Chloride (Cl) iis a major extracellular anion, distributed in blood plasma and interstitial fluid. Together, sodium and chloride represent the majority of the osmotically active constituents of extracellular fluid. It is involved in the regulation of osmotic pressure. Chloride ions in food are almost completely absorbed from the intestinal tract. They are filtered from plasma at the glomeruli and passively reabsorbed, along with Na+, in the proximal tubules. Increased levels of chloride in the sweat serve as an indicator of cystic fibrosis.