Acid Base Balance

Question 1

What is the most important type of PH buffering in the circulation?

Answer 1

HCO3 mediated buffering

H+ + HCO3- <=> H2CO3 <=> H2O + CO2

H+ + HCO3- <=> H2O + CO2
(lungs and kidney controls this)

Question 2

What is the Henderson-Hasselbalch equation?

Answer 2

PH =pKa + log10(base)/(acid)

Question 3

When does net hydrogen ion production occur?

Answer 3

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

Question 4

What happens to the metabolically produced H+?

Answer 4

• has to be removed from the body

- occurs by reaction with HCO3- producing CO2 (exhaled) (this results in the loss of HCO3-)

Question 5

How do we counteract the loss of HCO3-?

Answer 5

• the kidney reabsorbs the filtered HCO3-

- it also produces more to replace the losses occurring in the body

Question 6

Where does HCO3- absorption take place?

Answer 6

in the proximal tubule of the kidney

Question 7

How are HCO3- ions absorbed?

Answer 7

• H+ ions are transported out of the cell by the NaH transporter and enter the filtrate
• HCO3- ions react with H+ ions in the filtrate producing water and carbon dioxide (carbonic anhydrase)
• The carbon dioxide can pass into the epithelial cells
• CO2 reacts with water (carbonic anhydrase) to form H+ + HCO3-
• Bicarbonate travels by a transporter to re-enter the body

Question 8

How do we ensure that all filtered bicarbonate is reabsorbed but excess bicarbonate isn’t reabsorbed?

Answer 8

-proximal tubule reabsorption of HCO3 is tubular maximum limited and this limit depends on the H+ in the proximal tubule which in turn relies on the Na+/H+ exchanger

Question 9

How is new HCO3 produced?

Answer 9

• CO2 arrives from the body and moves into the epithelial cells
• It can react with H2O to produce H+ and HCO3- using the enzyme carbonic anhydrase

Question 10

How does distal tubule H+ secretion occur?

Answer 10

• primary active transport is the dominant mechanism for H+ secretion this is through apical:
• H+K+ ATPase
• H+ ATPase
• this occurs in the α-intercalated cells

Question 11

How is the filtrate buffered after the excess H+ ions enter it?

Answer 11

buffering by hydrogen phosphate

Question 12

Look at different forms of phosphate

Answer 12

on slides

Question 13

What is another way that H+ secretion into the filtrate can be balance?

Answer 13

• The ammonium ion (NH4+) is produced in the proximal tubule by conversion of glutamine to glutamic acid and α-ketoglutarate
• NH4+ is in equilibrium with NH3, which being small and uncharged is membrane permeable
• NH4+ reforms in the filtrate lumens acting as another reservoir for H+ (see diagram)

Question 14

What are the key acid-base problems?

Answer 14

respiratory acidosis

• respiratory alkalosis
• metabolic acidosis
• metabolic alkalosis

Question 15

Look at davenport diagram

Answer 15

on slides

Question 16

What happens in respiratory acidosis?

Answer 16

• caused by hypoventilation
• CO2 increased so H+ increases
• to compensate the kidney increases the production of HCO3, returning PH towards normal

Question 17

What happens in metabolic acidosis?

Answer 17

• usually caused by renal failure, lactic acidosis, ketoacidosis, poisoning
• H+ increases so HCO3- decreases (or HCO3- decreases so H+ increases)
• to compensate the CNS increases the ventilation rate decreasing CO2 returning the PH to normal