Systems responsible for maintenance of the acid-base balance or pH

Several systems maintain constant pH. The list below is made according to order when they act:

1) Chemical buffering systems provides immediate and short-term response to pH changes.

The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbon dioxide (CO₂) in order to maintain pH in the blood. In presence carbonic anhydrase, carbon dioxide (CO₂) reacts with water (H₂O) to form carbonic acid (H₂CO₃), which in turn rapidly dissociates to form a bicarbonate ion (HCO₃⁻) and a hydrogen ion (H⁺) as shown in the following reaction:

CO₂ + H₂O   ↔      H₂CO3 ↔ H⁺ + HCO₃^–

When the pH of the body falls, H⁺ and HCO₃^–  react to form H₂CO3 

and when pH rise H⁺ react with OH^– to form water leading to further ionization of H₂CO3

2) Respiratory system

Respiration reacts in 1-3 minutes. Respiratory system regulates carbon dioxide. Respiration is able to change pCO₂ by its elimination or retention. Respiratory centre is in brainstem.

3) Kidneys

Kidneys react in hours-days. In the kidney carbon dioxide react with water to form hydrogen ions (H⁺) and hydrogen carbonate (HCO₃^–)

CO₂ + H₂O   ↔      H₂CO3 ↔      H⁺ + HCO₃^–

When the pH is low the collecting duct and distal convoluted tubules secrete hydrogen ions and retain hydrogen carbonate ions producing urine with low pH.

Conversely, if blood too alkaline, then the collecting duct can secrete bicarbonate into urine and retain H⁺ lowering the pH of blood leading to alkaline urine.

4) Liver

Liver is pivotal organ of the energetic metabolism it also has important influence on the acid-base balance. Liver is the most important tissue where ammonium is detoxified in both urea cycle, and glutamine synthesis. Which one of these fates of ammonium is favored closely depends on status of the acid-base balance:

1. a) NH₄⁺ → urea + 2 H⁺ → acidification of the body

CO₂ + 2 NH₄⁺ → CO(NH₂)₂ + 2 H⁺ + H₂O

H⁺ + HCO₃^– → H₂O + CO₂ (consumption of bicarbonate^–) causing a rise in pH

1. b) NH₄⁺ → glutamine synthesis → H⁺ is not produced, glutamine is taken up by the kidneys. In the kidney     is H⁺ excreted as NH₄⁺

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Dr. Bbosa Science