic acid > dissociation into lactate and H+

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16
Q

How is excess H+ removed from the body?

A

reacts with HCO3- to form CO2 which can be exhaled *however this decreases circulating HCO3- = problem!

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17
Q

HCO3- reabsorption occurs in which part of the nephron?

A

proximal tubule *look back to previous lectures

18
Q

Is HCO3- reabsorption Tm limited?

A

yes - therefore an excess of HCO3- will not be reabsorbed > can quickly correct HCO3- concentration

19
Q

What is the kidneys response to CO2 entering through the vasa recta rather than the filtrate?

A
  • this suggests an excess of H+ in blood which reacts with HCO3- to produce CO2. This results in a lower concentration of HCO3- in the blood.
  • the kidney responds by ‘de novo’ HCO3- production i.e. production of new HCO3- within the kidney
  • this replaces HCO3- lost elsewhere in the body
  • H+ in circulation is also buffered by HPO42- to stop it from reacting with HCO3- to form more CO2 (and H2O)
20
Q

Describe new HCO3- production in the kidney.

A
  • CO2 enters epithelial cells of proximal tubule from interstitium (from vasa recta)
  • CO2 combines with H2O to form HCO3- and H+ using CA.
  • H+ is removed through Na+/H+ pump to stop it reacting
  • HCO3- moves into interstitium through 3HCO3-/Na+ co-transporter
  • increase in HCO3- in body
21
Q

How is H+ secreted into the distal tubule?

A

primary active transport:

  • H+ ATPase
  • H+K+ ATPase

*occur in alpha-intercalated cells of distal tubule

22
Q

How are excess H+ ions buffered in the filtrate?

A
  • No HCO3- to react with H+ as most is reabsorbed in proximal tubule
  • H+ buffered using hydrogen phosphate instead
  • helps to keep urinary H+ low (urine is not too acidic)
23
Q

State the equation for the reaction between H+ and hydrogen phosphate.

A

H+ + HPO4^2- H2PO4-

24
Q

At blood plasma, what is the predominate form of hydrogen phosphate present in solution?

A

higher pH (7.4) so HPO4^2- predominates as less H+ reacting with it

25
Q

In acidic urine, what is the predominate form of hydrogen phosphate present in solution?

A

lower pH (5) so H2PO4- predominates as excess H+ reacts with HPO4^2- –> buffering the pH.

*can maintain an only slightly acidic pH urine as H+ ions are removed by buffer, don’t need to have very low urinary pH to remove H+ ions.

26
Q

In extreme acidosis, what mechanism can the kidney use to get rid of H+ from the body?

A
  • in PT, glutamine converted into glutamic acid and then alpha-ketoglutarate
  • at each conversion stage, NH4+ is produced.
  • in cell, NH4+ is in equilibrium with NH3
  • NH3 can cross apical membrane into filtrate
  • the H+ ions (from NH4+ > NH3 + H+) pass into filtrate through Na+/H+ antiporter
  • NH4+ reforms in filtrate
  • this mechanism helps to remove some of excess H+

*this is not the main mechanism and only happens in extreme acidosis

27
Q

What is the urinary pH at the end of the proximal tubule?

A
  1. 9

* later down the nephron pH can be variable depending on how much acid is in the body, can get as low as 4.5

28
Q

What is the typical cause of respiratory acidosis?

A

hypoventilation

29
Q

Describe and explain the presentation of the blood for respiratory acidosis?

A
  • low pH and high HCO3-
  • low pH due to decreased respiratory rate > high CO2 in circulation
  • high CO2 leads to high H+, reaction favours:

CO2 + H2O –> HCO3- + H+

Kidney produces MORE HCO3- in compensation, thereby returning pH towards normal.

30
Q

What is the typical cause of respiratory alkalosis?

A

hyperventilation, high altitude

31
Q

Describe and explain the presentation of the blood for respiratory alkalosis?

A
  • high pH and low HCO3-
  • high pH due to increase in respiratory rate > low CO2 in circulation
  • low CO2 leads to low H+, reaction favours:

H+ + HCO3- –> CO2 + H2O

Kidney decreases production or recovery of HCO3- in compensation, thereby returning pH towards normal.

32
Q

What are typical causes of metabolic acidosis?

A

renal failure, lactic acidosis, ketoacidosis, poisoning (e.g. aspirin) *many many more!

33
Q

Describe and explain the presentation of the blood for metabolic acidosis?

A
  • low pH and low HCO3-
  • low pH due to increase in H+ in circulation
  • increase in H+ leads to decrease in HCO3-, reaction favours:

H+ + HCO3- –> CO2 + H2O

*can be vice versa i.e. decrease in HCO3- leads to increase in H+ > same result, low pH

H2O + CO2 –> H+ + HCO3-

CNS increases ventilation rate in compensation > decrease CO2, returning pH to normal

34
Q

What are typical causes of metabolic alkalosis?

A

vomiting, contraction alkalosis (increase in pH due to fluid losses)

35
Q

Describe and explain the presentation of the blood for metabolic alkalosis?

A
  • high pH and high HCO3-
  • high pH due to decrease in H+ in circulation
  • decrease in H+ leads to increase in HCO3-, reaction favours:

H2O + CO2 –> H+ + HCO3-

*can be vice versa i.e. increase in HCO3- leads to decrease in H+ > same result, high pH

H+ + HCO3- –> CO2 + H2O

CNS decreases ventilation rate in compensation > increase CO2, returning pH to normal

36
Q

What is the anion gap?

A

The measured difference between positive (cations) and negative (anions) charges in our bodies. The difference is due to anions that are not analysed in testing. There is a normal range for the anion gap and if the value is higher than the normal range (in serum), this can suggest metabolic acidosis.

37
Q

What is anion gap usually measured as?

A

[Na+] - [Cl-] - [HCO3-]

38
Q

What is the normal reference range for the anion gap?

A

3-11mmol/L *although this tends to vary between laboratories

39
Q

A higher-than-normal anion gap can be due to a high concentration of ions that are not being tested in sample. Which anions might they be?

A
  • lactate - anaerobic metabolism - lactic acidosis
  • ketones - diabetes or alcohol toxicity
  • sulfates, phosphates, urate and hippurate - renal failure
  • aspirin overdoes

*therefore causes of metabolic acidosis can change the anion gap

40
Q

NOTE

A

look at davenport diagrams on handout

renal (16 decks)

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