acidosis and alkalosis

To have acid-base homeostasis to maintain the blood pH of 7.4, both carbonic acid and metabolic acids must be excreted from the body via lungs (for CO2) and kidneys (for metabolic acids). The disorders of the pH of the blood, termed acidosis (blood pH below 7.4) and alkalosis (blood pH above 7.4), can be of two types:

  1. Alterations in the blood bicarbonate levels: These are metabolic acidosis and alkalosis.
  2. Alteration in Pco2 (which depends upon the ventilatory function of the lungs): These are respiratory acidosis and alkalosis.

METABOLIC ACIDOSIS

A fall in the blood pH due to metabolic component is brought about by fall of bicarbonate level and excess of H+ ions in the blood. This occurs in the following situations:

i) Production of large amounts of lactic acid (lactic acidosis) e.g. in vigorous exercise, shock.
ii) Uncontrolled diabetes mellitus (diabetic ketoacidosis).
iii) Starvation.
iv) Chronic renal failure.
v) Therapeutic administration of ammonium chloride or acetazolamide (Diamox).

High blood levels of H+ ions in metabolic acidosis stimulate the respiratory center so that the breathing is deep and rapid (air hunger or Kussmaul’s respiration). There is a fall in the plasma bicarbonate levels.

METABOLIC ALKALOSIS

A rise in the blood pH due to a rise in the bicarbonate levels of plasma and loss of H+ ions is called metabolic alkalosis. This is seen in the following conditions:

i) Severe and prolonged vomitings.
ii) Administration of alkaline salts like sodium bicarbonate.
iii) Hypokalaemia such as in Cushing’s syndrome, increased secretion of aldosterone.

Clinically, metabolic alkalosis is characterized by depression of respiration, depressed renal function with uremia, and increased bicarbonate excretion in the urine. The blood level of bicarbonate is elevated.

RESPIRATORY ACIDOSIS

A fall in the blood pH occurring due to raised PCo2 consequent to hypoventilation of lungs (CO2 retention) causes respiratory acidosis. This can occur in the following circumstances:

i) Air obstruction as occurs in chronic bronchitis, emphysema, asthma.
ii) Restricted thoracic movement e.g. in pleural effusion, ascites, pregnancy, kyphoscoliosis.
iii) Impaired neuromuscular function e.g. in poliomyelitis, polyneuritis.

Clinically, there is peripheral vasodilatation and raised intracranial pressure. If there is severe CO2 retention, patients may develop confusion, drowsiness, and coma. The arterial Pco2 level is raised. 

RESPIRATORY ALKALOSIS

A rise in the blood pH occurring due to lowered Pco2 consequent to hyperventilation of the lungs (excess removal of CO2) is called respiratory alkalosis. This occurs in the following conditions:

i) Hysterical over-breathing.
ii) Working at high temperatures.
iii) At high altitude.
iv) Meningitis, encephalitis.
v) Salicylate intoxication.

Clinically, the patients with respiratory alkalosis are characterized by peripheral vasoconstriction and consequent pallor, lightheadedness, and tetany. The arterial Pco2 is lowered.