Physiology of stomach and absorption

phases of gastric secretion

1. Cephalic phase 2. gastric phase 3. intestinal phase

What does the duodenum monitor?

1. Degree of distention of the duodenum 2. Presence of any degree of irritation of the duodenal mucosa 3. Degree of acidity of the duodenal chyme 4. degree of osmolarity of the chyme 5. presence of certain breakdown products in the chyme ( proteins and fats)

Motor functions of the stomach (3)

1. Storage of large quantities of food until the food can be processed in the stomach, duodenum, and lower intestinal tract. 2. Mixing of this food with gastric secretions until it forms a semifluid mixture called chyme. 3. Slow emptying of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.

Oxyntic (Gastric) Glands is composed of what kinds of cells

1. mucous neck cells: mucus 2. peptic/chief cells: secrete large quantities of pepsinogen 3. parietal cells: secrete HCL and intrinsic factor

What do these 3 things stimulate?

AcH released by the parasympathetic stimulation excites secretion of pepsinogen by peptic cells, HCL by parietal cells, and mucus from mucous cells. Gastrin and Histamine strongly stimulate HCL by parietal cells but nothing else.

Basic factors that stimulate gastric secretion are what 3 things?

Acetylcholine, gastrin, histamine

Storage function of the stomach

As food enters the stomach, it forms concentric circles of the food in the orad portion of the stomach, the newest food lying closest to the esophageal opening and the oldest food lying nearest the outer wall of the stomach. Normally, when food stretches the stomach, a "vagovagal reflex" from the stomach to the brain stem and then back to the stomach reduces the tone in the muscular wall of the body of the stomach so that the wall bulges progressively outward, accommodating food- completely relaxed stomach 1.5 liters. The pressure in the stomach remains low until this limit is approached.

Brunner's Glands

Compound mucus glands Secrete mucus to protect the dueodenal wall. The mucus contains bicarbonate, which helps to neutralize HCl Brunner's glands are in the wall of the first few cm of duodenum. They are located between the pyloris of the stomach and papilla of Vater (where pancreatic secretion and bile empty into duodenum).

Gastrin effect on stomach empyting

Digestive products of meat elicit the release of hormone gastrin from the antral mucosa. This has potent effects to cause secretion of highly acidic gastric juice by the stomach glands and mild to moderate stimulatory effects on motor functions in the body of the stomach. Most important, it enhances the activity of the pyloric pump.

Parietal cells of the oxyntic glands are the only cells that secrete HCL, the parietal cells operate in close association with what other type of cell?

Enterochromaffin like cells (ECL cells) whose primary function is to secrete histamine. The ECL cells lie in the deep recesses of the glands and therefore release histaime in direct contact with the parietal cells of the glands. The rate of formation and secretion of HCL by the parietal cells is directly related to the amount of histamine secreted by the ECL cells. ECL cells are stimulated to secrete histamine by the hormonal substance gastrin, which is formed almost entirely in the antral portion of the stomach in response to proteins in foods.

Digestive enzymes in small intestine

Enterocytes of mucosa contain digestive enzymes that digest specific food substances while they are being absorbed through epithelium. 1. Several peptidases are present in the small intestine for splitting small peptides into amino acids 2. Four enzymes are present for splitting disaccharides into monosaccharides: i. Sucrase, maltase, isomaltase and lactase 2. Small amounts of intestinal lipase are present for splitting neutral fats into glycerol and fatty acids

CCK effect on stomach emptying

Fat inhibits gastric empyting by stimulating the release of CCK. CCK is released from the mucosa of the jejunum in response to fatty substances in the chyme. This hormone acts as an inhibitor to block increased stomach motility caused by gastrin.

Mixing and Propulsion of food in the stomach

Gastric glands secrete digestive juices in the stomach. These secretions come immediately into contact with the food that is lining the stomach. As long as food is in the stomach, weak peristaltic *constrictor waves* called *mixing waves* begin in the mid to upper portions of the stomach wall and move toward the antrum about once every 15-20 seconds. These waves are initiated by the gut wall *basic electrical rhythm*, consisting of electrical slow waves, that occur spontaneously in the stomach wall. As the constrictor waves progress from the body of the stomach into the antrum they become more intense, some becoming extremely intense and providing powerful *peristaltic action potential*- driven constrictor rings that force the antral contents under higher and higher pressure toward the pylorus. Each time a perstaltic wave passes down the antral wall toward the pylorus, it digs deeply into the food contents in the antrum. Yet the opening of the pylorus is still small enough that only a few mL or less of antral contents are expelled into the small intestine with each wave. Also as each wave approaches the pyloric muscle itself contracts, which further impedes emptying, so most of the contents is squeezed from the bottom of the stomach up not through pylorus=MIXING.

Stimulation of acid secretion by gastrin

Gastrin is a hormone secreted by G cells in the pyloric glands in the distal end of the stomach. When protein containing foods reach the antral end of the stomach, some of the proteins from these foods have a special stimulatory effect on the gastrin cells in the pyloric glands to cause release of gastrin into the blood to be transported to the ECL cells of the stomach: causing release of histamine directly into the deep oxyntic glands. The histamine then acts quickly to stimulate gastric HCL secretion from parietal cells.

What are hunger pangs?

Hunger Contractions occur when the stomach is empty for several hours or more. They are rhythmical peristaltic contractions of the body of the stomach. When the successive contractions become extremely strong they often fuse to cause a continuing tetanic contraction that sometimes last 2-3 minutes. Most intense in young healthy people with high tone, and greatly increased with lower than normal levels of blood sugar. Mild pain=hunger pangs. Usually don't begin until 12-24 hours after last ingestion-greatest intensity at 3-4 days.

The effect of gastric food volume on the rate of stomach empyting

Increased food volume in the stomach promotes stomach emptying because stretching of the stomach wall elicits local myenteric reflexes in the wall that greatly accentuate activity of the pyloric pump and at the same time inhibit the pylorus.

Pyloric Pump

Intense atral peristaltic contractions during stomach empyting.

Secretion of intrinsic factor

Intrinsic factor is essential for the absorption of B12 in the ileum, and is secreted by parietal cells along with HCL. B12 helps RBCs mature->stimulates bone marrow

Cephalic phase of gastric secretion

Occurs before food enters the stomach, results from smell/sight/touch/taste of food. Neurogenic signals originate in the cerebral cortex and in appetite centers of the amygdala and hypothalamus. They are then transmitted through the dorsal motor nuclei of vagi and then through the vagus nerve to the stomach. This accounts for 30% of gastric secretions for a meal.

Gastric phase of gastric secretion

Occurs once the food enters the stomach it excites: 1. long vagovagal reflexes from stomach to brain and back to stomach 2. local enteric reflexes 3. gastrin mechanism all causes secretion of gastric juices during several hours while food remains in the stomach. Accounts for 60% of total gastric secretions for a meal.

Secretion of pepsinogen

Pepsinogen is secreted by chief cells (peptic cells) when pepsinogen first secreted it has no digestive activity. HCL activates pepsinogen to pepsin. Pepsin functions as an active proteolytic enzyme in a highly acidic medium (optimum 1.8-3.5) doesn't work if pH above 5-HCL is necessary for its function.

Intestinal phase of gastric secretions

Presence of food in upper intestine (duodenum) will continue to cause stomach secretion of small amounts of gastric juice, small amount of gastrin release by duodenum. Accounts for 10% of acid in response to a meal.

Inhibitory effect of enterogastric nervous reflexes from the duodenum

Signals from the duodenum strongly inhibit the "pyloric pump" propulsive contractions AND increase the tone of the pyloric sphincter.

Pyloric Glands

Structurally similar to oxyntic glands with few peptic cells and no parietal cells. Contain mostly mucus cells. Secrete mainly mucus for protection of pyloric mucosa from stomach acid. Also secrete gastrin (controls gastric secretion) from G cells (gastrin cells). Located on antral portion of stomach

Small intestine: secretion of the watery fluid. Secretion of watery fluid occurs by an unclear mechanism. Secretion involves at least two active secretory processes:

The exact mechanism that controls the marked secretion of watery fluid by the crypts of Leiberkuhn is still unclear. 1. Active secretion of Cl- ions into the crypts 2. active secretion of bicarb ions. The secretion of both ions causes electrical drag of positively charged sodium ions through the membrane and into the secreted fluid as well. Finally, all theses ions together cause osmotic movement of water into the lumen of the small intestine.

What are the reflexes especially sensitive to?

The inhibitory reflexes of the duodenum are especially sensitive to the presence of irritants and acids in the duodenal chyme- become activated in 30 seconds. 1. Whenever the pH of the chyme falls below 3.5 to 4, the reflexes frequently block further release of acidic stomach contents in the duodenum until the duodenal chyme can be neutralized 2. Breakdown products of protein digestion also elicit inhibitory enterogastric reflexes, by slowing the rate of stomach emptying, giving enough time for adequate protein digestion in the small intestine. 3. Either hypotonic or hypertonic fluids elicit inhibitory reflexes. Thus too rapid flow of nonisotonic fluids into the small intestine is prevented, thereby also preventing rapid changes in electrolyte concentrations in the whole body ECF during absorption of the intestinal contents.

Role of pylorus in controlling stomach empyting

The pylorus remains slightly contracted all the time. The pylorus is usually open enough for water and other fluids to empty from the stomach to the duodenum with ease. Conversely, the constriction usually prevents passage of food particles until they have become mixed in the chyme to almost fluid consistency. The degree of constriction of the pylorus is increased or decreased under the influence of nervous and humoral reflex signals from both the stomach and duodenum.

Why are the secretions and absorption by the enterocytes so important?

The secretions are almost pure ECF and have slightly alkaline pH. The secretions are rapidly absorbed by the villi. This flow of fluid from the crypts into the villi supplies a watery vehicle for absorption of substances from chyme when it comes in contact with the villi. The primary function of the small intestine is to absorb nutrients and their digestive products into the blood.

Basic anatomy of the stomach

The stomach is divided into 2 major parts (anatomically): 1. The body 2. The antrum The stomach is divided into 2 major parts physiologically: 1. Orad portion, first 2/3 of the body. 2. Caudad portion, remainder of the body plus the antrum.

Tubular glands of stomach

The stomach mucosa has 2 important types of tubular glands: 1. oxyntic (gastric glands): acid forming, secrete HCL, pepsinogen, intrinsic factor, and mucus 2. Pyloric glands: mainly mucus for protection and gastrin

Crypts of Lieberkühn

These are small pits that lie between intestinal villi in the small intestine. These crypts are located over the entire surface of the small intestine.

Surfaces of crypts and villi are covered by epithelium with two types of cells:

i. Goblet cells - secrete mucus that lubricates and protects the intestinal surfaces ii. Enterocytes: which in the crypts secrete large quantities of water and electrolytes and over the surfaces of the villi reabsorb water and electrolytes along with end products of digestion

Brunner's glands secrete large amounts of alkaline mucus in response to:

i. Tactile or irritating stimuli of duodenal mucosa ii. Vagal stimulation iii. Gastrointestinal hormones, especially secretin

Regulation of small intestine secretion is by

local stimuli i. Local enteric nervous reflexes 1. Initiated by tactile or irritative stimuli from chyme in intestines

Mucus-secreting cells in stomach

secrete large quantities of viscid mucus (coats the stomach) giving protection and lubrication for food transport.

Brunner's glands are inhibited by

sympathetic stimulation Excitable people (medical students/stressed people) → increased inhibition of brunner's glands → unprotected duodenum → enhanced susceptibility to peptic ulcers due to lack of mucus So do not stress!