Physiology of Gastrointestinal Tract PDF

PHYSIOLOGY OF GASTROINTESTINAL TRACT  Favourite food ?  The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients. To achieve this requires: (1) movement of food through the alimentary tract; (2) secretion of digestive juices and digest...

PHYSIOLOGY OF GASTROINTESTINAL TRACT  Favourite food ?  The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients. To achieve this requires: (1) movement of food through the alimentary tract; (2) secretion of digestive juices and digestion of the food; (3) absorption of water, various electrolytes, vitamins, and digestive nutritive products; (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances; and (5) control of all these functions by local, nervous, and hormonal systems  The gastrointestinal system comprises the organs from mouth to anus that form the gastrointestinal tract, the pancreas, which mainly secretes digestive juices into the small intestine, and the liver and biliary system, which perform vital metabolic functions in addition to their contribution to digestion and absorption of nutrients  The mouth and teeth are the first structures in this tract and are connected by a powerful muscular tube, the esophagus, to the stomach. The mouth and teeth admit food into the gastrointestinal tract. They cut and break large pieces, chop, grind and moisten what can be chewed, and prepare a smooth, round bolus that can be swallowed and passed onto the rest of the system. Salivary glands Lubrication is essential for chewing and the formation of a bolus of food that can be easily swallowed. The three main pairs of salivary glands are the parotid, submandibular and sublingual glands there are many smaller, unnamed glands lining the mouth. The larger glands have main ducts that transport the saliva to the oral cavity Function One to two liters of saliva are secreted each day and almost all is swallowed and reabsorbed. Secretion is under autonomic control. Food in the mouth stimulates nerve fibers that end in the nucleus of the tractus solitaries and, in turn, stimulate salivary nuclei in the mid-brain. Salivation is also stimulated by sight, smell and anticipation of food through impulses from the cortex acting on brainstem salivary nuclei. Intense sympathetic activity inhibits saliva production, which is why nervous anxiety causes a dry mouth  Saliva is composed of water and mucins, forms a gel-like coating over the oral mucosa and lubricates food Saliva also contains a-amylase, which begins the process of carbohy- drate digestion, although its overall contribution is probably minor. Saliva contains antibacterial enzymes, such as lysozyme, and immunoglobulins that may help to prevent serious infection, and maintain control of the resident bacterial flora of the mouth The tongue and pharinx The pharynx is a conduit for air, food and drink, and swallowing Oesophagus The oesophagus is a muscular tube, beginning at the pharynx and ending at the stomach. It traverses the neck and thorax, where it lies close to the trachea, the great vessels and the left atrium of the heart. Just above the gastro-oesophageal junction, the oesophagus traverses a natural hiatus or gap in the diaphragm, to enter the abdomen  The vagus nerve runs alongside the oesophagus and innervates oesophageal muscle directly and via intrinsic nerves in the myenteric nerve plexus Function The oesophagus conveys food, drink and saliva from the pharynx to the stomach, by peristalsis. Peristalsis comprises a coordinated wave of contraction behind the bolus of food, with relaxation ahead of it, propelling the food bolus forward. It is involuntary, resulting from intrinsic neuromuscular reflexes in the intestinal wall, independent of extrinsic innervation Ingestion of Food The amount of food that a person ingests is determined principally by intrinsic desire for food called hunger. The type of food that a person preferentially seeks is determined by appetite. The current discussion of food ingestion is confined to the mechanics of ingestion, especially mastication and swallowing. Mastication (Chewing) The teeth are admirably designed for chewing, the anterior teeth (incisors) providing a strong cutting action and the posterior teeth (molars), a grinding action. Chewing is important for digestion of all foods, but especially important for most fruits and raw vegetables because these have indigestible cellulose membranes around their nutrient portions that must be broken before the food can be digested. Digestive enzymes act only on the surfaces of food particles; therefore, the rate of digestion is absolutely dependent on the total surface area exposed to the digestive secretions Swallowing Swallowing is a complicated mechanism, principally because the pharynx subserves respiration as well as swallowing and respiration should not be compromised because of swallowing. primary peristalsis Is simply continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing. This wave passes all the way from the pharynx to the stomach in about 8 to 10 seconds. Food swallowed by a person who is in the upright position is usually transmitted to the lower end of the esophagus even more rapidly than the peristaltic wave itself, in about 5 to 8 seconds, because of the additional effect of gravity pulling the food downward. The secondary peristaltic waves If the primary peristaltic wave fails to move into the stomach all the food that has entered the esophagus, Then there is secondary peristaltic waves result from distention of the esophagus itself by the retained food; these waves continue until all the food has emptied into the stomach. The musculature of the pharyngeal wall and upper third of the esophagus is striated muscle. Therefore, the peristaltic waves in these regions are controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves. In the lower two thirds of the esophagus, the musculature is smooth muscle, but this portion of the esophagus is also strongly controlled by the vagus nerves acting through connections with the esophageal myenteric nervous system. Propulsion and mixing of food in the alimentary tract For food to be processed optimally in the alimentary tract, the time that it remains in each part of the tract is critical. Also, appropriate mixing must be provided. Because the requirements for mixing and propulsion are quite different at each stage of processing, multiple automatic nervous and hormonal feedback mechanisms stomach The stomach is the first intra- abdominal intestinal organ. It is adapted for mechanical churning, storage and digestion of food and contributes to neuro-endocrine coordination of intestinal  Gastric glands are tubular structures with specialized cells for the production of: Parietal cells are found in glands throughout the fundus, corpus and antrum. They secrete HCl,the glycoproteins intrinsic factor and gas- troferrin, which facilitate the absorption of vitamin B12 and iron, When stimulated, the parietal cells secrete an acid solution that contains about 160 mmol/L of hydrochloric acid, which is nearly isotonic with the body fluids. The pH of this acid is about 0.8, demonstrating its extreme acidity Function Food is mixed thoroughly by the churning action of gastric muscle against a closed pyloric sphincter. The pylorus opens only to allow semi-liquid material (chyme) through into the duodenum, preventing the passage of large food particles Rhythmic electric activity in the stomach produces regular peristaltic waves three times a minute, known as the gastric slow wave. Gastric secretions stimulated by the anticipation of food, the so-called cephalic phase, and by food reaching the stomach, the gastric phase. Acetylcholine and histamine, acting through M2 muscarinic and H2 receptors stimulate the secretion of HCl.  Chief cells are found predominantly in the corpus. They secrete pepsinogen Main entero-endocrine cells of the stomach are G cells, producing gastrin, D cells, producing somatostatin, Phases of Gastric Secretion Gastric secretion is said to occur in three “phases” a cephalic phase, a gastric phase, and an intestinal phase. Cephalic Phase. The cephalic phase of gastric secretion occurs even before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food, and the greater the appetite, the more intense is the stimulation. Neurogenic signals that cause the cephalic phase of gastric secretion originate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus. They are transmitted through the dorsal motor nuclei through the vagus nerves to the stomach. This phase of secretion normally accounts for about 30 percent of the gastric secretion associated with eating a meal. Gastric Phase Once food enters the stomach, it excites long vagovagal reflexes from the stomach to the brain and back to the stomach, local enteric reflexes, and the gastrin mechanism, all of which in turn cause secretion of gastric juice during several hours while food remains in the stomach. The gastric phase of secretion accounts for about 60 percent of the total gastric secretion associated with eating a meal and therefore accounts for most of the total daily gastric secretion of about 1500 milliliters. Intestinal Phase The presence of food in the upper portion of the small intestine, particularly in the duodenum, will continue to cause stomach secretion of small amounts of gastric juice, partly because of small amounts of gastrin released by the duodenal mucosa. This accounts for about 10 percent of the acid response to a meal. duodenum The duodenum is the first major digestive and absorptive region of the intestine, receiving food- chyme from the stomach and mixing it with bile, pancreatic juice and enteric secretions  Alkaline bile and pancreatic juices neutralize stomach acid. Powerful enzymes from the pancreas, which are activated in the lumen by autocatalysis and by the action of enterokinase released from duodenal enterocytes, support rapid and efficient digestion. the final stages of digestion occur in the brush border of enterocytes under the action of disaccharidases and peptidases.  Bile salts emulsify fatty foods, allowing digestive enzymes to act more efficiently. Transport proteins in the apical membrane actively absorb sugars, amino acids and electrolytes into the enterocyte. Fatty acids and cholesterol enter by direct diffusion across the lipid membrane iron and calcium in particular are preferentially absorbed in the duodenum  The small intestine is relatively free from resident bacteria and antimicrobial environment is maintained by the action of gastric acid and antibacterial substances produced by Brunner’s glands and Paneth cells. Entero-endocrine cells in the duodenum secrete cholecystokinin and secretinin response to food, stimulating gallbladder contraction and pancreatic secretion, and inhibiting gastric motility. Thus, the duodenum participates in neuro-endocrine coordination of gastrointestinal function Pancreas The pancreas lies transversely on the posterior abdominal wall and is covered by peritoneum. The headlines to the right, adjacent to the duo-denum, and the body and tail extend across the epigastrium to the spleen. The main pancreatic duct extends along the length of the gland and a smaller accessory duct drains the superior part of the head and may open separately into the duodenum. The main duct joins the common bile duct before opening into the duodenum through the ampulla of Vatery Function The pancreas is a powerful producer of digestive enzymes. Pancreatic secretion is stimulated by hormonal signals, particularly cholecystokinin, which is released when food enters the duodenum. trypsin; chymotrypsin procarboxypeptidases A and B; pro-elastase; phospholipase A; pancreatic lipase (and colipase); pancreatic amylase The pancreas secretes about 2L/day of a bicarbonate-rich alkaline fluid that helps to neutralize stomach acid and provides optimal conditions for digestion by pancreatic enzymes.  Trypsin and chymotrypsin split whole and partially digested proteins into peptides of various sizes but do not cause release of individual amino acids. The pancreatic enzyme for digesting carbohydrates is pancreatic amylase, which hydrolyzes starches, glycogen, and most other carbohydrates (except cellulose) to form mostly disaccharides and a few trisaccharides. The main enzymes for fat digestion are pancreatic lipase, which is capable of hydrolyzing neutral fat into fatty acids and monoglycerides; cholesterol esterase, which causes hydrolysis of cholesterol esters; phospholipase, which splits fatty acids from phospholipids Liver Regulating homeostasis of carbohydrate, lipid and amino acid metabolism. Storing nutrient ssuch as glycogen, fats and vitamins B12, Aand K. Producing and secreting plasma proteins and lipoproteins, including clotting factors and acute phase proteins. Synthesizing and secreting bile salts for lipid digestion. Detoxifying and excreting bilirubin, other endogenous waste products and exogenous metal ions, drugs and toxins Clearing toxins and infective agents from the portal venous blood whilst maintaining systemic immune tolerance to antigens in the portal circulation. In addition, hepatocytes retain the capacity to proliferate, so that the liver can regenerate dramatically after injury.  One of the many functions of the liver is to secrete bile, normally between 600 and 1000 ml/day. Bile serves two important functions. First, bile plays an important role in fat digestion and absorption, Second, bile serves as a means for excretion of several important waste products from the blood. These include especially bilirubin, an end product biliary system Macroscopically, the intrahepatic bile ducts, common hepatic duct, cystic duct, gallbladder and common bile duct constitute the biliary system  Bile is secreted continually by the liver cells, but most of it is normally stored in the gallbladder until needed in the duodenum. The maximum volume that the gallbladder can hold is only 30 to 60 milliliters. When food begins to be digested in the upper gastrointestinal tract, the gallbladder begins to empty, especially when fatty foods reach the duodenum about 30 minutes after a meal  Emulsifying function, bile salts help in the absorption of fatty acids, monoglycerides, cholesterol, and other lipids from the intestinal tract. They do this by forming small physical complexes with these lipids; the complexes are called micelles, and they are semisoluble in the chyme because of the electrical charges of the bile salts. The intestinal lipids are “ferried” in this form to the intestinal mucosa, where they are then absorbed into the blood, Without the presence of bile salts in the intestinal tract, up to 40 percent of the ingested fats are lost into the feces and the person often develops a metabolic deficit because of this nutrient loss Jejunum and ileum The main function is apsorption The jejunum begins at the junction with the duodenum at the ligament of Treitz and measures about 3.5· m. The ileum comprises the most distal 2.5· m of small intestine, terminating in the caecum. The jejunum and ileum are attached to the posterior abdominal wall by a long mesentery that allows free movement and rotation  The total quantity of fluid that must be absorbed each day by the intestines is equal to the ingested fluid (about 1.5 liters) plus that secreted in the various gastrointestinal secretions (about 7 liters). This comes to a total of 8 to 9 liters. All but about 1.5 liters of this is absorbed in the small intestine, leaving only 1.5 liters to pass through the ileocecal valve into the colon each day The combination of the folds of Kerckring, the villi, and the microvilli increases the total absorptive area of the mucosa , making a tremendous total area of 250 or more square meters for the entire small intestine—about the surface area of a tennis court Absorption from the small intestine each day consists of several hundred grams of carbohydrates, 100 or more grams of fat, 50 to 100 grams of amino acids, 50 to 100 grams of ions, and 7 to 8 liters of water. The absorptive capacity of the normal small intestine is far greater than this: as much as several kilograms of carbohydrates per day, 500 grams of fat per day, 500 to 700 grams of proteins per day, and 20 or more liters of water per day. The large intestine can absorb still additional water and ions, although very few nutrients. Absorption of Nutrients Carbohydrates Are Mainly Absorbed as Monosaccharides Essentially all the carbohydrates in the food are absorbed in the form of monosaccharides; only a small fraction is absorbed as disaccharides and almost none as larger carbohydrate compounds. The most abundant of the absorbed monosaccharides is glucose, usually accounting for more than 80 percent of carbohydrate calories absorbed. The remaining 20 percent of absorbed monosaccharides is composed almost entirely of galactose and fructose, the galactose derived from milk and the fructose as one of the monosaccharides digested from cane sugar. Monoccaharides are co-transported with sodium  Absorbed monosaccharides leave the enterocyte by facilitated diffusion, through selective channels in the basolateral surface. They then enter the circulation via the rich capillary network in the villus Absorption of Proteins as Dipeptides, Tripeptides, or Amino Acids Protein digestion begins in the stomach with the action of pepsin, although the pancreas secretes the bulk of important proteases. Trypsinogen, chymotrypsinogen and proelastase are endopeptidases that cleave at specific residues in the peptide chain,  Most proteins, after digestion, are absorbed through the luminal membranes of the intestinal epithelial cells in the form of dipeptides, tripeptides, and a few free amino acids. Most peptide or amino acid molecules bind in the cell’s microvillus membrane with a specific transport protein that requires sodium binding before transport can occur. After binding, the sodium ion then moves down its electrochemical gradient to the interior of the cell and pulls the amino acid or peptide along with it using the mechanism of co-transport (or secondary active transport)  Enterocyte-derived peptidases complete the digestion of peptides, producing single amino acids and di- and tripeptides that are absorbed. Amino acids enter enterocytes along with Na+ ions From the cytoplasm, amino acids enter the circulation via selective channels, and are carried to the circulation Absorption of Fats Unlike carbohydrates and proteins, which are water soluble and therefore easily accessible to digestive enzymes and membrane transporters, lipids require partition into a hydrophobic or amphipathic environment.  The main dietary lipids are triglycerides, Lipases, phospholipases and cholesterol esterases, which are synthesized by the pancreas, break down dietary lipids to fatty acids, monoacyl glycerol, lysophospholipids cholesterol.  These digested lipids are absorbed across the cell membrane into the enterocyte cytoplasm where they are re-esterified and complexed with proteins called apolipoproteins to form lipid-rich lipoprotein particles known as chylomicrons. Chylomicrons are actively secreted into the circulation via the thoracic duct and empty into the circulating blood. Colon The colon is divided into four parts. The ascending colon the transverse colon, the descending colon the sigmoid colon, the rectum and anus colon The major function of the colon is to reabsorb water from the liquid intestinal contents remaining after digestion and absorption in the jejunum and ileum. This converts the faecal stream into a semisolid mass that is then excreted. Muscular action in the colon mixes and squeezes faecal matter and propels it toward the rectum The colon contains 1012 bacteria/g of its content, which are normal commensals. There are about 500 different species of bacteria, including lactobacilli, bifidobacteriae, bacteroides and enterobacteriacae. Most colonic bacteria are anaerobes. Some are potential pathogens Absorption in the Large Intestine Formation of Feces About 1500 milliliters of chyme normally pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this chyme are absorbed in the colon, usually leaving less than 100 milliliters of fluid to be excreted in the feces. Also, essentially all the ions are absorbed, leaving only 1 to 5mEq each of sodium and chloride ions to be lost in the feces.  Most of the absorption in the large intestine occurs in the proximal one half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for feces storage until a propitious time for feces excretion and is therefore called the storage colon. Absorption and Secretion of Electrolytes and Water The mucosa of the large intestine, like that of the small intestine, has a high capability for active absorption of sodium, the electrical potential gradient created by absorption of the sodium causes chloride absorption as well. The tight junctions between the epithelial cells of the large intestinal epithelium are much tighter than those of the small intestine. This prevents significant amounts of back-diffusion of ions through these junctions, thus allowing the large intestinal mucosa to absorb sodium ions far more  The mucosa of the large intestine secretes bicarbonate ions while it simultaneously absorbs an equal number of chloride ions in an exchange transport process that has already been described The bicarbonate helps neutralize the acidic end products of bacterial action in the large intestine. Absorption of sodium and chloride ions creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.  The large intestine can absorb a maximum of 5 to 8 liters of fluid and electrolytes each day. When total quantity entering the large intestine through the ileocecal valve or by way of large intestine secretion exceeds this amount, the excess appears in the feces as diarrhea. Bacterial Action in the Colon Numerous bacteria, especially colon bacilli, are present even normally in the absorbing colon. They are capable of digesting small amounts of cellulose, in this way providing a few calories of extra nutrition for the body Other substances formed as a result of bacterial activity are vitamin K, vitamin B12, thiamine, riboflavin, and various gases that contribute to flatus in the colon, especially carbon dioxide, hydrogen gas, and methane. The bacteria-formed vitamin K is especially important because the amount of this vitamin in the daily ingested foods is normally insufficient to maintain adequate blood coagulation. Composition of the Feces. The feces normally are about three-fourths water and one-fourth solid matter that is composed of about 30 percent dead bacteria, 10 to 20 percent fat, 10 to 20 percent inorganic matter, 2 to 3 percent protein, and 30 percent undigested roughage from the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells.  The brown color of feces is caused by stercobilin and urobilin, derivatives of bilirubin. The odor is caused principally by products of bacterial action; these products vary from one person to another, depending on each person’s colonic bacterial flora and on the type of food eaten. The actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide. THE END CNS regulation The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility (i.e., milling, absorption, secretion, and defecation),