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We need to regulate pH as the function of enzymes and cell surface ion channels are affected by [H+].
– In addition, bone buffers [H+], so an increase in [H+] will demineralize bone
– An acidosis also may lead to a potassium extracellular shift causing hyperkalaemia.
The main way body pH is controlled is by buffering [H+] ions by the bicarbonate buffer system.
When [H+] ions are added, they are buffered by HCO3 ions and excreted by the lungs as CO2.
However, this is not sustainable unless the kidneys also produce HCO3 ions to replace the ones that are lost.
This is Henderson-Hasselbach equation:
– Partial pressure of carbon dioxide is controlled by alveolar ventilation, through chemoreceptors in the carotid and aortic bodies.
Renal control of pH
The kidneys are net producers of HCO3 ions and so all the freely filtered bicarbonate is reabsorbed.
– H+ is secreted into the PCT by NHE3.
– This drives a reaction to produce H2O + CO2 which diffuses into the cell.
– As the cell is more alkaline, the H2O + CO2 dissociate into ions H+ and HCO3–
– HCO3– transported out of cell and H+ secreted into tubule restarting the cycle.
– In the DCT, protons are secreted due to a H+/K+ ATPase, which exchanges H+ for K+
In addition to reabsorbing all the bicarbonate, the kidneys actively produce bicarbonate from the breakdown of glutamine.
– However, this would make the urine too acidic and so a buffer (not required by the body is needed)
– The main buffer in the body is inorganic phosphate which can be excreted in the urine
– In addition, the kidneys produce ammonia in the proximal tubule, allowing protons to be excreted as NH4
Much of the control over plasma pH is intrinsic, but it can be affected by some chemicals:
a) Low pH – this increases the expression of transporters, allowing increased H+ excretion
b) Cortisol (low pH) – increases transcription of NHE3 in the PCT
c) Angiotensin II – stimulates NHE3 in PCT
d) Aldosterone – stimulates the H+/K+ ATPase in DCT cells, increasing H+ secretion in exchange for potassium.
This is why both abnormalities in aldosterone (Conn’s Syndrome) and cortisol (Addison’s disease) lead to problems with pH and K+
