QS Study

The kidneys have two very vital roles in maintaining the acid-base balance: They reabsorb bicarbonate from urine. They excrete hydrogen ions into the urine. The kidneys also organize salt levels and the emission of urea. Water that is not put back into the blood is excreted in our urine.

Regulation of acid-base balance (H+ ion concentration).

A body has 3 primary lines of defense against changes in H+.

  • Acid-base buffer system.
  • Respiratory buffer system. [Which eliminates CO2 and therefore Carbonic acid (H2CO3)].
  • Kidney buffer system [Which can secrete H+, reabsorb HCO3 and produce new HCO3].

(1) Renal Control of Acid-base balance: The kidneys control acid-base balance by excreting either an acidic or a basic urine and 3 basic mechanisms are –

(i) Secretion of H+

(ii) Reabsorption of filtered HCO3

(iii) Production or news HCO3

Two buffer systems are playing their roles –

(i) Bicarbonate butter system (HCO3/PCO2)

(ii) NH3 and PO43- buffer System.

Fig: diagram depicts a cross-section of the left wall of a kidney proximal tubule

Effect of small arterial pressure

(1) A small increase in arterial pressure often because of a marked increase in urinary excretion of Na+ and water. These are pressure natriuresis and pressure diuresis. But arterial pressure between 75 – 160 mmHg, autoregulation occurs and there is no increase in urinary output,

(2) Increase renal arterial pressure – Increase peritubular capillary hydrostatic pressure – Increase renal interstitial fluid hydrostatic pressure – Backleak of Na+ into tubular lumen – Increase Na+ and H2O excretion – Increase urinary output.

(3) Increase renal arterial pressure – Decrease Angiotensin II secretion – Decrease Na+ and H2O reabsorption – Increase urinary output.

 

According to the Henderson-Hasselbalch equation –

pH = 6.1 + log [HCO3/ (0.03 x PCO2]

Therefore ↑ PCO2 →↓ pH.

Overall mechanism is –

Large numbers of HCO3 are filtered continuously into tubules. If they are excreted in the urine, this removes the base from the blood.

Large numbers of H+ are also secreted into the tubular lumen by the tubular epithelial cells, thus removing acid from the blood.

If more H+ is secreted than HCO3 are filtered, there will be a net loss of acid from the ECF.

Conversely, if more HCO3 is filtered than H+ is secreted, there will be a net loss of base.

  • The body produces 80 mEq nonvolatile acids per day.
  • The kidneys filter about 4320 mEq of HCO3 /day. The HCO3 must react with a secreted H+ to form H2CO3 before it can be reabsorbed.
  • So, 4320 + 80 = 4400 mEq H+ are required to be secreted in the tubular fluid.

In acidosis, when [H+] the extracellular fluid increases-

  • The kidneys do not excrete NCO3 in the urine,
  • Reabsorb all of the filtered HCO3 and,
  • Produce new HCO3, which is added back to the extracellular fluid.

All these reduce intracellular fluid [H+] toward normal.

In alkalosis, when [H+] in the extracellular fluid decreases –

The kidneys fail to reabsorb all of the filtered bicarbonate, thereby increasing excretion of HCO3

As HCO3 buffers H+ in the ECF, this loss of HCO3 is the same as adding an H+ to the extracellular fluid.

Thus in alkalosis, the removal of HCO3 raises the extracellular fluid (H+) and decreases pH back toward normal.

 

(2) Phosphate buffer system in renal tubule

As long as there are excess HCO3 in the tubular fluid, most of the secreted H+ combines with HCO3. However, once all the HCO3 has been reabsorbed and is no longer available to bind H+, any excess H+ can combine with HPO4.

After H+ combines with HPO4 to form H2HPO4, it can be excreted as a Na-salt (NaH2PO4), carrying with it the excess H+.

The HCO3generated in the tubular cell and enters the peritubular blood and represents a net gain of HCO3 by the blood, rather than merely a replacement of filtered HCO3.

Therefore, whenever an H+ secreted into the tubular lumen combines with a buffer other then HCO3, the net effect is an addition of a new HCO3 to the blood.

 

(3) Ammonia Buffer System:

In PCT, LOH, and DT, Glutamine (an amino acid) breaks down into 2NH4+ and 2HCO3

NH4+ is counter exchanged on the luminal membrane with Na+

HCO3 is transported along with Na+ into the renal interstitium and then to the peritubular capillary.

The HCO3 generated by this process constitutes new HCO3

Fig: (Ammonia buffer system in renal tubule)

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