Acidification Of Urine And Role Of Kidney In Acid-Base Balance Introduction
The kidney plays an important role in the maintenance of acid-base balance by excreting hydrogen ions and retaining bicarbonate ions. Normally, urine is acidic in nature with a pH of 4.5-6. The metabolic activities in the body produce a lot of acids (with a lot of hydrogen ions) which threaten to push the body toward acidosis. However, kidneys prevent this by excreting hydrogen ions (H+) and conserving bicarbonate ions (HCO–3).
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Read And Learn More: Medical Physiology Notes
- Conservation or reabsorption of HCO–3 is an important process because a large quantity of HCO–3 is filtered and all are likely to be excreted through urine.
- About 4320 mEq of HCO–3 is filtered by the glomeruli every day. It is called the filtered load of HCO–3.
- Excretion of this much HCO–3 through urine will affect the acid-base balance of body fluids.
- So, HCO–3 must be taken back from the renal tubule by reabsorption.
- The reabsorption of filtered HCO–3 occurs by the secretion of H+ in the renal tubules.
- About 4380 mEq of H+ appears every day in the renal tubule by means of filtration and secretion.
- Not all the H+ is excreted through urine. Out of 4380 mEq, about 4280-4330 mEq of H+ is utilized for the reabsorption of filtered HCO–3.
- Only the remaining 50-100 mEq is excreted. It results in the acidification of urine.
Secretion Of Hydrogen Ions
- Secretion of H+ into the renal tubules occurs through the formation of carbonic acid.
- Carbon dioxide formed in the tubular cells combines with water to form carbonic acid.
- Carbon dioxide enters the cells from tubular fluid also. Carbonic anhydrase is essential for the formation of carbonic acid This enzyme is available in large quantities in the epithelial cells of the renal tubules.
- The carbonic acid immediately dissociates into H+ and HCOf H+ from the tubular cells and is secreted into the lumen of the proximo convoluted tubule, distal convoluted tubule, and collecting duct.
- The distal convoluted tubule and collecting duct have a special type of cells called intercalated cells (I cells) that are involved in handling hydrogen and bicarbonate ions.
There are two mechanisms for the secretion of H+:
- Sodium-Hydrogen antiport pump
- ATP-driven proton pump.
Sodium-Hydrogen Antiport Pump
- When sodium ion (Na+) is reabsorbed from the tubular fluid into the tubular cell, H+ is secreted from the cell into the tubular fluid in exchange for Na+.
- The sodium-hydrogen antiport pump present in the tubular cells is responsible for the exchange of Na+ and H+.
- This type of sodium-hydrogen countertransport occurs predominantly in the distal convoluted tubule.
Atp Driven Proton Pump
This is an additional mechanism of H+ secretion in the distal convoluted tubule and collecting duct. This pump is operated by obtaining energy from ATP.
Removal Of Hydrogen Ions And Acidification Of Urine
Role of Kidney in Preventing Metabolic Acidosis
The kidney plays an important role in preventing metabolic acidosis by excreting H+. The excretion of H+ occurs by three mechanisms:
- Bicarbonate mechanism
- Phosphate mechanism
- Ammonia mechanism.
Bicarbonate Mechanism
All the filtered HCO–3 into the renal tubules is reabsorbed. About 80% of it is reabsorbed in the proximal convoluted tubule; 15% in Henle’s loop and 5% in the distal convoluted tubule and collecting duct.
- The reabsorption of HCO–3 utilizes the H+ secreted into the renal tubules.
- The H+ secreted into the renal tubule, combines with filtered HCO–3 forming carbonic acid.
- Carbonic acid dissociates into carbon dioxide and water in the presence of carbonic anhydrase.
- Carbon dioxide and water enter the tubular cell.
- In tubular cells, carbon dioxide combines with water to form carbonic acid.
- It immediately dissociates into H+ and HCO–3. HCO–3 from the tubular cell enters the interstitium.
- Simultaneously Na+ is reabsorbed from the renal tubule under the influence of aldosterone.
- HCO–3 combines with Na+ to form NaHCOs. Now, the H+ is secreted into the tubular lumen from the cell in exchange for Na+.
- Thus, for every hydrogen ion secreted into the lumen of the tubule, one bicarbonate ion is reabsorbed from the tubule, in this way, kidneys conserve the HCO–3.
- The reabsorp- itof: of filtered HCO–3 is an important factor in maintaining the pH of the body fluids.
Phosphate Mechanism
- In the tubular cells, carbon dioxide combines with water which forms carbonic acid.
- It immediately dissociates into H+ and MCOt. HCO–3 from the tubular cell enters the interstitium.
- Simultaneously, Na+ is reabsorbed from the renal tubule under the influence of aldosterone.
- Na+ enters the interstitium and combines with HCO–3. The H+ is secreted into the tubular lumen from the cell in exchange for Na+.
- The H+, which is secreted into renal tubules, reacts with the phosphate buffer system.
- It combines with sodium hydrogen phosphate to form sodium dihydrogen phosphate.
- Sodium dihydrogen phosphate is excreted in the urine. The H+, which is added to urine in the form of sodium dihydrogen, makes the urine acidic.
- It happens mainly in the distal tubule and collecting duct because of the presence of a large quantity of sodium hydrogen phosphate in these segments.
Ammonia Mechanism
- This is the most important mechanism by which kidneys excrete H+ and make the urine acidic.
- In the tubular epithelial cells, ammonia is formed when the amino acid glutamine is converted into glutamic acid in the presence of the enzyme glutaminase. Ammonia is also formed by the deamination of some of the amino acids such as glycine and alanine.
- The ammonia (NH3) formed in tubular cells is secreted into the tubular lumen in exchange for sodium ions.
- Here, it combines with H+ to form ammonium (NH4). The tubular cell membrane is not permeable to ammonium.
- Therefore, it remains in the lumen and combines with sodium aceto- acetate to form ammonium acetoacetate.
- Ammonium acetoacetate is excreted through urine. Thus, H+ is added to urine in the form of ammonium compounds resulting in acidification of urine.
- This process takes place mostly in the proximal convoluted tubule because glutamine is converted into ammonia in the cells of this segment.
- I am by excreting H+ and conserving HCO–3, kidneys produce urine and help to maintain the acid-base balance of body fluids.
Applied Physiology
In renal disease, the kidney fails to excrete metabolic acids resulting in metabolic acidosis.
When the kidney excretes a large number of H+, metabolic alkalosis occurs
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