Contraction alkalosis

Contraction alkalosis refers to the increase in blood pH that occurs as a result of fluid losses. The change in pH is especially pronounced with acidic fluid losses caused by problems like vomiting.

Extracellular fluid (ECF) volume contraction is associated with decreased blood volume and decreased renal perfusion pressure. Three compensation mechanisms engage as a result:

1. Renin secretion is increased
2. Production of angiotensin II is increased
3. Secretion of aldosterone is increased

Increases in angiotensin II causes increased Na⁺-K⁺ exchange and increased HCO₃^– (bicarbonate) reabsorption in the proximal tubule. Increased aldosterone secretion causes increased collecting duct – alpha intercalated cell H⁺ secretion, worsening the metabolic alkalosis. Additionally, increased aldosterone secretion causes increased distal tubule K⁺ secretion, causing the hypokalemia seen with contraction alkalosis.

Treatment consists of NaCl infusion to correct ECF volume contraction and administration of K⁺ to replace urinary losses.

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