Acid-Base Excretion

Question 1

How is blood pH buffered in the short term?

Answer 1

Blood pH is buffered in the short term by buffers in the red blood cells and the respiratory system.

Blood buffers: Oxyhaemoglobin and carbonic acid (also phosphoric acid, etc.).

Respiration: Respiratory centres in the brain that respond to changes in PCO2 & blood pH induce deeper or shallower breathing to moderate expiration of carbon dioxide.

Question 2

How is blood pH buffered in the longer term?

Answer 2

Kidney excretion of acids/bases (bicarbonate)

Question 3

What is the physiological plasma concentration of bicarbonate in a normal healthy animal?

Answer 3

22 to 28 mMol

Question 4

What is meant by the alkali reserve?

Answer 4

In animals, blood concentration of bicarbonate is in the normal range of 22 to 28 mM and must be maintained. The HCO3- represents the conjugate-base component of the carbonic acid-bicarbonate buffer system.

It is lost through respiration through the dissociation of carbonic acid when red blood cells reach the lungs under high oxygen tension:

H⁺ +HCO₃^-⇆ H₂CO₃⇆ CO₂ +H₂O

The bicarbonate that would normally be dissociated into by the carbonic acid instead isn’t made in the lungs and is instead depleted as the carbonic acid instead is broken down to water and carbon dioxide, which are lost during respiration.

Thus, for every proton excreted as part of water during respiratory regulation of pH, one bicarbonate is also lost. The kidney then has to take over to replace the alkali reserve.

Question 5

How does the kidney maintain the alkali reserve?

Answer 5

Reabsorption of bicarbonate from the proximal tubules:

All bicarbonate filtered through the glomerulus is reabsorbed in the proximal tubules. The PCT cells’ brush border reabsorbs bicarbonate into the blood, maintaining alkali reserve.

Acid is excreted via neutral water in urine, so urine isn’t acidic. This mechanism is so efficient that all the bicarbonate that passes into the glomerular filtrate has been re-absorbed by the time it reaches the distal tubules. When all the bicarbonate is reabsorbed, this mechanism cannot be used to excrete any more acid.

Question 6

How is acid removed in the distal convoluted tubules? What buffer is used?

Answer 6

Protons bind to HPO₄^2- in the DCT lumen

H₂PO₄^- is formed, binds with Na+ to form NaH₂PO₄

This is excreted, making urine ACIDIC

Question 7

How does ammonia help the kidneys excrete acid?

Answer 7

Even though ammonia is a strong base, acid is excreted in this mechanism.

This is important in prolonged acidosis, after reabsorption of bicarbonate from the proximal tubules and excretion of acid from distal tubules.

The steps are similar to the reabsorption of bicarbonate and excretion of acid up to the point that carbonic acid dissociates into bicarbonate and hydrogen ion in the distal-tubule cell. Ammonia gas NH_3(g) generated from metabolism, diffuses easily across the cell membrane into the lumen and is used to excrete acid as it combines with the dissociated proton immediately to form ammonium ion NH₄⁺. Since it’s charged, it can’t get back across into the cell and is thus excreted in the urine with an anion.