Acid-base regulation Flashcards
Give 2 reasons why the regulation of H+ is more complex and tighter than for other ions.
H+ alters protein activity, especially enzymes
H+ alters binding of other ions e.g. a low H+ increases Ca2+ binding to albumin
What are the 3 main mechanisms to minimise changes in pH?
Buffer systems
Respiratory regulation
Renal regulation
By what mechanism does respiratory regulation help minimise changes in pH?
Adjustment of CO2 levels
What is a buffer?
Any substance that can reversibly bind H+
What are the 3 main buffer systems?
Bicarbonate buffer system
Phosphate buffer system
Protein buffer system
Where does the bicarbonate buffer system take place?
Extracellular
Where does the phosphate buffer system take place?
Intracellular and in urine
Where does the protein buffer system take place?
Mainly intracellular
What is the chemical reaction of the bicarbonate buffer system?
H+ + HCO3- = H2CO3 (carbonic acid) = CO2 + H2O
What is the chemical reaction of the phosphate buffer system?
HPO42- + H+ = H2PO4-
What is the chemical reaction of the protein buffer system?
Pr- + H+ = HPr
Is urine usually acidic or alkaline? Why?
Acidic – due to excretion of H+ from non-volatile acid production
What are the 2 main processes by which the kidneys regulate extracellular fluid pH?
Reabsorption of filtered HCO3-
Excretion of H+
How many mmol of HCO3- to the kidneys filter each day?
4500 mmol
Describe how HCO3- is reabsorbed in the PCT.
It cannot be absorbed directly. H+ is secreted into the tubule lumen by a Na+/H+ antiporter. The H+ combines with bicarbonate to form carbonic acid (H2CO3). Carbonic anhydrase then converts carbonic acid into CO2 and H2O. The CO2 diffuses across the cell membrane into the PCT cell, where it combines with H2O to form carbonic acid, which then dissociates into H+ and HCO3-. The HCO3- is transported across the basolateral membrane into the bloodstream via Na+/HCO3- co transporter. The H+ is secreted back into the tubule lumen, continuing the cycle
The secretion of H+ in the late distal tubule and collecting ducts occurs through the action of which cells?
Type A intercalated cells
Describe how H+ is secreted into the late distal tubule and collecting tubules.
H+ ATPase and H+/K+ATPase to pump H+ into the tubule lumen against its concentration gradient
What are the 2 main urinary buffers?
Phosphate
Ammonia
In urine, phosphate exists in which two forms?
Monohydrogen phosphate (HPO42-)
Dihydrogen phosphate (H2PO4-)
In the urinary phosphate buffer system, H+ is secreted with phosphate and what else?
Sodium
Draw the formula for the urinary ammonia buffer.
NH3 + H+ = NH4+
Ammonium is synthesised in the PCT from what?
Glutamine
How does a decrease in pH affect the urinary ammonia buffer system?
It causes the kidneys to metabolise more glutamine, resulting in higher levels of ammonia, which can combine with H+ to be excreted, therefore reducing the acidity of the blood
What 4 things stimulate H+ secretion by the kidneys?
Increase in pCO2 of the extracellular fluid
A decreased pH of the extracellular fluid
Increased aldosterone levels
Hypokalaemia
What is respiratory acidosis?
Low pH due to increased CO2
Over what general time frame and how is respiratory acidosis compensated for?
Slowly (days) by the kidneys to increase the production of bicarbonate
What is metabolic acidosis?
Low pH due to decreased HCO3-
What are the 3 main mechanisms which cause metabolic acidosis?
Addition of exogenous acid (e.g. methanol)
Addition of endogenous acid (e.g. lactic or keto acids)
Failure of H+ excretion or loss of HCO3- (e.g. severe prolonged diarrhoea)
Over what general time frame and how is metabolic acidosis compensated for?
Rapidly by the lungs to increase ventilation and thus decrease CO2
