• Usually caused by a rise in plasma bicarbonate levels.
• Rise of bicarbonate above 24 mmol/L will typically result in renal excretion of excess bicarbonate.
• Caused by a loss of hydrogen ions or a gain of bicarbonate. It is due mainly to problems of the kidney or gastrointestinal tract

Causes

• Vomiting / aspiration (e.g. Peptic ulcer leading to pyloric stenosis, nasogastric suction)
• Diarrhoea - e.g. chloride secreting villous adenoma
• Diuretics (hypokalaemia)
• Liquorice, carbenoxolone
• Primary hyperaldosteronism
• Cushing's syndrome
• Congenital adrenal hyperplasia

Mechanism of metabolic alkalosis

• Activation of renin-angiotensin II-aldosterone (RAA) system is a key factor
• Aldosterone causes reabsorption of Na in exchange for H in the distal convoluted tubule
• ECF depletion (vomiting, diuretics) → Na and Cl loss → activation of RAA system → raised aldosterone levels
• In hypokalaemia, K shift from cells → ECF, alkalosis is caused by shift of H into cells to maintain neutrality