Nutrients from the diet are absorbed across the mucosa of the small intestine concomitant with water and electrolytes which play a critical role in maintaining body hydration and it's acid-base balance. An important process that takes place in the small intestine to make absorption possible is the establishment of an electrochemical gradient of sodium across the epithelial cell from the lumen to vasculature. For cells to remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases "pumps" - embedded in the basolateral membrane. These pumps which require energy (ATP) export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane. In the rat, there are about 150,000 sodium pumps per small intestinal enterocyte which equates to roughly 4.5 billion sodium ions being pumped out of each cell per minute (J Membr Biol 53:119-128, 1980). This active transport of sodium is ultimately responsible for water, amino acid, and carbohydrate absorption as a secondary active transport process via the driving force of sodium. Measuring sodium absorption by the small and large intestine in different regions gives insight into overall nutrient absorption.