Digestive processes occurring inthe mouth, pharynx, andesophagus Mastication is the mechanical breakdown of food (chewing). Deglutition is the transport of food from the mouth to the stomach (swallowing).
The buccal phase of swallowing occurs inthe mouth, and involves the coordinatedmovements of the tongue and palate toforce food into the oropharynx.
The pharyngeal-esophageal phase ofswallowing occurs after the buccal phase,and includes blockage of the mouth by thetongue, rising of the soft palate and theepiglottis.
Digestive processes occurring inthe stomach Pepsin is the primary enzyme secreted by the stomach. Intrinsic factor is required for intestinal absorption of vitamin B12, which stimulates the production of erythrocytes.
Phases of gastric secretion The cephalic (reflex) phase The gastric phase The intestinal phase
The enterogastric reflex inhibits the vagalnuclei in the medulla, inhibits local reflexes,and activates sympathetic fibers that causethe pyloric sphincter to tighten and preventfurther food entry into the small intestine.
Gastric motility and emptying Receptive relaxation occurs in response to food entry, and causes the smooth muscles of the stomach to relax. Adaptive relaxation also occurs in response to food entry, but is initiated by stretch receptors in the wall of the stomach and involves nitric oxide.
Plasticity is the intrinsic ability of visceralsmooth muscle to exhibit the stress-relaxation response, which is to stretchwithout increasing tension.The basic electrical rhythm refers to the rateof the stomach’s peristaltic waves, which isapproximately three/minute, and is inducedby spontaneous pacemaker cells.
Emesis (vomiting) is stomach emptyingthrough the mouth due to stretching, or thepresence of irritants.The emetic center of the medulla initiatesthe contraction of skeletal muscles of theabdominal wall and diaphragm.
Hormone Site of Production Stimulus Target Organs ActivityGastrin Stomach mucosa Food Stomach Causes gastric glands to increase secretory activity Stimulates gastric emptying Small intestine Stimulates contraction of intestinal muscle Ileocecal valve Relaxes ileocecal valve Large intestine Stimulates mass movementsSerotonin Stomach mucosa Food Stomach Causes contraction of stomach muscleHistamine Stomach mucosa Food Stomach Activates parietal cells to release HCLSomatostatin Stomach mucosa & Food Stomach Inhibits gastric secretion, Duodenal mucosa motility, and emptying Pancreas Inhibits secretion Small intestine Inhibits GI blood flow Gallbladder Inhibits contractin & bile release
Hormone Site of Production Stimulus Target Organs ActivitySecretin Duodenal mucosa Acidic Stomach Inhibits gastric gland secretion chyme and motility Pancreas Increases output of pancreatic juice rich in bicarbonate ions Liver/pancreas Increases bile outputCholecystokinin Duodenal mucosa Fatty Liver Augments secretins actions chyme Pancreas Increases output of enzyme rich pancreatic juice Gallbladder Stimulates organ to contract and expel stored bile Hepatopancreatic Relaxes to allow entry of bile sphincter and pancreatic juice into duodenum
Intestinal juice: Composition andcontrol Intestinal juice has a pH similar to that of blood, is isotonic with blood plasma, and is composed of water, and mucus, but is enzyme poor.
Regulation of bile release intothe small intestine Gallstones can obstruct the flow of bile from the gall bladder, and form as a result of the crystallization of cholesterol due to either too much cholesterol or too few bile salts.
Digestive processes occurring inthe small intestine The migrating mobility complex is the pattern of peristaltic activity initiated in the duodenum, whose waves sweep along the intestine 10-70 cm before dying out. Enhanced activity of the stomach initiates the gastroileal reflex, which is a long reflex that enhances the force of segmentation in the ileum.
Digestive processes occurring inthe large intestine Haustral contractions are slow segmenting movements that occur every 30 minutes, stimulated by distension of its muscles. Mass movements are powerful contractile waves that move over large areas of the colon three or four times daily, and force the contents toward the rectum.
The gastrocolic reflex is the colon’sequivalent to the gastroileal reflex in thesmall intestine, and is responsible forinitiating mass movements.Diverticulitis is a result of lack of bulk inthe colon, causing small herniations of thecolon called diverticula which becomeinflamed.
Defecation reflex causes emptying of therectum due to stretching of the rectal wall asa result of mass movements.Diarrhea is watery stools, which resultsfrom any condition that rushes food residuethrough the large intestine before that organhas had sufficient time to absorb theremaining water.
When food remains in the colon forextended periods, too much water isabsorbed and the stool becomes hard anddifficult to pass, resulting in a conditioncalled constipation.
Mechanism of chemicaldigestion: Enzymatic hydrolysis Chemical digestion is a catabolic process in which large food molecules are broken down to monomers small enough to be absorbed by the GI tract lining. The enzymatic breakdown of any type of food molecule is called hydrolysis, due to the addition of a water molecule to each molecular bond broken.
Chemical digestion of specificfood groups The monomers of carbohydrates are called monosaccharides. Salivary amylase splits starch into smaller fragments of two to eight linked monosaccharides.
Carbohydrates that escape being brokendown by salivary amylase are acted on bypancreatic amylase.Dextrinase and glucoamylase are brushborder enzymes that act on polysaccharidescomposed of three simple sugars.
Maltase, sucrase, and lactase are brushborder enzymes that hydrolyze maltose,sucrose, and lactose respectively into theirconstituent monosaccharides.
Pepsin begins protein digestion in thestomach, when it is converted in an acidicenvironment, from pepsinogen.Rennin is a stomach enzyme that coagulatesmilk protein, but is not produced in adults.
Trypsin and chymotrypsin are pancreaticenzymes secreted into the duodenum thatcleave protein fragments into smallerpeptides.
Carboxypeptidase, secreted by both theintestinal brush border and the pancreas,splits off one amino acid at a time from theend of the polypeptide chain with thecarboxyl group.Aminopeptidase and dipeptidase are brushborder enzymes that liberate the final aminoacid products.
Lipases are fat-digesting enzymes secretedby the pancreas, that hydrolyze triglyceridemolecules, yielding free fatty acids andmonoglycerides.
Nucleic acids are hydrolyzed into theirnucleotide monomers by pancreaticnucleases.Nucleotides are broken down bynucleosidases and phosphatases, which areintestinal brush-border enzymes.
Absorption of specific nutrients Micelles are collections of fatty elements clustered together with bile salts in such a way that the polar ends of the molecules face the water and the nonpolar portions from the micelle core.
Chylomicrons are composed of triglyceridescombined with small amounts ofphospholipids and a cholesterol, and coatedwith proteins to form water-solublelipoprotein droplets.The triglycerides of chylomicrons arehydrolyzed to free fatty acids and glycerolby lipoprotein lipase in the bloodstream.
Iron binds with a protein called ferritin afterit is transported into mucosal cells, which islike a storehouse for iron.Iron binds with a protein called transferrinin the blood, which transport it in thecirculation.
Calcium absorption is regulated by theactive form of vitamin D, which acts as acofactor to facilitate calcium absorption.
Malabsorption of nutrients Malabsorption can result from anything that interferes with the delivery of bile or pancreatic juice to the small intestine, as well as factors that damage the intestinal mucosa.