APHY 201 Module 13 Digestive System Part 2 Ivy Tech

hepatopancreatic sphincter

(aka sphincter of Oddi) is a muscular valve that controls the flow of digestive juices (bile and pancreatic juice) through the ampulla into the duodenum. It is relaxed by the hormone cholecystokinin (CCK).

exocrine function of pancreas

Acini (clusters of secretory cells) secrete pancreatic juice stimulated by CCK though pancreatic duct into the lumen of the duodenum (A)

endocrine function of pancreas

Pancreatic islets secrete insulin (to lower blood sugar) and glucagon (to raise blood sugar) (B)

mass movement

The shift of some of the colonic faecal contents into the rectum in a single process occasioned by contraction of a long segment of colon. Mass movement is quickly followed by the desire to defaecate.

gastroileal reflex

This is stimulated by the presence of food stretching the stomach. Initiation of the reflex causes peristalsis in the ileum and the opening of the ileocecal valve (which allows the emptying of the ileal contents into the large intestine, or colon). This in turn stimulates colonic peristalsis and an urge to defecate.

increase surface area for nutrient absorption

What are the purpose of the structural modifications (circular folds, villi, microvilli) of the small intestine?

chylomicrons

_______________are lipoprotein coated lipid globules that are designed for the transport of lipids in the circulation. The intestinal cells absorb fatty acids and glycerol through the luminal membrane by diffusion. The intestinal cells produce recombine glycerol into triglycerides and cover with lipprotein then exocytose the ______________ through the basolateral membrane. They are absorbed into lacteals and travel through lymph before being transported to the blood.

feces

consist largely of water (75%) and undigested material (25%), mucus, and bacteria. About 30% of the solid waste is bacteria. The color of the feces is due to bile salts.

gastrin

hormone that stimulates gastric cell secretion of HCl, pepsinogen, gastric lipase, and enhances gastric motility

enterogastrones

hormones released from the small intestine to inhibit gastrin release. Ex: cholecystokinin (CCK) and secretin

intestinal phase

inhibitory effects (enterogastric reflex and enterogastrones) due to chyme with H+, fats, peptides, irritating substances in small intestine causing inhibition to slow gastric secretion and motility

intestinal crypts

intestinal equivalent of gastric pits; secretory cells that produce intestinal juice - enteroendocrine cells, goblet cells, Paneth cells, and stem cells divide to produce intestinal cells. (6)

intestinal juice

1-2 L secreted daily; slightly alkaline; largely water; enzyme-poor (enzymes of small intestine only in brush border); contains mucus. Facilitates transport and absorption of nutrients.

pancreatic juice

1200 - 1500 ml/day; Watery alkaline solution (pH 8) that neutralizes chyme; Electrolytes (primarily HCO3-); Enzymes: amylase, lipases, proteases secreted in inactive (zymogen) form and activated in duodenum. It is the only digestive organ that produces enzymes that can digest all manner of foodstuff.

gastric juice

3L of colorless, watery, acidic digestive fluid that is secreted each day by various glands in the mucous membrane of the stomach and consists chiefly of hydrochloric acid, pepsin, and mucin

micelles

After digestion, monoglycerides and fatty acids associate with bile salts and phopholipids to form ________. They are necessary because they transport the poorly soluble monoglycerides and fatty acids to the surface of the enterocyte where they can be absorbed.

bile

As both an excretory product and a digestive secretion, ______________ functions to emulsify fats and excrete bilirubin formed from the breakdown of old red-blood cells.

mucosal barrier

Due to harsh digestive conditions in stomach, this is produced to protect. Consists of a thick layer of bicarbonate-rich mucus, tight junctions between epithelial cells to prevent juice seeping underneath tissue, and damaged epithelial cells quickly replaced by division of stem cells (surface cells replaced every 3-6 days).

gastrocolic reflex

It stimulates an increase in motility of the colon in response to stretch in the stomach and byproducts of digestion in the small intestine. Thus, this reflex is responsible for the urge to defecate following a meal. The small intestine also shows a similar motility response.

small intestine

Major organ of digestion and absorption from pyloric sphincter to ileocecal valve. Subdivisions: duodenum, jejunum, and ileum.

What are the modifications to the layers of the stomach?

Muscularis externa modified to have three layers of smooth muscle (oblique, circular, and longitudinal). The inner oblique layer allows stomach to churn, mix, move, and physically break down food. Mucosa is modified with simple columnar epithelium composed of mucous cells that secrete two-layer coat of alkaline mucus. The surface layer traps bicarbonate-rich fluid beneath it for protection from acid. Mucosa also dotted with gastric pits (depressed areas) that produce gastric juice (2-3L per day) which consists mainly of water, HCl and pepsin.

regulation of gastric secretion

Neural control by parasympathetic (vagus nerve) stimulation increases secretion and motility and sympathetic stimulation decreases secretion and motility. Hormonal control largely gastrin. Gastrin enhances enzyme and HCl secretion and motility, whereas small intestine hormone secretions (secretin, CCK) inhibit gastrin.

enterogastric reflex

Neuronal reflex to decrease gastric secretion and motility by inhibiting parasympathetic fibers and stimulating sympathetic fibers. Causes decreased gastric activity to protect small intestine from excessive acidity. Part of intestinal phase of gastric secretion.

small intestine functions

Neutralization of acid chyme to alkali Completes the digestion of nutrients Absorbs the productions of digestion Transports the residues to the large intestine

intrinsic factor

Only stomach function essential to life is secretion of _____ _____ for vitamin B12 absorption. Made in the parietal cells. B12 needed to produce mature red blood cells. Lack of intrinsic factor causes pernicious anemia and must be treated with B12 injections.

enterokinase

Pancreatic zymogens are normally only activated after they reach the small intestine. A brush border enzyme called __________ cleaves a peptide from trypsinogen, forming the active enzyme trypsin. It then activates the other enzymes. ("Brush border" is another term for the microvilli at the apical surface of enterocytes, where brush border enzymes are located).

hepatocyte functions

Process bloodborne nutrients. Convert excess glucose to glycogen for storage. Synthesize glucose from amino acids and glycerol (gluconeogenesis). Convert fatty acids and glycerol to fat and cholesterol. Convert amino acids to plasma proteins. Store iron, copper, etc. Store fat-soluble vitamins (A, B, D, E, K). Perform detoxification. Filter toxic substances from the incoming blood. Detoxify alcohol, antibiotics, other drugs. Produce ~900 ml bile per day.

gastric glands

Produce gastric juice up to 3 L gastric juice/day from parietal cell secretions of hydrochloric acid (HCl) and intrinsic factor, and chief cell secretions of pepsinogen - inactive protein-digesting enzyme and gastric lipases.

pancreatic lipase

Released from acinar cells stimulated by CCK to digest fats in the lumen of the small intestine into glycerol and free fatty acids.

enteroendocrine cells of stomach

Secrete chemical messengers like gastrin into lamina propria to act as hormones

Enterogastrones

Secretin and cholecystokinin (CCK) are gastrin-inhibiting hormones released by small intestine to inhibit gastrin secretion causing decreased gastric enzymes and HCl secretion and inhibit gastric motility.

gastric phase

Stimulated by distension, protein, gastrin (major stimulus) to increase gastric secretion and motility

digestive processes in stomach

Store, deliver (fluid) chyme to small intestine; Mechanical breakdown; Denaturation of proteins by HCl; Enzymatic digestion of proteins by pepsin ; Lipid-soluble alcohol and aspirin absorbed into blood; Secretes intrinsic factor for vitamin B12 absorption.

receptive relaxation

Stretches to accommodate incoming food with pressure constant until 1.5 L food ingested. Reflex-mediated and coordinated by swallowing center of brain stem.

secretin, cholecystokinin

Substances (hormones) that inhibit gastric secretion and motility.

fats

The macronutrient that most significantly delays emptying of the stomach.

volume, fluidity

The rate of gastric emptying is directly related to the _______________________ and ___________________ of chyme.

secretin

a hormone released from enteroendocrine intestinal cells exposed to acidic chyme; It inhibits gastrin secretion from the stomach and causes secretion of (bicarbonate-rich) pancreatic juice by duct cells. Bicarbonate neutralizes the acid of the chyme as it enters the duodenum (where enzymes optimally function at a neutral pH).

cholecystokinin

a hormone released from enteroendocrine intestinal cells exposed to fatty chyme and protein. It inhibits gastrin secretion from the stomach and stimulates the gallbladder to contract to release bile. It also stimulates secretion of (enzyme-rich) pancreatic juice. The enzymes found in the lumen of the small intestine were produced in the pancreas. CCK also causes hepatopancreatic sphincter to relax so bile and pancreatic juice are released into the duodenum.

bile

a yellow-green, alkaline solution that emulsifies fats to aid in absorption of fatty acids, cholesterol, and fat-soluble vitamins

gastric lipase

an enzyme produced by chief cells in stomach to digest triglycerides

protease

an enzyme that breaks down proteins and peptides.

pepsin

an enzyme that breaks down proteins into smaller peptides (that is, a protease). It is produced in the stomach by the chief cells. The digestive power of pepsin is greatest at the acidity of normal gastric juice (pH 1.5-2.5). In the intestine the gastric acids are neutralized (pH 7), and pepsin is no longer effective.

zymogen

an inactive substance that is converted into an enzyme when activated by another enzyme. Digestive proteases are released in inactive forms called ________. This is necessary to prevent the digestive enzymes from digesting the cells that produce them. In this form, part of the protein blocks the active site of the enzyme. Cleaving off this peptide activates the enzyme.

cephalic phase

conditioned reflex triggered by aroma, taste, sight, thought to increase gastric secretion and motility

phases of gastric secretion

cephalic, gastric, and intestinal

bacterial flora of intestine

colonize colon and synthesize B complex vitamins and vitamin K

hydrochloric acid

denatures proteins in the stomach lumen to prepare them for digestion and kills bacteria that enter the stomach. It converts the inactive enzyme pepsinogen into the active enzyme pepsin, which is responsible for digesting proteins in your stomach. Ultimately, it is neutralized by bicarbonate secreted into the duodenum once the chyme passes through the pyloric sphincter.

ileum

end portion and longest section of small intestine that joins large intestine at ileocecal valve.

recycles

enterohepatic circulation _______ bile salts

lipid digestion

fatty acids and glycerol are absorbed with cholesterol and fat-soluble vitamins. Digestive enzymes involved are lingual lipase (mouth), gastric lipase (stomach), pancreatic lipase (most effective due to emulsification of fat by bile). Absorption into intestinal cells is by diffusion. Then recombined to form triglycerides and surrounded by protein to form a chylomicron. Chylomicrons are exocytosed and absorbed into lacteals (lymph vessels).

villi

fingerlike extensions of mucosa with capillary bed and lacteal for absorption

duodenum

first portion and shortest section of small intestine. Bile duct (from liver) and main pancreatic duct (from pancreas) meet and join at hepatopancreatic ampulla (controlled by hepatopancreatic sphincter). Opening the hepoatopancreatic sphincter (due to CCK) secretes bile and pancreatic juice into the lumen of the small intestine.

appendix

masses of lymphoid tissue that is part of the immune system. Acts as a bacterial storehouse to recolonizes gut when necessary like after a round of antibiotic.

microvilli (brush border)

microscopic cellular membrane protrusions that increase the surface area of cells and contain enzymes (produced by the small intestine) for carbohydrate and protein digestion

jejunum

middle portion of small intestine that extends from duodenum to ileum and is responsible for most digestion and absorption.

protein digestion

only amino acids can be absorbed. Digestive enzymes involved are pepsin (stomach), pancreatic proteases, and brush border peptidases (small intestine). Absorption mainly due to cotransport with Na+ into intestinal cells. Enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

liver

only digestive function is bile (a fat emulsifier) production

carbohydrate digestion

only monosaccharides can be absorbed. Digestive enzymes involved are salivary amylase (mouth), pancreatic amylase, and brush border enzymes (small intestine). Absorption mainly due to cotransport with Na+ into intestinal cells. Enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

defecation reflex

parasympathetic signals stimulate contraction of sigmoid colon and rectum; relax internal anal sphincter. Conscious control allows relaxation of external anal sphincter .

gastric emptying

peristaltic waves move toward pylorus at rate of 3 per minute. Distension and gastrin increase force of contractions. Chyme is either delivered in ~3 ml spurts through the pyloric sphincter into the duodenum, or forced backward into stomach.

circular folds

permanent folds that force chyme to slowly spiral through lumen to cause more nutrient absorption

bilirubin

pigment byproduct of the break down of heme (from hemoglobin) that is removed from blood by the liver and added to bile for excretion

parietal cells

produce hydrochloric acid (HCl) with a pH 1.5-3.5 to denature protein, activate pepsin, break down plant cell walls, kill many bacteria; Also produce intrinsic factor - a glycoprotein required for absorption of vitamin B12 in ileum of small intestine.

chief cells

produce pepsinogen - inactive enzyme (zymogen) activated to pepsin by HCl for protein digestion and gastric lipases that digest ~10-15% of lipids. (B)

enteroendocrine cells of intestine

produce the enterogastrones (CCK and secretin) which are hormones secreted into blood

release

the _____ of bile is stimulated by CCK from the small intestine

emulsification

the breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol. It is assisted by the action of the bile salts.

chyme

the watery, semidigested tiny pieces into which the bolus (food) is converted in the stomach and intestine

gallbladder

thin walled muscular sac that stores and concentrates bile; stimulated to contract by CCK