Water absorption.minerals,vitamines

Water absorption.minerals,vitamines

1. Absorption of water,vitamins,minerals and electrolytes. Movement of GI Tract. Rahana Moideen Koya.V.K Asst.Professor of Zoology,Farook College.

2. Absorption of Vitamins- Vitamins  are organic molecules necessary for normal metabolism in animals, but either are not synthesized in the body or are synthesized in inadequate quantities and must be obtained from the diet. Essentially all vitamin absorption occurs in the small intestine. Absorption of vitamins in the intestine is critical in avoiding deficiency states, and impairment of intestinal vitamin absorption can results from a number of factors, including intestinal disease, genetic disorders in transport molecules, excessive alcohol consumption and interactions with drugs.

3. Absorption of Water-soluble Vitamins Most water soluble vitamins are available for intestinal absorption from two sources: 1) the diet, and 2) synthesis by microbes in the large intestine or, in the case of ruminants, the  rumen . These dual-origin vitamins include biotin, folic acid, pantothenic acid, riboflavin and thiamin . Ascorbic acid can be synthesized by many animals, but not by primates or guinea pigs, in which it is a true vitamin and must be obtained from dietary sources. Niacin is also a bit different - it can be synthesized within the body from tryptophan but is also absorbed in the intestine from dietary sources.

4. Water soluble vitamins of dietary origin are absorbed predominantly in the small intestine, whereas those synthesized by microbes in the large intestine are absorbed there. For most of these vitamins, specific carrier-mediated transport systems have been identified that allow uptake from the intestinal lumen into the enterocyte and for export from the basolateral surface of the enterocyte. Some of these transporters are sodium-dependent, while others are not. Most of the vit are absorbed in upper part of small intestine and vit B12 cobalamin(prod of RBC and DNA,NS functioning) absorbed in i leum .Absorption of water soluble vit. Is faster.

5. Absorption of Fat-soluble Vitamins The fat soluble vitamins A, D, E and K are absorbed from the intestinal lumen using the same mechanisms used for  absorption of other lipids . In short, they are incorporated into mixed micelles with other lipids and bile acids in the lumen of the small intestine and enter the enterocyte largely by diffusion. Within the enterocyte, they are incorporated into chylomicrons and exported via exocytosis into lymph.

6. Overview GI Water and Electrolyte Absorption The GI system secretes nearly 8L of fluid into the GI tract which is combined with the nearly 2L of ingested fluid; however, only a 0.1-0.2L of fluid is excreted in the feces. Therefore, the GI system must absorb nearly 9L of fluid , composed of water and electrolytes, each day. Water and electrolyte absorption primarily occurs in the small and large intestines. Dysregulation of this absorption can either lead to  diarrhea  or constipation.

7. Large intestine —Water, Electrolytes, Organic substances like glucose , alcohol, Drugs like anesthetic agents, sedatives and steroids. Bacterial flora of large intestine synthesizes Folic acid , Vit B12,Vit.K. By this function large intestine contributes in erythropoietic activity and blood clotting mechanism. Vitamin B12  occurs in foods that come from animals. Normally,  vitamin B12  is readily  absorbed  in the last part of the small intestine (ileum), which leads to the large intestine. .

8. However, to be  absorbed , the  vitamin  must combine with intrinsic factor, a protein produced in the stomach. Two steps are  required  for the body to  absorb vitamin B12  from food. First, hydrochloric acid in the stomach separates  vitamin B12  from the protein to which  vitamin B12  is attached in food. After this,  vitamin B12  combines with a protein made by the stomach called intrinsic factor and is  absorbed  by the body. Once bound with IF,  vitamin B-12  is resistant to further digestion. A carrier glycoprotein ( Castle's  gastric  intrinsic factor ) binds to  vitamin B12  and delivers it to receptor sites on the brush border of the ileal mucosal cell, to which it attaches in the presence of calcium and alkaline pH, thence to be  absorbed .

9. In small intestine Na absorbed actively . It is responsible for absorption of glucose ,a acids and other substances by means of sodium co-transport. Water moves in and out of the intestinal lumen until the osmotic pressure of intestinal contents becomes equal to that of plasma. In ileum , Chloride ion is actively absorbed in exchange for bicarbonate Ca is is actively absorbed mostly in upper part of small intestine.

10. Luminal membrane Na +  resorption occurs via a number of symporters and antiporters throughout the small and large intestines. In all cases, the luminal membrane resorption is powered by a NaK ATPase on the basolateral enterocyte membrane. This NaK ATPase actively transports Na +  past the basolateral membrane, thus reducing intracellular Na +  concentration, which subsequently creates an electrochemical gradient for inward Na +  transport on the luminal membrane.

11. Duodenum and Jejunum: Luminal resorption occurs on a variety of Na-Nutrient symporters, which include monosaccharides, as well as amino acids, dipeptides, and tripeptides. These mechanisms of sodium resorption are constitutively active and are not physiologically regulated.

12. Large Intestine Na +  absorption in the large intestine is very similar to that occurring in the Principal Cells during late distal tubule and collecting duct transport. diffusion of Na +  through luminal membrane ion channels is powered by a basolateral NaK ATPase. As in the late distal tubule and collecting duct aldosterone significantly enhances sodium resorption in the large intestine by increasing expression of the basolateral NaK ATPase and luminal Na +  ion channels.

13. Water absorption- -- Water is always absorbed in the alimentary tract through passive osmosis via a mostly paracellular route between enterocyte tight junctions. Consequently, water absorption is primarily actuated (motivated) by active absorption of osmotic electrolytes, especially sodium. In cases where a high concentration of unabsorbable solutes remain in the GI lumen, water cannot be resorbed and thus causes an  osmotic diarrhea .

14. Chloride resorption- -- Absorption of Cl -  largely occurs through passive diffusion via a paracellualr route. Substantial resorption of Na +  may create a lumen negative charge, thus creating a strong electrochemical gradient for passive resorption of Cl - . The majority of chloride is resorbed in the small intestine, especially the duodenum and jejunum.

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16. Bicarbonate resorption- A large amount of bicarbonate is secreted during pancreatic secretion and maintenance of proper acid-base balance requires that some must be resorbed. A CO 2  molecule is converted to H +  and HCO 3 -  by  carbonic anhydrase  in the enterocytes. The HCO 3 -  is transported past the basolateral membrane while the H +  is transported into the intestinal lumen on an Na + -H +  Antiporter. The H +  probably then combines with luminal bicarbonate, and is then converted to CO 2  which then diffuses into the blood and is breathed off by the respiratory system. The net effect is resorption of a bicarbonate ion.

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18. Mouth—tongue Nasopharynx—Elevation of soft palate along with extension called uvula Larynx--Epiglottis

19. Esophagial stage-peristaltic waves.-Peristalsis—a wave of contraction followed by wave of relaxation of muscle fibers of GI tract which travel aboral direction-away from mouth. When bolus reaches the esophagus .the peristaltic waves are initiated ,which propel the bolus into the stomach .Two types of peristaltic contractions produced by esophagus. 1– primary-when bolus reaches the upper part of esophagus ,the peristalsis starts. perstatalic contractions pass down through the rest of the esophagus propelling bolus towards stomach.. The pressure developed during this contraction propel the bolus.

20. Secondary peristaltic contractions—if primary contraction unable to propel bolus this occur ,this contractions arise in esophagus locally due to distention of upper esophagus by the bolus . Role of lower esophageal sphincter— The distal 2 to 5 cm of esophagus acts like a sphincter-lower esophagial sph.When bolus enter this part of esophagus ,this sphincter relaxes and contents enter stomach.

21. Deglutition Reflex-Swallowing at the Beginning it voluntary act ,later involuntary and is carried out by a reflex action called deglutition reflex,.It occurs during the pharyngeal and esophageal stages. Stimulus—When the bolus enters oropharyngeal region ,the receptors present in this region stimulated, Afferent fibers—afferent impulses from oropharyngeal receptors pass via the glossopharyngeal nerve fibers to the deglutition centre . Centre—Deglutition centre is at the floor of vthe fourth ventricle in medulla oblongata . Efferent fibres —The impulses from centre travel through glossopharyngeal(Pharyngeal stage) and vagus neves ( esophagial stage) to soft palate,pharynx,and esophagus. Response—The reflex causesupward movt . Of soft palate,to close nasopharynxand upward movt.of larynx to close respiratory passage so that bolus enters the esophagus .Now peristalsis occurs in esophagus pushing bolus into stomach.

22. Movements of stomach- Hunger contractions—the activities of smooth muscles of stomach stomach increased during gastric digestion –stomach filled with food and when stomach empty. The movts of empty stomach are related to the sensation of hunger.so this movts -hunger contractions.hunger contractions are peristaltic waves superimposed over the contractions of gastric muscle as a whole.the digestive peristaltic contraction occur in body and pyloric part of stomach.persaltic contractions of empty stomach involve the entire stomach. contractions disappear when food consumed

23. Type1-when the tone of the stomach is low it appears lasts for 20 sec..interval bet con is 3-4 sec. Type11 hunger contraction—appear when the tonus of stomach is stronger,the tonus increases in stomach if food intake is postponed,even after the appearance of type1 con it lasts for 20 sec. Type111—or incomplete tetanus appear when the hunger becomes severe and tonus increases to a great extent. these contractions are rare we take food before this. Lasts for 1-5 minutes Normal type 1 followed by type 11 –soon after food is consumed hunger contractions disappear.

24. Receptive relaxation—Relaxation of upper portion of stomach when bolus enters stomach from esophagus It involves the fundus and upper part of the body of stomach. Peristalsis of stomach---When food enters the stomach ,the peristaltic contraction appears with a frequency of 3/minute .it strats from the lower part of stomach passes through the pylorus till the pyloric sphinctor .

25. Initially contractions appear as a slight indentatation on the greater and lesser curvatures and travel towards pylorus..finally it ends with constriction of pyloric sphincter.this type of peristaltic contraction is called digestive peristalsis.because it is responsible for grinding mixing. Filling and emptying of stomach- Filling—the first eaten food is placed againt the greater curvature in the fundus.and body of the stomach. the successive layers of food particles lie nearer the lesser curvature until the last portion of food eaten lies near the upper end of lesser curvature adjecent to cardiac sphincter. the liquid remains near lesser curvature and flows towards the pyloric end of stomach along a V shaped groove formed of smooth muscles it is called magenstrasse

26. Emptying of stomach---Food remains in stomach for 3 hrs ..chyme emptied into intestine slowly by peristaltic contraction. the peristaltic waves in the body and pyloric part of of the stomach and simultaneous relaxation of pyloric sphincter necessary for gastric emptying. Regulation of gastric emptying---emptying of stomach stops due to the inhibition of gastric motility . the inhibition of motility by nervous and hormonal factor.

27. Nervous factor is enterogastric reflex—when chyme enters intestine ,the gastric muscle is inhibited and motility stops and arrest of gastric emptying.. this is called enterogastric reflex, this reflex involves vagus nerves. Hormonal factors-when chyme enters duodenum ,duodenal mucosa releases hormones which enter stomach thrugh blood and inhibit gastric motility. Hormones inhibit motility and emptying of stomach VIP,GIP,Secretin,Cholecystokinin,somatostatin PeptideYY