A&P II CH 25: Digestive System 25.4 -

intestinal juice

- 1-2 L per day secreted by the intestinal crypts in response to acid, hypertonic chyme, and distension of the intestine - fluid has a pH of 7.4-7.8 contains water and mucus but relatively little enzyme

absorption and motility of the large intestine

- 12-24 hours to reduce residue of meal to feces - feces is usually 75% water and 25% solids - solids are 30% bacteria, 30% undigested fiber, 10-20% fats, and smaller amounts of protein, sloughed epithelial cells, salts, mucus, and other digestive secretions - motility by haustral contractions and mass movements triggered by gastrocolic and duodenocolic reflexes

exocrine part of the pancreas

- 99% of the pancreas which secretes 1,200 to 1,500 mL of pancreatic juice per day - cells of secretory acini exhibit high density of rough ER and secretory vesicles; open into a system of branched ducts that eventually converge on the main pancreatic duct

chylomicrons

- Golgi complex combines to make triglycerides again, and with a small amount of cholesterol coats the complex with a film of phospholipids and protein to form droplets - packages chylomicrons into secretory vesicles that migrate to the basal surface of the cell and release their contents into the core of the villi - taken up by the more porous lacteal into the lymph; white, fatty intestinal lymph flows through larger and larger lymphatic vessels of the mesenteries, eventually passing through to the thoracic duct and into the left subclavian vein

bile

- a green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids. - principal pigment is bilirubin - liver secretes about 500-1000 mL of bile per day

parasympathetic defecation reflex

- a spinal reflex - principal events are that stretch signals are transmitted to the spinal cord, and motor signals return by way of the pelvic nerves to intensify peristalsis in the descending and sigmoid colon and rectum and to relax the internal anal sphincter

pancreatic juice

- alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other eletrolytes

trypsin

- an autocatalytic enzyme secreted into the intestinal lumen and converted by enterokinase, an enzyme secreted by the mucosa of the small intestine - it also converts the other two zymogens into chymotrypsin and carboxypeptidase ***primary role is protein digestion.

parts of colon

- ascending and descending colon are retroperitoneal and have a serosa only on the anterior surface, whereas the transverse and sigmoid colon are entirely enclosed in serosa and anchored to the posterior abdominal wall by the mesocolon

liver

- body's largest gland, weighing in about 1.4 kg (3 lb.) - secretion of bile contributes to digestion - hepatic sinusoids have hapatocytes that function to absorb glucose, amino acids, iron, vitamins, and other nutrients from the blood for metabolism or storage; they also remove and degrade hormones, toxins, bile pigments, and drugs - secrete albumin, lipoproteins, clotting factors, angiotensinogen, and other products into the blood

emulsification droplets

- broken up fat globules by bile acids - agitation produced by intestinal segmentation breaks the fat up into droplets and a coating of lecithin and bile acids keeps it broken up, exposing far more surface to enzymatic action

brush border enzymes

- carry out some of the final stages of chemical digestion - not secreted into the lumen; instead, the chyme must contact the brush border for digestion to occur (Contact digestion) ***reason it is important that intestinal contractions churn the chyme and ensure that it all contacts the mucosa

minerals

- chloride ions are actively transported by the distal ileum by a pump that exchanges them for bicarbonate ions, reversing the chloride-bicarbonate exchange that occurs in the stomach - the K+ concentration of the chyme rises as water is absorbed from it, creating a gradient favorable to K+ absorption. ***In diarrhea, when water absorption is hindered, potassium ions remain in the intestine and pass with the feces; therefore, chronic diarrhea can lead to hypokalemia

Acetylcholine (ACh)

- coming from the vagus and enteric nerves - stimulates the pancreatic acini to secrete their enzymes even during the cephalic phase of gastric control - the enzymes remain stored in the pancreatic acini and ducts, however, in preparation for release later when chyme enters the duodenum

two brush border enzymes

- dextrinase and glucoamylase - hydrolyze oligosaccharides that are three or more residues long - maltase hydrolyzes maltose to glucose - sucrose and lactase digest sucrose and lactose respectively

ribonuclease and deoxyribonuclease

- digest RNA and DNA respectively - these enzymes are not altered after secretion but become fully active upon exposure to bile or ions in the intestinal lumen

lipases

- digest fats; lingual, gastric, and pancreatic lipase - 10-15% of fats digested before reaching duodenum - removes the first and third fatty acids from the glycerol backbone and usually leaves the middle one; thus two free fatty acids and a monoglyceride

starch

- digested first to oligosaccharides up to eight glucose residues long, then into the disaccharide maltose, and finally to glucose, which is absorbed by the small intestine - process begins in the mouth (6.8-7.0 pH) - amylase does not function at a pH lower than 4.5 and is thus denatured in the stomach - 50% is digested before it reaches the small intestine

ileum

- last 60% of the postduodenal intestine - thinner, less muscular, less vascular, and has a paler pink color than the jejunum - Peyer patches: prominent lymphatic nodules in clusters which are readily visible to the naked eye and become progressively larger approaching the large intestine

pathway of bile

- liver secretes bile into narrow channels, the bile canaliculi, between the back-to-back layers of hepatocytes. bile passes from there into the small bile ductules between lobules, and these converge to ultimately form right and left hepatic ducts. - the converge on the inferior side of the liver to form the common hepatic duct. - farther on, this is joined by the cystic duct coming from the gallbladder - their unions for the bile duct, which descends through the lesser omentum toward the duodenum - near duodenum, the bile duct joins the duct of the pancreas and forms an expanded chamber called the hepatopancreatic ampulla - then to major duodenal papilla which contains a muscular hepatopancreatic sphincter (sphincter of Oddi), which regulates the passage of bile and pancreatic juice into the duodenum

micelles

- made in the liver, consist of 20-40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward - pass down the bile duct into the duodenum, where they absorb fat-soluble vitamins, more cholesterol, and the FFAs and monglycerides produced by fat digestion - lipids transported to the surfaces of the enterocytes, where the lipids leave the micelles and diffuse through the plasma membrane into the cells.

flatus

- mostly from swallowed air, but also from bacteria - 7-10 L in large intestine, 500 mL /day expelled - composed of N2, CO2, H2, CH4, H2S, and two amines: indole and skatole - indole, skatole, and H2S produce the odor of flatus and feces, whereas the others are odorless

nucleic acids

- nucleases of pancreatic juice hydrolyze these to their constituent nucleotides - nucleosidases and phosphatases of the brush border then decompose the nucleotides into phosphate ions, ribose or deoxyribose, and nitrogenous bases. - products transported across the intestinal epithelium by membrane carriers and enter the capillary blood of the villus

haustral contractions

- occur about every 30 minutes - distention of a haustrum with feces stimulates it to contract - it churns and mixes the residue, promotes water and salt absorption, and passes the residue distally to another haustrum.

intrinsic defecation reflex

- operates entirely within the myenteric nerve plexus - stretch signals travel through the plexus to the musclaris of the descending and sigmoid colon and the rectum activating a peristaltic wave that drives feces downward, and it relaxes the internal anal sphincter - this reflex is relatively weak, however, and usually requires the cooperative action of the parasympathetic defecation reflex.

endocrine part of pancreas

- pancreatic islets, which secrete insulin and glucagon

gallbladder

- pear shaped sac on the underside of the liver that serves to store and concentrate bile - between meals, the gallbladder absorbs water and electrolytes from the bile and concentrates it by a factor of 5-20 times. -

migrating motor complex

- peristaltic wave begins in the duodenum, travels 10-70 cm, and dies out, only to be followed by another wave that begins a little farther down the tract than the first one did - successive, overlapping waves of contraction

secretin

- produced by the same regions of the small intestine, mainly in response to the acidity of chyme from the stomach - secretin stimulates the ducts of both the liver and the pancreas to secrete an abundant sodium bicarbonate solution; in the pancreas this also flushes the enzymes into the duodenum - sodium bicarbonate buffers the HCl arriving from the stomach - what is left in the small intestine is saltwater (NaCl and H20)

large intestine

- receives about 500 mL of indigestible food/day and reduces it to about 150 mL of feces by absorbing water and salts, and eliminates the feces by defacation

cholecystokinin (CCK)

- secreted by the mucosa of the duodenum and proximal jejunum, primarily in response to fats in the small intestine - also stimulates the pancreatic acini to secrete enzymes, but it is named for its strongly stimulatory effect on the gallbladder - induces contractions of the gallbladder and relaxation of the hepatopancreatic sphincter, discharging bile into the duodenum

accessory pancreatic duct

- smaller accessory pancreatic duct that branches from the main pancreatic duct and opens independently into the duodenum and the minor duodenal papilla - bypasses the sphincter and allows pancreatic juice to be released into the duodenum even when bile is not

pancreas

- spongy retroperitoneal gland posterior to the greater curvature of the stomach - head encircled by the duodenum, midpoint called the body, and blunt, tapered tail on the left - both an endocrine and exocrine gland

mass movements

- stronger contractions occur one to three times a day, last about 15 minutes and move residue several centimeters at a time; often triggered by gastrocolic and duodenocolic reflexes, which filling of the stomach and duodenum stimulates motility of the colon - occur especially in the transverse to sigmoid colon, often within an hour after breakfast

bacterial flora

- the large intestine harbors about 800 species of bacteria - the bacteria digest cellulose, pectin, and other plant polysaccharides for which we have no digestive enzymes, and we absorb the resulting sugars

segmentation

- the most common type of intestinal contraction; a movement in which stationary ringlike constrictions appear at several places along the intestine and then relax as new constrictions form elsewhere - rhythm set by pacemaker cells of the musclaris externa (12 times/min in duodenum and 8-9 times/min in the ileum

enterohepatic circulation

- the route of secretion, reabsorption, and resecretion; reused bile acids tow or more times during the digestion of an average meal

gastroileal reflex

- triggered by food in the stomach along with release of gastrin, both of which enhance segmentation in the ileum and relax the valve - as the cecum fills with residue, the pressure pinches the valve shut and prevents the reflux of cecal contents into the ileum

fate of peptides in the small intestine

- trypsin and chymotrypsin hydrolyze polypeptides into even shorter oligopeptides - taken apart one amino acid at a time by: 1) carboxypeptidase: removes amino acids from the carboxyl end of the chain 2) aminopeptidase: removes them from the amino end 3) dipeptidase: splits depeptidase in the middle and releases two free amino acids ***aminopeptidase and dipeptidase are brush border enzymes, while carboxypeptidase is a pancreatic secretion

three sources of amino acids absorbed by the small intestine

1) dietary proteins 2) digestive enzymes digested by each other 3) sloughed epithelial cells digested by these enzymes

three functions of intestinal contractions

1) mix chyme with intestinal juice, bile, and pancreatic juice, allowing these fluids to neutralize acid and digest nutrients more effectively 2) churn chyme and bring it into contact with the mucosa for contact digestion and nutrient absorption 3) move residue toward the large intestine

SGLT

80% of the absorbed sugar is glucose, taken up by this sodium-glucose transporter like that of the kidney tubules

lacteal

a lymphatic capillary embedded in the areolar tissue of the lamina propria of the core of a villus - lacteals absorb most of the lipids

enterocytes

absorptive cells covering the villi

three stimuli chiefly responsible for release of pancreatic juice and bile

acetylcholine (ACh), cholecystokinin (CCK), and Secretin

procarboxypeptidase

becomes carboxypeptidase, which hydrolyzes the terminal amino acid from the carboxyl end of small peptides

cecum

blind pouch in the lower right abdominal quadrant inferior to the ileocecal valve

three internal folds or projections in the small intestine to increase its surface area for reabsorption

circular folds, villi, and microvilli - the circular folds increase the surface area by a factor of 2-3; the villi by a factor of 10; and the microvilli by a factor of 20

appendix

densely populated with lymphocytes and is a significant source of immune cells

proteases

enzymes that digest proteins - absent from saliva, but begin work in the stomach - 10-15% digested in stomach by pepsin into shorter polypeptides and a small amount of free amino acids - pesin's optimal pH is 1.5-3.5 so it is inactivated when it passes into the duodenum

intestinal crypts (crypts of Lieberkuhn)

extend as far as the muscularis mucosae, consist of enterocytes and goblet cells, dividing stem cells

jejunum

first 40% of small intestine beyond the duodenum - most digestion and nutrient absorption occur here

duodenal (Brunner) glands

in the submucosa - secrete an abundance of bicarbonate-rich mucus, which neutralizes stomach acid and shields the mucosa from its erosive effects

constipation

occurs when fecal movement is slow, too much water is reabsorbed, and the feces become hardened; can result from lack of dietary fiber, lack of exercise, emotional upset, or long-term laxative abuse

Diarrhea

occurs when the large intestine absorbs too little water; occurs when the intestine is irritated by bacteria and feces passes through too quickly for adequate reabsorption, or when the feces contain abnormally high concentrations of a solute such as lactose that opposes osmotic absorption of water

colon

part of the large intestine between the ileocecal junction and the rectum (not including the cecum, rectum, or anal canal)

hemorrhoids

permanently distended veins that protrude into the anal canal or form bulges external to the anus

pancreatic duct

runs lengthwise through the middle of the gland and joins th ebile duct at the hepatopancreatic ampulla - the hepatopancreatic sphincter thus controls the release of both bile and pancreatic juice into the duodenum

Paneth cells

secrete lysozyme, phospholipase, and defensins--defensive proteins that resist bacterial invasion of the mucosa

What is the most digestible carbohydrate?

starch

bile acids (bile salts)

steroids synthesized from cholesterol - aid in fat digestion and absorption along with lecithin - about 80% are reabsorbed in the ileum, the last portion of the small intestine, and returned to the liver, where the hepatocytes absorb and resecrete them - the 20% that are not reabsorbed is excreted in the feces ***The body's only way of eliminating excess cholesterol (liver synthesizes new bile acids from cholesterol to replace the quantity lost in the feces)

transverse rectal folds (rectal valves)

three infoldings that allow the rectum to retain feces while passing gas

pancreatic ymogens

trypsinogen, chymotrypsinogen, and procarrboxypeptidase