Acid-base balance-Lecture-1- significance of pH

Acid-base balance

An acid is a substance that can donate hydrogen ions (H+), and a base is a substance that can accept H+ ions, regardless of the substance’s charge.

H₂ CO₃ (acid) «H⁺ + HCO3 ^– (base)

Strong acids are those that are completely ionized in body fluids, and weak acids are those that are incompletely ionized in body fluids.

HCl «H⁺ + Cl^–

Hydrochloric acid (HCl) is considered a strong acid because it is present only in a completely ionized form in the body, whereas H₂ CO₃ is a weak acid because it is ionized incompletely, and, at equilibrium, all 3 reactants are present in body fluids.

H₂ CO₃ (acid) «H⁺ + HCO3 ^– (base)

In body fluids, the concentration of hydrogen ions ([H^+]) is maintained within very narrow limits, with the normal physiologic concentration being 40nEq/L. The concentration of HCO₃^– (24mEq/L) is 600,000 times that of [H⁺]. The tight regulation of [H⁺] at this low concentration is crucial for normal cellular activities.

Significance of pH

1) Specific tautomeric forms exist at physiologic pH. This helps in proper hydrogen bonding between the complementary base pairs in the structure of DNA.

2) The solubility and biologic activity of proteins depend upon their 3D structure and that depends upon net charge on protein for the maintenance of hydrogen and ionic interactions. The net charge depends upon the pH of the medium.

3) The movement of ions across the membrane depends upon their net charge as determined by the pH.

4) Ionic state of the nucleic acids, lipids, and mucopolysaccharides is also determined by the physiological pH

5) All enzymes function best within an optimum pH range.

6) Nerve conduction and muscle contractions are also pH-dependent

7) All metabolic processes are pH-dependent.

8) Oxygen and CO₂ transport, release or gaseous exchange are pH-dependent.

Maintenance of pH is important for the proper physiological functioning of cells and tissues. Any changes in pH can alter enzyme activity, cellular uptake, incorporation and use of minerals and metabolites, uptake and release of oxygen, and the formation of biological structural components.

Normal plasma pH = 7.40 (±0.05). The pH range that is compatible with life is from 6.8 to 7.8. The body can comfortably tolerate a shift in pH of about 0.04. Most cells of the body have a pH = 7.0, but RBC’s boast a pH of 7.2. The pH of the body affects its acid-base balance and the pH of blood has the greatest effect.

Sources for pH disturbances

1) Organic acids- The most common sources for pH disturbances are the body’s production of organic acids (acetic, acetoacetate, propionic, butyric, lactic, etc.), which are the major sources of hydrogen ion.

2) Carbonic acid is the chief acid produced in the body by the metabolic processes in the body. Approximately 300 liters of CO₂ are produced and eliminated daily in the body of an adult.

3) Sulphuric acid- it is produced during the oxidation of sulfur-containing amino acids and vitamins.

4) Phosphoric acid- is produced from the metabolism of dietary phosphoproteins, phospholipids, nucleic acids and hydrolysis of phosphoesters.

Mechanism of maintenance of Physiological pH

Under normal conditions, acids and, to a lesser extent, bases are being added constantly to the extracellular fluid compartment but still a physiologic [H+] of 40 nEq/L is maintained and the following 3 processes must take place:

• Buffering by extracellular and intracellular buffers
• Alveolar ventilation, which controls PaCO₂
• Renal H⁺ excretion, which controls plasma [HCO₃ ^–]