The Stomach PDF

Dr P De Gabriele MD DipWH(ICGP) FMCFD The Stomach  Cardia  This surrounds the lower oesophageal sphincter.  Fundus  the rounded portion above and to the left of the cardia  Body  the large central portion of the stomach  Pylorus  the narrow inferior region of the stomach that...

Dr P De Gabriele MD DipWH(ICGP) FMCFD The Stomach  Cardia  This surrounds the lower oesophageal sphincter.  Fundus  the rounded portion above and to the left of the cardia  Body  the large central portion of the stomach  Pylorus  the narrow inferior region of the stomach that communicates with the duodenum of the small intestine via a sphincter called the pyloric sphincter The Stomach Linings  The stomach wall is composed of the same 4 basic layers as the rest of the GIT, with certain modifications.  The Mucosa  The mucosa lies in large folds called rugae when the stomach is empty.  It is a layer of simple columnar epithelium containing many narrow openings that extend down into the lamina propria.  These pits are the gastric glands. Gastric Glands The gastric glands are lined with several kinds of secreting cells:  ZYMOGENIC (PEPTIC) CELLS  These secrete the enzyme precursor, pepsinogen.  PARIETAL (OXYNTIC) CELLS  These secrete hydrochloric acid and intrinsic factor.  MUCOUS CELLS  These secrete mucus.  ENTEROENDOCRINE CELLS  These secrete stomach gastrin. Gastric Glands  The Submucosa  This is composed of loose areolar connective tissue, which connects the mucosa to the muscularis.  The Muscularis  Unlike other areas in the GI tract, the muscularis has 3 layers of smooth muscle:  an outer longitudinal layer  a middle circular layer  an inner oblique layer  This arrangement allows the stomach to contract in a variety of ways to churn food, break it into small particles, mix it with gastric juice and pass it into the duodenum. GASTRIC JUICE  Gastric juice is a collection of secretions of  the zymogenic (peptic) cells  the parietal cells  the mucous cells  2500 ml of gastric juice are produced daily. Composition of Gastric Juice  Cations – Na+, K+, Mg2+, H+  Anions – Cl-, HPO42-, SO42-  Pepsins I – III  Gelatinase  Mucus  Intrinsic factor – This is involved in the absorption of vitamin B12 for red cell production.  Water  Hydrochloric acid Hydrochloric Acid  This is found within gastric juice from the combination of H+ and Cl- ions.  Hydrochloric acid converts pepsinogen to the active enzyme pepsin. The glands in the body of the stomach secrete it.  It has the following functions:  kills many ingested bacteria  aids protein digestion  stimulates the flow of bile and pancreatic juice Regulation of Gastric Secretion  The secretion of gastric juice is regulated by both nervous and hormonal mechanisms.  Parasympathetic impulses from nuclei in the medulla are transmitted via the vagus (X) nerves and stimulate the gastric glands to secrete  Pepsinogen,  Hydrochloric acid,  Mucus  Stomach gastrin (This is also secreted by gastric glands in response to certain foods that enter the stomach.)  Emotions such as anger, fear and anxiety may slow down digestion in the stomach because they stimulate the sympathetic nervous system. Stimulation of Gastric Secretion  Regulation is divided into 3 phases:  CEPHALIC (REFLEX) PHASE  GASTRIC PHASE  INTESTINAL PHASE  This phase produces relatively small amounts of gastric juice. Cephalic Phase SIGHT, SMELL & TASTE OF FOOD  CEREBRAL CORTEX or FEEDING CENTRE IN THE HYPOTHALAMUS  MEDULLA  Impulses over the parasympathetic fibres in the VAGUS NERVE  GASTRIC GLANDS secrete. Gastric Phase  Once the food reaches the stomach, both nervous and hormonal mechanisms ensure that gastric secretion continues. DISTENSION  RECEPTORS in the wall of the stomach  Impulses to the MEDULLA and back to the gastric glands  Stimulation of the flow of gastric juice  Protein food and alcohol stimulate the pyloric mucosa to secrete the hormone stomach gastrin that:  stimulates the gastric glands to secrete large amounts of gastric juice  contracts the lower oesophageal sphincter  increases the motility of the GIT  relaxes the pyloric sphincter and ileocaecal sphincter. Intestinal Phase Partially digested proteins in the duodenum  Duodenal mucosa  Release of ENTERIC GASTRIN  GASTRIC GLANDS continue their secretion Inhibition of Gastric Secretion  The presence of food in the small intestine during the intestinal phase initiates an enterogastric reflex.  Nerve impulses carried to the medulla from the duodenum return to the stomach and inhibit gastric secretion.  These impulses ultimately inhibit parasympathetic stimulation and stimulate sympathetic activity.  Stimuli that initiate this reflex are:  Distension of the duodenum  The presence of acid or partially digested proteins in food in the duodenum  Irritation of the duodenal mucosa Hormonal Inhibition of Gastric Secretion  The intestinal mucosa secretes hormones that inhibit gastric secretion and decrease motility of the GIT:  Secretin  Cholecystokinin (CCK)  Gastric inhibiting peptide (GIP)  Secretion of these hormones is stimulated by:  Presence of acid or partially digested proteins  Fats  Hypertonic or hypotonic fluids  Irritating substances in chyme Digestion in the Stomach  Digestion in the stomach occurs in 2 ways:  MECHANICALLY  CHEMICALLY  Mechanical Digestion  Several minutes after food enters the stomach, gentle, ripping peristaltic movements called mixing waves pass over the stomach every 15 to 25 seconds.  These waves  macerate food,  mix it with the secretions of the gastric glands,  reduce it to a thin liquid called chyme.  Foods remain in the fundus of the stomach for an hour or more without being mixed with gastric juice. During this time, salivary digestion continues.  As digestion proceeds in the stomach, more vigorous mixing waves begin at the body of the stomach and intensify as they reach the pylorus.  At the pylorus, each mixing wave forces a small amount of the gastric contents into the duodenum through the slightly open pyloric sphincter.  Most of the food is forced back into the body of the stomach, where it is subjected to further mixing.  Chemical Digestion  The principal chemical activity of the stomach is to start the digestion of proteins.  In the adult, digestion of proteins is primarily achieved through the enzyme pepsin. How does pepsin work?  Pepsin is most effective in the very acidic environment of the stomach (pH 2).  Pepsin breaks certain peptide bonds between the amino acids making up proteins.  The zymogenic cells produce pepsinogen.  The parietal cells secrete hydrochloric acid.  When pepsinogen comes into contact with the hydrochloric acid, it is converted into active pepsin.  Mucus protects the stomach mucosa from being digested by pepsin. Further Chemical Digestion  Gastric lipase is also present in the stomach.  This operates best at pH 5 to 6.  It has a limited role of splitting the butterfat molecules found in milk.  The infant stomach also secretes rennin, which is important in the digestion of milk.  Rennin and calcium act on the casein of milk to produce a curd.  The coagulation prevents too rapid passage of milk from the stomach. Regulation of Gastric Emptying  Gastric emptying is stimulated by 2 principal factors:  Nerve impulses in response to distension  Stomach gastrin releases in the presence of certain types of food  The stomach empties all its contents into the duodenum within 2 to 6 hours after ingestion.  Foods rich in carbohydrates spend the least time in the stomach.  Protein foods are somewhat slower.  Emptying is slowest after a meal containing large amounts of fat. Stimulation of Gastric Emptying  Inhibition of Gastric Emptying  Stomach emptying is inhibited by  the enterogastric reflex  the hormones  secretin, CCK and GIP  The rate of stomach emptying is limited to the amount of chyme that the small intestine can process. The diagram below summarises the factors that inhibit gastric emptying. Inhibition of Gastric Emptying