Acid Base Homeostasis

Question 1

Normal arterial blood pH range?

Answer 1

7.35 - 7.45

Question 2

Minimum and maximum ranges of pH that are compatible with human life?

Answer 2

6.7 to 7.9

Question 3

What does the plasma [H+] normally sit around?

Answer 3

40nmol/L

Question 4

What are the normal arterial and venous bicarb ranges?

Answer 4

Arterial 22-26mEq/L

Venous 19-25mEq/L

Question 5

How is bicarb reabsorbed into the PCT?

Answer 5

It can’t be freely absorbed.
Protons pumped out, react with bicarb in filtrate, form CO2 and H2O using carbonic anhydrase
CO2 can pass through cells and react with water and carbonic anhydrase, reform HCO3- and H+
HCO3- formed is pumped out of cell into interstitium.

Question 6

How can H+ enter the proximal convoluted tubule cell?

Answer 6

H+ ATPase

Sodium Proton antiporter

Question 7

How can HCO3- be pumped out of the proximal convoluted tubule cell?

Answer 7

Chloride-bicarbonate exchanger
Sodium-bicarbonate co-transporter
Chloride is then pumped back out of the cell by Cl- channels

Question 8

What is the henderson hasselback equation?

Answer 8

pH = pK + log10 ([HCO3-]/[CO2])

Question 9

Where are acid secreting cells found?

Answer 9

Intercalating cells of distal convoluted tubule, or collecting duct

Question 10

What happens in acid secreting cells?

Answer 10

H+ pumped into lumen.
H+ react with bicarb result in carbonic acid formation and breakdown.
CO2 passes into cell (formed by H+ + HCO3- -> H2O + CO2 by carbonic anhydrase) and this CO2 is used to form bicarbonate (re-absorbed) and H+ (acid excreted)

Question 11

How is HCO3- reabsorbed in acid secreting cells

Answer 11

Via AE1 transporters - chloride bicarbonate exchanger that allows Cl- into cell and this Cl- leaks out which can cotransport water

Question 12

What cells are bicarb secreting in kidneys?

Answer 12

Intercalating cells of distal convoluted tubule and collecting duct

Question 13

What happens in bicarb secreting cells?

Answer 13

CO2 and H2O combine into carbonic acid and dissociate into bicarbonate and acid
Bicarbonate secreted and acid reabsorbed

Question 14

What secretes bicarb and reabsorbs acid in bicarb secreting cells?

Answer 14

HCO3-/Cl- bicarbonate exchanger secrete bicarb

H+ ATPase reabsorbs acid

Question 15

How is bicarb generated when none is available in the filtrate - acid phosphate excretion?

Answer 15

CO2 + H2O with carbonic anhydrase makes HCO3- and H+
H+ secreted into filtrate (H+ ATPase), bind to phosphate to create acid phosphate
HCO3- reabsorbed through HCO3-/Cl- bicarb exchanger AE1

Question 16

How is bicarb generated when there is none available in the filtrate - ammonium salt excretion?

Answer 16

Glutamine split into bicarbonate and ammonium salts
Ammonium salt excreted using sodium ammonium antiports
Bicarb absorbed using AE1 and the Na+/K+ ATPase maintains the cytosolic concentrations

Question 17

What are the 3 buffering mechanisms of bicarbonate?

Answer 17

Phosphate buffer - mainly intracellular fluid buffering
Protein buffer
Bicarb buffer - mainly extracellular fluid buffering

Question 18

What are the limits of urine acidity and alkalinity?

Answer 18

Urine is a regulator so it has a lot wider of a normal pH range
5-8 pH

Question 19

Define respiratory acidosis

Answer 19

Hypoventilation
Reduced diffusion gradient for CO2
Greater PCO2 in post alveolar blood
Decreased pH and normal base excess

Question 20

Define respiratory alkalosis

Answer 20

Hyperventilation
Increased gradient for CO2
lower pCO2 in post alveolar blood
Increased pH and normal base excess

Question 21

Define metabolic acidosis

Answer 21

Diarrhoea or H+ gaining/bicarb losing
Increased dissociation of carbonic acid
pH reduction with normal PCO2 and low base excess

Question 22

Define metabolic alkalosis

Answer 22

Vomiting or H+ losing/bicarb gaining
Increased bicarbonate
high pH normal PCO2 and high base excess

Question 23

What is meant by respiratory compensation for acid base distubances

Answer 23

Increasing or decreasing ventilation rate to alter diffusion gradient and affect PCO2 in metabolic acidosis/alkalosis to compensate and normalise

Question 24

What is meant by renal compensation for acid base disturbances

Answer 24

Increasing or decreasing bicarbonate leading to altered base excess leading to more/less H+ due to shifting carbonic acid equilibrium and therefore stabilising pH