Acid-base balance

Acid-base balance

The normal plasma pH is 7.42 (range 7.38 – 7.44) corresponding to H+ concentration of 40 nmol/L (range 36 - 43 nmol/L). It is maintained within these narrow limits through strict regulations by respiratory mechanisms, renal tubular mechanisms and body buffers (carbonic acid, dihydrogen phosphate, organic acids and proteins).
Acidosis results when hydrogen ion is greater than 44 nmol/L (pH <> 7.45). Compensatory mechanisms start immediately through alteration of the rate of ventilation, followed by activation of body buffer system, lastly by the renal mechanism.

Biochemical changes in acid-base imbalance:
a) Respiratory alkalosis (CO2 deficit):
- Causes: over breathing e.g. hysterical, over ventilation in a mechanical respirator or injury to the respiratory center e.g. trauma.
- Compensatory mechanisms; body buffer systems, followed by loss of bicarbonate in the urine and decrease in H+ excretion.
b) Respiratory acidosis (CO2 retention):
· Non- respiratory (metabolic) disturbances:
a) Non-respiratory alkalosis:
Causes: ingestion of large amounts of alkali for treatment of peptic ulcer and loss of acid from the stomach (prolonged vomiting). Compensatory mechanism: hypoventilation followed by renal loss of bicarbonate
b) Non-respiratory acidosis:
Causes: renal disease leading to failure to excrete hydrogen ions e.g. renal tubular acidosis, acute and chronic renal failure, failure to excrete an excessive load of hydrogen ions as in ketoacidosis, lactic acidosis and uretero-colic anastomosis, and loss of base by excessive loss of GIT secretions.
Compensatory mechanisms: hyperventillation followed by renal excretion of H+ (if the kidney is healthy) and fall in plasma bicarbonate due to its elimination with H+ ® H2 CO3 ® CO2, which is washed by the lungs.
Laboratory assessment of acid -base balance:
Arterial or capillary blood samples are used. Samples are with­drawn in a syringe using heparin as anticoagulant and transferred immediately to the laboratory. If there is some delay (15-30 min.), samples must be kept in iced containers. For the precise investigation of acid-base balance, the three parameters: [H+], bicarbonate concentration and PCO2 must be determined.