Highest concentration of potassium is seen in
Correct Answer: Rectum
Description: K+ channels are present in the luminal as well as the basolateral membrane of the enterocytes of the colon, so K+ is secreted into the colon. In addition, K+ moves passively down its electrochemical gradient. The accumulation of K+ in the colon is paially offset by H+-K+ ATPase in the luminal membrane of cells in the distal colon, with resulting active transpo of K+ into the cells. Neveheless, loss of ileal or colonic fluids in chronic diarrhea can lead to severe hypokalemia. When the dietary intake of K+ is high for a prolonged period, aldosterone secretion is increased and more K+ enters the colon. This is due in pa to the appearance of more Na+- K+ ATPase pumps in the basolateral membranes of the cells, with a consequent increase in intracellular K+ and K+ diffusion across the luminal membranes of the cells. The intestines are presented each day with about 2000 mL of ingested fluid plus 7000 mL of secretions from the mucosa of the gastrointestinal tract and associated glands. Ninety-eight percent of this fluid is reabsorbed, with a daily fluid loss of only 200 mL in the stools. In the small intestine, secondary active transpo of Na+ is impoant in bringing about absorption of glucose, some amino acids, and other substances such as bile acids (see above). Conversely, the presence of glucose in the intestinal lumen facilitates the reabsorption of Na+. In the period between meals, when nutrients are not present, sodium and chloride are absorbed together from the lumen by the coupled activity of a sodium/hydrogen exchanger (NHE) and chloride/bicarbonate exchanger in the apical membrane, in a so-called electroneutral mechanism. Water then follows to maintain an osmotic balance. In the colon, moreover, an additional electrogenic mechanism for sodium absorption is expressed, paicularly in the distal colon. In this mechanism, sodium enters across the apical membrane an ENaC (epithelial sodium) channel that is identical to that expressed in the distal tubule of the kidney. This underpins the ability of the colon to desiccate the stool and ensure that only a small poion of the fluid load used daily in the digestion and absorption of meals is lost from the body. Following a low-salt diet, increased expression of ENaC in response to aldosterone increases the ability to reclaim sodium from the stool. The pancreatic juice is alkaline and has a high HCO3 content (approximately 113 mEq/L vs. 24 mEq/L in plasma). About 1500 mL of pancreatic juice is secreted per day. Bile and intestinal juices are also neutral or alkaline, and these three secretions neutralize the gastric acid, raising the pH of the duodenal contents to 6.0 to 7.0. By the time the chyme reaches the jejunum, its pH is nearly neutral, but the intestinal contents are rarely alkaline. Bile is made up of the bile acids, bile pigments, and other substances dissolved in an alkaline electrolyte solution that resembles pancreatic juice.REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO:435,438,441,442
Category:
Physiology
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