Urinary: Acid Base Balance Flashcards Preview

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Flashcards in Urinary: Acid Base Balance Deck (19)
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1
Q

What is the normal range of plasma pH?

A

7.35 - 7.45

2
Q

What is the consequence of alkalaemia?

A

Lowers free calcium by causing Ca2+ to come out of solution, which increases neuronal excitability and therefore leads to tetany.

More dangerous than acidaemia because tetany can affect resp muscles or cause laryngeal spasm

3
Q

What is the consequence of acidaemia?

A

Increases plasma potassium concentration which can cause life threatening arrhythmias

Increased H+ can denature proteins

4
Q

What is the Henderson-Hasselbach equation?

A

CO2 + H20 H+ + HCO3-

Provides a way of calculating pH
pK (6.1) + Log ([HCO3-] / pCO2 x 0.23))

As long as 20:1 ratio of bicarb to co2 is maintained pH will be 7.4

5
Q

How does a change in ventilation affect pH?

A

Hypoventilation leads to hypercapnia which causes a fall in pH = respiratory acidaemia

Hyperventilation leads to hypocapnia which causes a rise in pH = respiratory alkalaemia

6
Q

What is the role of chemoreceptors?

A

Central chemoreceptors change ventilation rate to correct respiratory disturbances to pCO2. This is a slow response but produces a large effect.

Peripheral chemoreceptors detect changes in pCO2 AND pH of plasma - they respond rapidly but overall have a smaller effect.
(if pH change is due to bicarbonate, this will only be detected by peripheral chemoreceptors)

7
Q

How can the kidney compensate for respiratory changes in pH?

A

Respiratory acidosis = kidneys reabsorb more HCO3-

Respiratory alkalosis = kidneys reabsorb less HCO3-

8
Q

How do changes in plasma HCO3- affect pH?

A

Acid produced at metabolising tissues reacts with HCO3- to produce CO2 which is blown off at the lungs. This causes a fall in HCO3- therefore a fall in pH = metabolic acidosis (and visa versa)

9
Q

How is metabolic acidosis compensated for?

A

Compensated for by increasing ventilation to blow off CO2

10
Q

How is metabolic alkalosis compensated for?

A

Compensated for by decreasing ventilation to increase CO2

However metabolic acidosis can only be partially compensated for because hypoventilation will cause hypoxia

11
Q

How do the kidneys correct respiratory changes to pH?

A

The kidneys can vary excretion of HCO3- and if required make more HCO3-

12
Q

How do the kidneys make HCO3-?

generally

A

The kidneys have a high metabolic rate and produce lots of CO2 which reacts with water to produce HCO3- and H+

  • -> the HCO3- enters the plasma
  • -> the H+ enters the urine

Kidneys also make HCO3- from AA

13
Q

Outline the recovery of HCO3- in the kidneys

eg ion channels

A

80% of the reabsorption occurs in the PCT
It is driven by the Na+ gradient established by Na-K-ATPase
.
Na-H exchanger pumps H+ into the lumen which reacts with bicarb to produce CO2 and water which diffuse freely into the tubular cell.
They then react to form HCO3- and H+ (the H+ is pumped out again by NHE) and the HCO3- is pumped out the basolateral membrane by HCO3- -Na+ cotransporter

14
Q

Outline the creation of HCO3- in the kidneys

eg ion channels

A

In the PCT:
Glutamine in the tubule cell is broken down into alpha ketoglutarate and NH4+. The NH4+ dissociates and the NH3 and H+ freely diffuse out for excretion into urine (combine back to NH4+ which cant diffuse across membrane so is trapped for excretion. The HCO3- is transported across the basolateral by HCO3- Na+ cotransporter.

In the DCT:
By now all of the HCO3- is usually recovered, so HCO3- has to be created. It is important to secrete H+ ions so HCO3- can keep being produced (from CO2 and H20). H+ is pumped out of apical and buffered by filtered HPO4+ or excreted NH3+

15
Q

How does a reduction in pH (acidosis) affect ion channels in the kidney?

A
  • enhances Na H exchanger
  • enhances ammonium production in PCT
  • enhances H+ ATPase in distal tubule
  • incfreases capacity to export HCO3- from tubular cells to ECF
16
Q

What is the anion gap?

A

Metabolically produced acids (lactic acid, keto acid etc) have an anion associated with them

The anion gap is the difference between [Na] + [K} and [Cl] + [HCO3]

H+ reacts with HCO3- to produce CO2 which is breathed out. Therefore the gap increases if HCO3- is replaced by an anion from the acids

Indicated HCO3- replaced by another anion other than Cl-

17
Q

How does the conc of HCO3- change after persistent vomiting?

A

HCO3- can increase but is very easy to correct because it can be excreted very rapidly.

Except if there is also volume depletion because the capacity to loose HCO3- is reduced due to high rate of Na+ recovery.

18
Q

How do changes in pH affect plasma K+ levels?

A

Metabolic acidosis = hyperkalaemia
This is because H+ is taken up by the cell to correct pH in return for K+

Metabolic alkalosis = hypokalaemia
H+ moves out of cells in exchange for K+

19
Q

How can you distinguish between the different types of metabolic acidosis?

A

Look at the anion gap:
Normal anion gap = loss of HCO3-
Large anion gap = production of other organic acids eg diabetic ketoacidosis

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