Oxygen Transport Blood Buffers  Elmhurst College
Carbon Dioxide Transport Respiratory Acidosis Metabolic Acidosis  Chemistry Department
Buffers in the Kidneys Respiratory Alkalosis Metabolic Alkalosis  Virtual ChemBook

Buffers in the Kidneys

.HYDROGEN PHOSPHATE KIDNEY BUFFER

ACIDIFICATION OF URINE

The pH of blood plasma is kept within normal limits by controlling the
excretion of H+ ions in the urine and the reabsorption of bicarbonate into blood plasma. If acid is excreted in the urine, its is in effect removed from the blood when an equal quantity of bicarbonate is added to the blood. Bicarbonate (as a base) neutralizes hydrogen ions in the blood. If the blood is too acidic more hydrogen ions are excreted, if the blood is too basic, then less hydrogen ions are excreted.

HCO3- + H+ <===> H2CO3 <===> CO2 + H2O

The renal tubules excrete hydrogen ions by an unknown series of reactions into
the tubular urine. The amount of hydrogen ions excreted is controlled by the
concentration of H+ (pH), bicarbonate, and the partial pressure of CO2 (pCO2)
in the blood plasma and by the amount of Na+ and bicarbonate in the developing urine.

Hydrogen ions and sodium ions exchange places throughout the formation of urine. For every H+ which enters the urine, one sodium ion is reabsorbed from the urine into the blood and is conserved. For every H+ ion excreted and every Na+ ion conserved, one bicarbonate ion is also reabsorbed into the blood. The charges on
sodium and bicarbonate are thus always balanced.

 

QUES. 4: If the pH of the blood is lower than normal, explain how the kidneys attempt to raise the pH of the blood.



PHOSPHATE BUFFER IN URINE

Normally phosphate is the only buffer in urine, although carbonic acid/ bicarbonate is also present. The developing urine contains NaH2PO4/Na2HPO4 in the same concentration as present in blood plasma. Na2HPO4 is actually the "salt" in the following dissociation reaction:

H2PO4- <===> H+ + HPO4 -2

In the developing urine the ratio of H2PO4-/HPO4 -2 is l : 4, therefore the right side of the equilibrium is favored.

When the urine is acidified (hydrogen ions added), the increase in H+ ion causes the equilibrium to shift left to form H2PO4-.

QUES. 5: Explain how one half of the sodium ions are conserved, ultimately in the blood by the addition of hydrogen ions to the urine. The urine starts out with mostly Na2(HPO4) present (Hint: Look at the charge on the two types of phosphate ions. How many sodium ions are needed to balance them?