Kidney function in maintaining acid-base balance

Maintaining a constant pH is closely related to maintaining the concentration ratio of conjugated acids and bases of plasma buffers, especially bicarbonate buffer. Plasma concentration of conjugated acid (H 2 CO 3, i.e. CO 2 ) is regulated by controlling pCO 2 by changes in pulmonary ventilation. The role of the kidneys is therefore to maintain the concentration of the conjugate base HCO 3 − .

Under normal circumstances, more bicarbonate is returned from the kidneys than is supplied to them by the a. renalis. This is due to the kidney's ability to produce urine with a low pH (4.5-5.0).

In addition, this amount serves to neutralize non-carbonic acids that were created by metabolism or are from food. In addition, by supplying bicarbonate, the kidneys prevent the decrease of buffering bases.

Bicarbonate is freely bufferable in the glomeruli – if it were to be excreted in the urine, its reserves would soon be depleted. Therefore, it is intensively resorbed.

The kidneys therefore maintain acid-base balance (hereafter referred to as ABR) by two mechanisms:


 * 1) by regulating the reabsorption of bicarbonate ;
 * 2) by excreting 40–80 mmol H + per day – not excreted freely (urine pH would not correspond to this) – buffered by urinary buffers and bound to NH 3.


 * The kidneys carry out these two processes by secreting H + into the tubular fluid, where bicarbonate is formed from CO 2 and water in the tubular cells.



Tubular transport mechanism

 * Bicarbonate resorption is coupled with the reaction (catalyzed by carbonic anhydrase ):


 * H 2 O + CO 2  H 2 CO 3  [H] + + [HCO 3 ] − .


 * Formation of H + : either according to the above-mentioned reaction, or the water molecule is divided into:


 * H + and OH −
 * While OH − reacts with CO 2 to form bicarbonate (carbonic anhydrase). It follows from both possibilities that the amount of formation and secretion of H + depends on the activity of carbonic anhydrase and pCO 2.


 * The proximal tubule reabsorbs up to 90% of the bicarbonate and reduces the pH of the tubular fluid. H + go into the lumen against the electrochemical gradient secondarily by active transport by the Na + -H + transporter (sodium from the lumen into the cell, in exchange for H + ).
 * Bicarbonate diffuses into the peritubular fluid.
 * Balance of the entire process: secretion of H + into the lumen – acidification of the tubular fluid, return of Na + and HCO 3 − ions to the blood and thus also to ECT.
 * Bicarbonate is additionally resorbed in the ascending limb of the loop of Henle, in the distal tubule and the collecting duct.
 * What does this imply? As a result of resorption in the urine, the concentration of bicarbonate gradually decreases, and as a result of secretion, the concentration of H + increases.

Regulation of bicarbonate resorption

 * Resorption is not constant - it rises when there is a lack of bicarbonate in the body and vice versa.
 * Resorption is affected by:
 * The amount of HCO 3 − in the tubules and thus the luminal pH:
 * with a change in concentration, resorption changes (in the same direction),
 * as the pH of the tubular fluid increases, the secretion of H + increases ,
 * Peritubular blood pH :
 * the resorbed proportion of bicarbonate rises with a fall and falls with a rise in peritubular blood pH (opposite direction).
 * Changes in ECT volume:
 * Hypovolemia (reduced ECT content) causes an effort in the kidneys to retain Na + (its concentration in the urine decreases and, in order to maintain electroneutrality, the same amount of Cl − as the main anion of the glomerular filtrate must be resorbed . Once the concentration of Na + in the filtrate is, for example, 140 mmol/ la Cl −  115 mmol/l, only 115 mmol/l Na + can be resorbed. The other moles must be resorbed together with the secretion of H + and or K + .)
 * Changes in plasma concentration of K + :
 * Reciprocal relationship between plasma K + concentration and bicarbonate resorption.
 * Aldosterone :
 * stimulates the distal secretion of H + + stimulates the proton pump.

Acid secretion

 * Secreted H + is responsible for the resorption of bicarbonate – when more H + is added to the urine, more bicarbonate is formed in the tubular cells, which moves into the blood (for every 1 mmol of H + that reacts with a non- bicarbonate buffer in the tubular fluid, in the tubular cell will form 1 mmol of bicarbonate).
 * The most important non-bicarbonate buffers in the tubular fluid: phosphate buffer, NH 3 , creatinine.
 * An important mechanism for the excretion of H + is the production of NH 3 (the ability of tubular cells to increase ammonia production is the main adaptive response to acidosis ).
 * NH 3 easily diffuses through the membrane, it is soluble in fats, the ionized form is also in water (however, it diffuses less through the membranes – when it forms in the lumen, it cannot get out).
 * It arises in the tubular cell mainly from glutamine.
 * The main factor regulating the movement of NH 3 into the lumen is the pH of the tubular fluid.

Related Articles

 * Kidney (histological preparation)
 * Kidneys
 * Nephron
 * Blood flow through the kidneys and its autoregulation
 * Acid-base balance
 * Acidosis
 * Alkalosis