Urinary: Acid Base Balance

Question 1

What is the normal range of plasma pH?

Answer 1

7.35 - 7.45

Question 2

What is the consequence of alkalaemia?

Answer 2

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

Question 3

What is the consequence of acidaemia?

Answer 3

Increases plasma potassium concentration which can cause life threatening arrhythmias

Increased H+ can denature proteins

Question 4

What is the Henderson-Hasselbach equation?

Answer 4

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

Question 5

How does a change in ventilation affect pH?

Answer 5

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

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

Question 6

What is the role of chemoreceptors?

Answer 6

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)

Question 7

How can the kidney compensate for respiratory changes in pH?

Answer 7

Respiratory acidosis = kidneys reabsorb more HCO3-

Respiratory alkalosis = kidneys reabsorb less HCO3-

Question 8

How do changes in plasma HCO3- affect pH?

Answer 8

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)

Question 9

How is metabolic acidosis compensated for?

Answer 9

Compensated for by increasing ventilation to blow off CO2

Question 10

How is metabolic alkalosis compensated for?

Answer 10

Compensated for by decreasing ventilation to increase CO2

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

Question 11

How do the kidneys correct respiratory changes to pH?

Answer 11

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

Question 12

How do the kidneys make HCO3-?

generally

Answer 12

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

Question 13

Outline the recovery of HCO3- in the kidneys

eg ion channels

Answer 13

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

Question 14

Outline the creation of HCO3- in the kidneys

eg ion channels

Answer 14

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+

Question 15

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

Answer 15

• 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

Question 16

What is the anion gap?

Answer 16

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-

Question 17

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

Answer 17

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.

Question 18

How do changes in pH affect plasma K+ levels?

Answer 18

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+

Question 19

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

Answer 19

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