GLOSSARY

Sodium is the most important cation of extracellular fluid, and its normal blood concentration is about 135–145 mmol/L. Sodium is the main determinant of body fluid volume, blood pressure and plasma osmolality.

Functions

• Blood volume: “water follows sodium”, so the body’s total sodium determines extracellular fluid volume.
• Excitability: the rapid entry of sodium into cells produces the action potential in nerve and muscle.
• Nutrient uptake: sodium co-transport drives absorption of glucose and amino acids in the gut and kidney.

Kidney Control

The kidney filters about 25,000 mmol of sodium a day and reabsorbs more than 99 percent of it. Aldosterone and angiotensin II increase reabsorption, while ANP reduces it.

Disorders