Gastro intestinal physiology system

Gastro intestinal physiology system

The gastrointestinal system evolved as a portal to permit controlled nutrient uptake in unicellular organisms. It is functionally continuous with the out side environment and is defended by a well developed mucosal immune system. Never-theless, the gut usually lives in harmony with an extensive commensurable micro flora, particularly in the colon. Digestive secretions serve to chemically alter the components of meals ( particularly macromolecules ) such that their constituents can be absorbed across the epithelium. Meal components are acted on sequentially by saliva, gastric juice, pancreatic juice, and bile, which contain enzymes, ions, water, and other specialized components. The intestine and the organs that drain into it secrete about 8 L of
fluid per day, which are added to water consumed in food and beverages. Most of this fluid is reabsorbed, leaving only approximately 200 mL to be lost to the stool. Fluid secretion and absorption are both dependent on the active epithelial transport of  ions, nutrients, or both. Gastrointestinal functions are regulated in an integrated fashion by endocrine, paracrine,and neurocrine mechanisms. Hormones and Paraclete factors are released from enteroendocrine cells in response to signals coincident with the intake of meals.The epicenter nervous system conveys information from the central nervous system to the gastrointestinal tract but also often can activate programmed responses of secretion and motility in an autonomous fashion. The intestine has an ususual circulation, in that the majority of its venous outflow does not return directed initially to the liver via the portal vein. A typical mixed meal consists of carbohydrates, proteins, and lipids ( the latter largely in the form of triglycerides ). Each must be digested to allow its uptake into the body. Specific transporters carry the products of digestion into the body. In the process of carbohydrate assimilation, short peptides can be absorbed in addition to amino acids. The protein assimilation machinery,which
rests heavily on the proteases in pancreatic juice, is arranged such that these enzymes are not activated until they reach their substrates in the small intestinal lumen.This is accomplished by the restricted  localization of an activating enzyme,enterokinase. Lipids face special challenges to assimilation given their hydrophobicity. Bile acids solubilize the products of lipolysis in micelles and accelerate their ability to diffuse to the epithelial surface. The assimilation of triglycerides is enhanced by this mechanism,

Key points of Gastrointestinal physiology

Whereas that of cholesterol and fat-soluble vitamins absolutely requires it. The catabolism of nutrients provides energy to the body in a controlled fashion, via stepwise oxidation's and other reactions. A balanced diet is important for health,and certain substances  obtained from the diet are essential to life. The caloric value of dietary intake must be approximately equal to energy expenditure for homeostasis.
Motility : The regulatory factors that govern  gastrointestinal secretion also regulate its motility to soften the food, mix it with secretions, and propel it along the length of the tract. Two major patterns of motility are peristalsis involves coordinated contractions and relaxations above and below the food bolus. The membrane potential of the majority of gastrointestinal smooth muscle undergoes rhythmic fluctuations that sweep along the length of the gut. The rhythm varies in different gut segments and is established by pacemaker cells known as interstitial cells of the Cajan. This basis electrical rhythm provides for sites of muscle contraction when stimuli superimpose spike potentials on the depolarizing portion of the BER waves. In the period between meals the intestine is relatively quiescent, but every 90 minute or so it is swept through by a large peristaltic wave triggered by the hormone motility. This migrating motor complex presumably serves a '' housekeeping'' function. Swallowing is triggered centrally and is coordinated with a peristaltic wave along the length of the esophagus that drives the food bolus to the stomach, even against gravity. Relaxation of the lower esophageal sphincter is timed to just precede the arrival of the bolus, there by limiting reflux of the gastric contents. Nevertheless, gastrointestinal reflux disease is one of the most common gastrointestinal complaints. The stomach accommodates the meal by a process of receptive relaxation. This permits an increase in volume without a significant increase in pressure. The stomach then serves to mix the meal and to control its delivery to downstream segments. Luminary contents move slowly through the colon, which enhances water recovery. Distension of the rectum causes reflex contraction of the internal anal sphincter and the desire to defecate. After toilet training, defecation can be delayed till a convenient time via voluntary contraction of the external anal sphincter.

Regulation of gastric secretion

Gastric motility and secretion are regulated by neural and humeral mechanisms. The neural components are local autonomic reflexes, involving choline neurons, and impulses from the CNS by way of the vague nerves. The humeral components are gastric enterogastrone. Va gal stimulation increases gastric secretion by release of gastric-releasing peptide. Other Val fibers release acetyl choline, which acts directly on the cells in the glands in the body and the fund us to increase acid and pepsin secretion. Stimulation of the vague nerve in the chest or neck increases acid and pepsin secretion but vasectomy does not abolish the secretory response to local stimuli. For convenience, the physiologic regulation of gastric secretion is usually discussed in terms of cephalic, gastric, and intestinal influences, although these overlap. The cephalic influences are vaginally mediated responses induced by activity in the CNS. The gastric influences are primarily local reflex responses and responses to gastric. The intestinal influences are the reflex and hormone feedback effects on gastric secretion initiated from the mucosa of the small intestine.                

Secretions of the large intestine
1.Mucus secretion : The mucosa of the large intestine, like that of the small intestine, has many crypts of Lieberman, but in this mucosa, unlike that of the small intestine, there are no villi. Also, the epithelial cells contain almost cells that secrete only mucus.Therefore, the great preponderance of secretion in the large intestine is mucus. This mucus contains large amounts of bicarbonate ions caused by active transport through other epithelial cells that lie between the mucus-secreting epithelial cells.
Function of mucus : Mucus in the large intestine protects the wall against excoriation, but in addition, it provides the adherent medium for holding fecal matter together. Furthermore, it protects the intestinal wall from the great amount of bacterial activity that take place inside the feces, and it plus the alkalinity of the secretion ( pH of 8.0 caused by large amounts of sodium bicarbonate ) provides a barrier to keep acids formed deep in the feces from attacking the intestinal wall.Regulation of secretion of mucus : The rate of secretion of mucus is regulated principally by direct, tactile stimulation of the mucous is regulated principally by direct, tactile stimulation of the mucous cells on the surface of the mucosa and by local
nervous reflexes to the mucous cells in the crypts of Lieberman. Stimulation of the pelvic nerves, which carry the parasympathetic innervation to the distal one half to two thirds of the large intestine, also causes marked increase in the secretion of mucus.This occurs along with an increase in motility. Therefore, during extreme parasympathetic stimulation, often caused by emotional disturbances, so much mucus may be secreted into the large intestine that the person has a bowel movement of ropy mucus as often as every 30 minutes ; this mucus contains little or no fecal material.
2.Secretion of water and electrolytes in response to irritation : Whenever a segment of the large intestine becomes intensely irritated, as occurs when bacterial infection becomes intensely irritated, as occurs when bacterial infection becomes rampant during enteritis, the mucosa secretes large quantities of water and electrolytes in addition to dilute the irritating factors and to cause rapid movement of the feces toward the anus. The usual result is diarrhea, with loss of large quantities of water and electrolytes in addition to the normal viscid solution of alkaline mucus. This acts to the dilute the irritating factors and to cause rapid movement of the feces toward the anus. The usual result is diarrhea, with loss of large quantities of water electrolytes. But the diarrhea also washes away the irritant factor, which promotes earlier recovery from the disease than would otherwise occur.