7. Bicarbonate Flashcards
What is the principal buffer of the ECF and blood?
Bicarbonate
pH is in proportion to…
HCO3-/pCO2
What is the equation that regulates the acid/base balance in cells?
Which components of this equation are:
a. Regulated by the resp system
b. Added to the body by diet and metabolism
c. Regulated by the kidneys and lost in faeces
CO2 + H2O H2CO3 H+ + HCO3-
a. Regulated by the resp system = CO2
b. Added to the body by diet and metabolism = H+
c. Regulated by the kidneys and lost in faeces = HCO3-
4 action of the kidney on bicarbonate?
Bicarbonate filtration Bicarbonate reabsorption Bicarbonate regeneration by: 1. Titratable acid excretion 2. Ammonium excretion
What happens to the pH of the tubule contents as it passes through nephron>?
Increases in acidity
i.e. pH drops
Where are the points of bicarbonate reabsorption?
Proximal convoluted tubule (85-90%)
Remaining 10-15% in distal convoluted tubule and collecting tubule
What is the process of HCO3- reabsorption in the distal and proximal convoluted tubule?
- H+ is secreted into the tubular lumen.
CO2+ H2O HCO3 + H+ catalysed by carbonic anhydrase - The HCO3 can then be absorbed into the blood.
Difference between proximal and distal HCO3 reabsorption?
Proximal:
Secondary active transport of H+ out of cell and Na+ in at apical cell. Steep inward directed Na gradient.
When HCO3 leaves the cell into the blood it does so via Na co-transporter. 3 x HCO3 with 1 x NA
Distal:
H+ leaves cell into lumen via ATPase transporter (primary)
HCO3 leaves cell into blood via Cl- anti porter. 1 x HCO3 with 1 x Cl-
Why must bicarbonate be regenerated?
As protons consume bicarbonate in the buffered system
What is the process of titratable acid excretion?
The H+ leaving the cell into the tubular lumen is buffered by non-bicarbonate buffers.
Non-bicarbonate buffers:
- Phosphate **
- Urate
- Creatinine
- Beta-hydroxybutyrate
Relatively constant.
Process of ammonium excretion?
Proximal tubule: Glutamine in the proximal tubule cells breaks down to ammonium and alpha-keto-glutarate.
The ammonium is excreted and the a-k-g is broken down the HCO3-.
Collecting tubule: H2O breaks down to H+ and OH-. The H+ is actively transported into lumen, combines with ammonium and forms ammonium for excretion. The OH- reacts with CO2, forming HCO3- for absorption
Whole process has potential to increase markedly
What is the renal compensation for resp academia?
- Rise in pCO2
- Parallel change inside the renal tubule cells
- Intracellular acidaemia
- Increases uptake & use of glutamine & hence ammonium excretion
- Increasing bicarbonate regeneration
- Low intracellular pH increases tubular proton secretion and ensures optimum reabsorption of bicarbonate
What is the renal compensation for resp alkalaemia?
- Fall in pCO2
- Parallel change inside the renal tubule cells
- Intracellular rise in pH
- Proton secretion falls
- Bicarbonate reabsorption falls
6 changes that result in increase in bicarb reabsorption and regeneration?
Increasing PCO2 Increased H+ Decreasing ECF volume Increasing angiotensin II Increasing aldosterone Hypokalaemia
6 changes that result in decrease in bicarb reabsorption and regeneration?
Decreasing PCO2 Decreased H+ Increasing ECF volume Decreasing angiotensin II Decreasing aldosterone Hyperkalaemia
