The Renal System and Acid-Base Balance

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Kidneys

Physically remove H+ from body

Excrete <100 mEq fixed acid per day

Also control excretion or retention of HCO3

If blood is acidic, then more H+ are excreted and all the HCO3 is retained, vice versa

While lungs can alter [CO2] in seconds, the kidneys require hours to days change HCO3  and affect pH.

Role of urinary buffers in excretion of excess H+

Once H+ has reacted with all the available HCO3, the excess reacts with phosphate and ammonia.

If all urinary buffers are consumed, further H+ filtration ends when pH falls to 4.5.

Activation of ammonia buffer system enhances Cl loss and HCO3 gain.

 

Even though the chemical buffer systems can inactivate excess acids and bases momentarily, they are unable to eliminate them from the body. 

Similarly, although the respiratory system can expel the volatile carbonic acid by eliminating CO2, it cannot expel other acids generated by cellular metabolism

Only the renal system can rid the body of acids such as phosphoric acids, uric acids, lactic acids, and ketone acids (also called fixed acids).

Only the renal system can regulate alkaline substances in the blood and restore chemical buffers that are used in managing H+ levels in extracellular fluids

Some HCO3, which helps to adjust H+ concentrations, is lost from the body when CO2 is expelled from the lungs.

When the extracellular fluids become acidic, the renal system retains HCO3 and excretes H+ ions into the urine: This causes the blood pH to increase. 

When the extracellular fluids become alkaline, the renal system retains H+ and excretes basic substances primarily HCO3 into the urine: This causes the blood pH to decrease

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