Acid-Base regulation Flashcards
(38 cards)
What is pH?
A measure of the concentration of H ions
pH = -log10[H]
What is the circulation buffered by?
HCO3
What are the arterial pH, Hco3 and CO2 levels?
pH = 7.4 HCO3 = 24 CO2 = 40
What are the venous pH, HCO3 and CO2 levels?
pH = 7.35 HCO3 = 25 CO2 = 46
Why is there a difference between arterial and venous values?
Venous has a higher CO2 as it is removing the waste produced by tissues. CO2 is converted to HCO3 + H so pH is more acidic and there is a higher concentration of HCO3
Why is arterial blood used for a blood gas sample?
Venous blood is variable depending on which capillary bed is used
What is the equation for pH?
pH = pk + log10 x [Hco3] / (0.03 x PCO2)
Hco3 is the base
0.03 x PCO2 can also be replaced by acidic H2CO3
Why is pH maintenance in the kidney required?
To regulate levels of HCO3 for buffering the blood.
What is H a waste product of?
Hydrolysis of ATP
Lactate production in anaerobic respiration
Production of ketones
Ingestion of acids
How is H removed from the body?
Reacts with Hco3 to produce CO2 which can be exhaled.
What is the issue with H removal and how does the body compensate for this?
Removal of H leads to a loss of Hco3 so the kidney must reabsorb Hco3 being filtered or produce new.
What is pH proportional to?
[Hco3] / [H2CO3]
If Hco3 increases, pH increases
How is Hco3 absorbed in the PT?
Using an Na/H exchanger, Na moves down its concentration gradient and produces energy to allow H to cross the apical membrane in the opposite direction.
H reacts with HCO3 in the filtrate to produce neutral CO2 and H2O that can cross the membrane. Once inside the cell the neutral substances redissociate back into H and Hco3 via CA. H is recycled.
Hco3 reabsorption across the basal membrane drives the reabsorption of Na.
What produces the Na gradient in a PT cell?
Na/K ATPase pumps Na out of the cell and K in. K is recycled back into the intersitium for continuous production of the gradien.
What is the limit of Hco3 absorption?
It is freely filtered but Tm limited at 25mM. Its level varies depending on the amount of H in the tubule. Excess HCo3 will then be excreted to correct the plasma levels.
What happens if there is a loss of HCo3 in the body?
CO2 will be sourced from the vasa recta rather than the filtrate for HCo3 production inside the PT cell to reabsorb HCo3 levels to normal.
Luminal H is buffered by HPO4 instead to regulate plasma pH until HCo3 levels rise again.
Why does new bicarb production not affect the pH?
Because H is produced alongside as CO2 and H2O dissociate, so there is no net change in pH, this is why HPO4 is required in the meantime.
What prevents rejoining of H and HCO3?
HCO3 is reabsorbed and H is secreted into the filtrate
What mechanism allows H secretion in the DT?
H secretion through primary active transport using apical H/K ATPase and H ATPase in the alpha intercalated cells of the DT. H is secreted into filtrate making urine acidic.
How is excess H in the filtrate buffered to prevent acidic urine?
HPO4(2-) buffers as there is no HCO3 left in the filtrate after reabsorption. Phosphate is excreted through the kidney making it readily available.
Where does filtrate buffering with HPO4 bring the pH?
pH = 6.8 as the buffering stops at the intermediate substance of H2PO4(-) as its pka value is the closest to normal pH, compared to H3PO4.
What does the HPO4(2-) : H2POa(-) ratio show?
Ratio also seen as [HPO4] / [H2PO4]
Normally HPO4 predominates so the ration is >1. Under normal conditions it equals 4.
When urine is acidic the ratio is <1 as H2PO4 dominates due to the increase in H buffering.
How is ammonium ion produced?
NH4+ is produced inside the PT cells by converting glutamine to glutamic acid and alphaKG.
What is the function of ammonium ion?
NH4+ is at equilibrium with membrane permeable NH3 so NH4+ can reform in the filtrate using the excess H to produce a reservoir of H to reduce the acidity under extremely acidic conditions.
