Acid-base balance Flashcards
Why must H+ concentration be tightly regulated in the body?
Metabolic reactions are highly sensitive to pH. If pH gets too high you get alkalosis and if it gets too low you get acidosis.
How is H+ concentration regulated?
Through ventilation and renal clearance.
Whats the fast response to increased H+?
Via the carbonic acid buffer system, H+ gets converted into CO2 - this is then detected by central chemoreceptors, increasing ventilation.
Whats the slower response to increased H+?
The kidneys filter and secrete H+ and bicarbonate in urine.
Carbonic anydrase is found in a range of tissues like the lungs and kidneys - what does it do?
Carbonic anhydras is an enzyme that catalyses the inter-conversion of CO2 and H2O with the dissociated ions of carbonic acid.
Whats total blood CO2?
Total blood CO2 is the sum of the 3 ways in which CO2 is transported around the body - dissolved, bound to haemoglobin and as bicarbonate ions (through reaction with water and catalysed by carbonic anhydrase).
As most CO2 transported around the body is in the form of bicarbonate, what happens when it reaches the lungs?
The bicarbonate ions are converted back to CO2 by carbonic anhydrase. As the blood co2 is greater than alveolar co2 there is net diffusion of co2 into the alveolus where it can be exhaled.
What is hypoventilation?
This is when ventilation is inadequate and doesnt allow normal elimination of CO2. This means arterial co2 increases (hypercapnia) and H+ increases causing respiratory acidosis.
What is hyperventilation?
This is when you exhale more than you inhale. Consequently you get lower arterial PCO2 and lower H+ concentration causing respiratory alkalosis.
When CO2 levels rise, both peripheral and central chemoreceptors increase ventilation. How does this happen?
Central chemoreceptors are stimulated by an increase in brain extracellular H+ caused by increased brain PCO2.
Peripheral chemoreceptors respond to an increase in arterial H+ concentration due to increased PCO2.
What if the change in pH is not due to CO2?
The H+ ions will primarily activate the peripheral chemoreceptors as H+ ions do not easily cross the blood brain barrier.
If there is a decrease in H+ there will be depression of the peripheral chemoreceptor firing.
How do the kidneys regulate H+ concentration?
By altering plasma HCO3- levels.
Bicarbonate is completely filtered at the renal corpuscles. Significant amounts are then reabsorbed in the PCT, loop of Henle and cortical collecting duct. There is also some further secretion in the collecting duct.
Carbonic anhydrase is a family of enzymes with different isoforms. What are these?
Intracellular enzymes - e.g red blood cells and kidney epithelial cells lining the tubules.
Extracellular enzymes - e.g found in the brush border surface of epithelial cells in kidney.
In tubular epithelial cells, CO2 and H2O are converted to H2CO3 then to HCO£- and H+. What happens to these ions?
The hydrogen ions diffuse into the tubular lumen and react with filtered bicarbonate ions to form carbon dioxide and water.
The bicarbonate ions diffuses into the interstitial fluid through facilitated diffusion.
What is required for the diffusion of hydrogen ions into the lumen?
- H+ ATPase
- H+/K+ ATPase
- NA+/H+ antiporter.
