Bicarbonate Flashcards
What is the principal buffer of ECF and blood?
Bicarbonate
What is the chemical equation relating to acid base balance and bicarbonate inn the body?
CO1 + H2O H2CO3 H+ + HCO3-
How is pH calculated (write out the equation)?
pH ~ HCO3-/pCO2
What happens to bicarbonate levels in a metabolic acidemia?
Fall in HCO3-
For every bicarbonate ion secreted into the gut below the pylorus what is secreted into the bloodstream?
H+ ion secreted into bloodstream
For every H+ ion secreted into the stomach above the pylorus what is secreted into the bloodstream?
HCO3- secreted into bloodstream
Overall what is substance is secreted into the bloodstream more from the gut the bicarbonate or protons?
Acid (so if nothing were to happen i.e kidneys did not exist the body would become more acidic)
What are the 4 ways by which the kidney controls bicarbonate?
- Bicarbonate filtration
- Bicarbonate reabsorption
- Bicarbonate regeneration by:
Titratable acid excretion
Ammonium excretion
What is the pH at the start of the proximal tubule?
~ 7.4
What is the GFR?
180 litres/day
What is the pH at the thick descending limb?
6.9
What is the pH at the collecting duct?
4.5
What is the normal plasma bicarbonate?
24 mmol/l
Where is 85 - 90% of filtered bicarbonate reabsorbed?
Proximal convoluted tubule
Where is the remaining 10 - 15% of bicarbonate reabsorbed?
Distal convoluted tibule and collecting tubule
How does bicarbonate reach the blood?
- CO2 diffuses from the tubular lumen into the tubular cells
- Carbonic anhydrase converts CO2 converts to CO2 to bicarbonate
- Bicarbonate then diffuses into blood
How is H+ excreted from the cell into the tubular lumen and what happens to it?
- Inward Na+ drives H+ out (co-transporter)
- H+ can then through carbonic anhydrase bind with HCO3- to create CO2 and H2O
- CO2 can then diffuse into tubular cells
What maximises potasium inside the cell and Na+ outwith?
Na+/K+ATPase on the basolateral membrane
Where is carbonic anhydrase present?
In the proximal tubular lumen not in the distal
What is bicarbonate exchanged for moving into the blood from the tubular cells?
Cl- ion
What are the 2 processes by which bicarbonate is regenerated and what is more important?
- Titratable acid excretion (accounts for ~ 1/3)
- Ammonium excretion (more significant)
What is the main source of phsophate in the body?
Dietary (processed meat, pheasant)
At what percentage is most of the phosphate present at a pH of 7.4?
80%
What are some of the buffers in the urine?
- Phosphate
- Urate
- Creatinine
- Beta-hydroxybutyrate
What buffer is produced in large quantities in diabetic ketoacidosis?
Beta-hydroxybutyrate
What is the main buffer of protons in the urine?
Phosphate
How much can ammonium excretion increase in the face of an acid load?
10x
What amino acid does the renal tubule cell take up more of?
Glutamine
What is glutamine broken down into?
- Alpha-keto-glutarate then HCO3-
- NH4+
Process uses up protons
How is ammonium excreted?
Na+ facilitated secondary active transport
What bicarbonate regeneration mechanism can inincrease markedly?
- Ammonium excretion
- NOT titratable acid excretion as it is dependant on diet so remains relatively constant
How is respiratory acidaemia managed by the kidney?
- Rise in pCO2
- Parallel change inside the renal tubule cells
- Intracellular acidaemia
- Increases uptake and use of glutamine and hence ammonium excretion
- Increasing bicarbonate regeneration
- Low intracellular pH increases tubular proton secretion and optimum reabsorption of bicarbonate
What is an example of a respiratory alkalaemia?
- Panic attack
- Pulmonary embolism
- High altitude (chronic)
How is respiratory alkalaemia managed by the kidneys?
- Fall in pCO2
- Parallel change inside the renal tubule cells
- Intrcellular rise in pH
- Proton secretion falls
- Bicarbonate reabsorption falls
What increases bicarbonate reabsorption and regeneration?
- Increasing pCO2
- Increasing H+
- Decreasing ECF volume
- Increasing angiotensin II
- Increasing aldosterone
- Hypokalaemia
What decreases bicarbonate reabsorption and regeneration?
- Decreasing pCO2
- Decreasing H+
- Increasing ECF volume
- Decreasing angiotensin II
- Decreasing aldosterone
- Hyperkalaemia
What is the function of carbonic anhydrase tablets which can be given at extreme altitudes?
Increases loss of bicarbonate and respiratory rate and tidal volume
What happens to ammonium secretion by the tubule if kidney function is impaired?
Decreased (results in metabolic acaedemia)
What are the 3 types of renal tubule acidosis?
- Type 1 (distal RTA)
- Type 2 (Proximal RTA)
- Type 4 (Hyperkalaemic RTA)
What is happens in type 1 (distal RTA)?
- Lowers capacity to conc. H+ in distal tubule
- pH above 5.5
- Plasma K+ usually low
- Normal GFR
- Bicarbonate < 10 mmol/l
What is happens in type 2 (proximal RTA)?
- Impairs prox reabsorption of bicarbonate
- Urine pH is variable
- Plasma K+ usually low
- GFR normal
- Plasma bicarb 15-20mmol/l
What is happens in type 4 (Hyperkalaemic RTA)?
- Caused by reduced aldosterone. Inhibits NH4+ production
- Urine pH below 5.5
- Plasma K+ is high
- Plasma bicarb 15-20 mmol/l
What type of renal tubule acidosis is most common in children?
Type 2 (Proximal RTA)
What type of renal tubule acidosis is most common in adults?
Type 4 (Hyperkalaemic RTA)
What is the rarest type of RTA?
Type 1 (Distal RTA)
How long does it take for maximal adaptation after a significant acid-base insult?
5-6 days
