Oral Rehydration Therapy - Physiological Basis

Physiological Basis

Fluid from the body is normally pumped into the intestinal lumen during digestion. This fluid is typically isosmotic with blood because it contains a high concentration of sodium (approx. 142 mEq/L). A healthy individual will secrete 20-30 grams of sodium per day via intestinal secretions. Nearly all of this is reabsorbed by the intestine, helping to maintain constant sodium levels in the body (homeostasis).

Because there is so much sodium secreted by the intestine, without intervention, heavy continuous diarrhea can be a very dangerous and potentially life-threatening condition within hours. This is because liquid secreted into the intestinal lumen during diarrhea passes through the gut so quickly that very little sodium is reabsorbed, leading to very low sodium levels in the body (severe hyponatremia). This is the motivation for sodium and water replenishment via ORT.

Sodium absorption via the intestine occurs in two stages. The first is at the outermost cells (intestinal epithelial cells) at the surface of the intestinal lumen. Sodium passes into these outermost cells by co-transport via the SGLT1 protein. From there, sodium is pumped out of the cells (basal side) and into the extracellular space by active transport via the sodium potassium pump.

The co-transport of sodium into the epithelial cells via the SGLT1 protein requires glucose or galactose. Two sodium ions and one molecule of glucose/galactose are transported together across the cell membrane through the SGLT1 protein. Without glucose or galactose present, intestinal sodium will not be absorbed. This is the reason glucose is included in ORSs. For each cycle of the transport, hundreds of water molecules move into the epithelial cell, slowly rehydrating the affected individual.