Question

What are the mechanisms used by our body to regulate the blood pH? Explain breifly the mechanisms. What is the importance of maintenance of a pH?

Answer 1

Importance of maintenance of pH:

• The activities of almost all enzyme systems in the body are influenced by hydrogen ion concentration.
• Changes in hydrogen ion concentration alters - all cell and body functions, the conformation of biological structural components, uptake and release of oxygen.
• To maintain all these in proper manner the pH is maintained in the normal range.
  Blood pH is altered because-
• There are 2 important things that alter the blood pH.
• They are the acids and bases which are produced through normal metabolism.
  Metabolic sources of Acids:
• Acids are of 2 types - 1.Fixed or non-volatile acids and 2. Volatile acids.
• Fixed acids are Phosphoric acid, Sulphuric acid, Pyruvic acid, Lactic acid and keto acids.
• These acids are the source of protons, if they are present in excess, they decrease the blood pH.
• Volatile acids breathe out through the lungs:
  Metabolic sources of Bases:
• Citrate salts of fruit juices may produce Bicarbonate salt.
• Deamination of amino acids produces Ammonia.
• Formation of Bis-phosphate also contributes to alkalinizing effect.
  Mechanisms regulating blood pH:

This includes Buffer mechanism (1st line of defense), Respiratory mechanism (2nd line of defense) and Renal mechanism (3rd line of defense).
1.Buffer mechanism:

• Buffer- Buffers are the solutions which can resist changes in pH when acid or base is added.
• The blood contains 3 buffer systems which include Bicarbonate buffer, Phosphate buffer and Protein buffer.
  Bicarbonate buffer system:
• It is the most important extracellular buffer.
• It plays an important role in maintaining blood pH, because of its high concentration.
• The 2 elements of this system HCO₃- and H₂CO₃ are regulated by the kidneys, and lungs respectively.
• It is evident that at a blood pH 7.4, the ratio of bicarbonate to carbonic acid is 20:1.
• Thus, the bicarbonate concentration is much higher (20m times) than carbonic acid in the blood.
• This is referred to as alkali reserve and is responsible for the effective buffering of H+ ions, generated in the body.
• The 2 elements of this buffer system are regulated by:
  a. Increasing or decreasing the rate of reabsorption of HCO₃- by the kidneys.
  b. By altering the rates of removal or retention of H₂CO₃ by the lungs.
  Phosphate buffer system:
• It is not important as a blood buffer.
• It plays a major role in buffering renal tubular fluid and intracellular fluids.
• It consists of 2 elements - alkaline phosphate and acid phosphate.
• The normal ratio in plasma is 4:1.
• This ratio is kept constant with the help of the kidneys.
• Thus, this system is directly linked up with the kidneys.
  Protein Buffer:
• In the blood, plasma proteins especially albumin acts as buffer.
• In acid solution, the basic amino group NH2 takes up excess  H+ ions forming (NH3+).
• In basic solution, the acidic group (COOH) give up hydrogen ion forming OH-.of alkali to water.
• The other buffer groups of proteins in the physiological pH range are the imidazole groups of histidine.
• Each albumin molecule contains 16 histidine residues.
  Hemoglobin buffer:
• It is the major intracellular buffer of blood which is present in erythrocytes.
• Each Hb molecule contains 38 molecules of histidine.
• The imidazole group of histidine has a pKa of approximately 7.3, fairly close to 7.4.
• It buffers carbonic acid.
• The transport of an appreciable quantity of the CO₂released from the tissues without change in pH is called Isohydric transport of CO₂
• Most of the CO₂ is transported in the plasma as bicarbonate.
• Because Bicarbonate is much more soluble in blood plasma than is  CO₂, this indirect route increases the blood's capacity to carry CO₂ from the tissues to the lungs.

2. Respiratory mechanism:

• This 2nd line of defense is against acid-base disturbances.
• It functions by regulating the concentration of carbonic acid in blood and other body fluids by lungs.
• The respiratory center regulates the removal or retention of  CO₂  and thereby carbonic acid from the extracellular fluid by the lungs.
• Increase in H+ or H₂CO₃ stimulates the respiratory center to increase the rate of respiratory ventilation and excess acid in the form of  CO₂ is quickly removed.
• Increase in OH- or HCO3- depresses respiratory ventilation and release of CO2 from the blood.

The increased blood CO2 will result in the formation of more H₂CO₃acid to neutralize excess alkali (HCO3).

3. Renal mechanism:

• Renal mechanism is the 3rd line of defense in acid base balance.
• Long term acid-base control is exerted by renal mechanisms.
• Kidney participates in the regulation of acid-base balance by conservation of HCO3- (alkali reserve) and excretion of acid.
• The pH of the initial glomerular filtrate is approximately 7.4 whereas the average urinary pH is approximately 6.0 due to excretion of non-volatile acids produced by metabolic processes.
• The pH of the urine may vary from 4.5 - 8.0 corresponding to the case of acidosis or alkalosis.
• The ability to excrete variable amounts of acid or base makes the kidney the final defense mechanism against change in body pH.
• The major kidney mechanisms are:
  1. Excretion of H+
  2. Reabsorption of filtered bicarbonate (recovery of bicarbonate)
  3. Excretion of titratable acid and
  4. Excretion of NH4+ (ammonium ions)

• Acids are the substances which are capable of donating protons.
• Bases are the substances which accept the protons.
• The normal blood pH is maintained within a remarkable constant level of 7.38-7.42.