 Dear students, in this topic we shall discuss the absorption of water and electrolytes in the intestine. You know that digestive juice secretion involves release of huge amount of water along with various electrolytes. In humans, this amount is over 7 liters per day which is about 1.5 times the total blood volume but it cannot afford to lose this huge quantity of water with electrolytes. So nearly all this secreted water along with the ingested water and electrolytes are absorbed by the intestine. Water absorption is in the intestine but most of it is absorbed in lower part of small intestine or colon. Water absorption from all parts of the intestine into the villi is entirely a passive process in which energy is not used. The force that is used to absorb water is the osmotic pressure. The osmotic pressure in epithelial cells of villi is produced because of the active transport of salts, sugars and amino acids which when absorbed in the villi they set up this osmotic gradient. The elevated osmotic pressure in the villi draws in the water and the water is passively absorbed. Dear students, In this way, the absorption of water sometimes becomes more than necessary and if excessive uptake of water is done then that results in abnormally dry, luminal contents. In luminal contents, indigested wastes are saved and if the excess water is absorbed then it becomes dry up. This causes constipation. Therefore, it is necessary to regulate the quantity of the absorption of water. Normally, a lot of factors regulate the absorption of water and the absorption of electrolytes and prevent excessive absorption. The various factors which control the absorption of water include the gastrin hormone which acts indirectly to inhibit water absorption in the small intestine. In addition, secretine and cholecystokinine reduce the uptake of electrolytes, sodium, potassium and chloride ions in the upper gjunum. Moreover, bile acids and fatty acids also inhibit absorption of water and electrolytes. Dear students, now we shall discuss the mechanism of absorption of sodium and chloride ions. Most of the sodium ions are actively absorbed by the cell. Sodium absorption occurs in the epithelial cells lining the tip of each villus. Chloride ions follow the sodium ions passively. When sodium ions are actively absorbed then chloride ions passively enter with them. The sodium and chloride absorption is enhanced when there is high concentration of glucose present in the lumen because for the absorption of glucose the core transport mechanism is used in which the sodium ions are absorbed. Therefore, when the glucose is high then the sodium ions are absorbed and the chloride ions e-absorption also increases. Dear students, calcium ions are absorbed from the intestine through a special active transport mechanism. In this mechanism, calcium ions are first found to a calcium binding protein present in the microvilli. This complex formation is an energy-consuming process. The presence of calcium binding protein in the microvilli is regulated by calcitriol hormone also known as vitamin D3. In this process, the parathyroid hormone is released in the calcium-absorptive cell after the absorption of calcium. The vitamin D12, which has a molecular weight of 13.57, is the largest in the world and is the largest water soluble essential nutrient which is taken up intact. This is a large molecule but it is absorbed from the lumen of the intestine with this large size into the epithelial cells. This absorption is at the distal end of the lumen.