DIGESTIVE SYSTEMFUNCTIONS

1. Ingestion 2. Mastication 3. Propulsion 4. Mixing 5. Secretion 6. Digestion 7. Absorption 8. Elimination

HISTOLOGY OF DIGESTIVE TRACT1) Mucosa - innermost tunic, consists of three layers.

a) Mucosa epithelium, stratified squamous epithelium b) Lamina propria, loose connective tissue. c) Muscularis mucosae, outer thin smooth muscle.

2) Submucosa – thick connective tissue layer containing nerves blood vessels and small glands. 3) Muscularis – inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle.

a) Exceptions superior esophagus has striated muscle, and the stomach has three muscular layers. 4) Serosa or Adventitia – connective tissue.

ORAL CAVITY   STRUCTURE FUNCTIONLips & Cheeks   Lips Mastication, speech, & entranceFrenula Mucosal folds attach upper lip to alveolar process of the maxilla, and

from the alveolar process of the mandible to lower lip.Cheeks Mastication ,speech, & facial expressionPalate   Palate (hard & soft) Mastication, soft palate prevents food from entering nasal cavity.Tongue Movement of food in mouth for mastication, swallowing, taste, speech.Intrinsic muscles Flattening and elevating tongue for swallowing. Extrinsic muscles Protrude and retract, side to side, and shape change.

Teeth Mastication and speechIncisors Cutting or nipping off foodCanines Tear and piercePremolars Grinding and crushingMolars Grinding and crushing    Salivary glandsStructure Location Parotid glands Just anterior to the ear, bilaterally.Submandibular glands Inferior border of the posterior mandibleSublingual glands Immediately below the mucus membrane in the floor of the mouth  Saliva

Secretion rate 1 to 1.5L/day Salivary amylase – serous saliva breaks down apart glucose molecules in starch and other

polysaccharides into disaccharides. Prevents bacterial infection together with lysozyme Mucin – proteoglycan that gives saliva a lubricating quality. Stimulated primarily by the parasympathetics and less by sympathetics.  

ESOPHAGUSAnatomy

Approx. 25 cm. Between pharynx and stomach Located in the mediastinum anterior to vertebrae and posterior to trachea. Esophageal hiatus – opening in diaphragm and ends at stomach. 4 tunics, mucosa, submucosa, muscularis, and adventitia. Upper esophageal and Lower esophageal sphincter – regulate the movement of materials into and

out of esophagus.   Histology

Mucosa stratified squamous epithelium with mucous glands that secret a thick lubricating mucus.   STOMACHAnatomy

Enlarged segment in left superior part of abdomen. Gastroesophageal opening (cardiac) – opening of stomach from esophagus. Cardiac region – located around the cardiac opening Fundus – left and superior to the cardiac region. Body – largest part of stomach curves to the right forming the greater and lesser curvature. Pyloric region – region where body narrows Pyloric opening - joins to the small intestine. Pyloric sphincter – thick smooth muscle surrounding the pyloric opening.

Histology Serosa – visceral peritoneum, outer most layer. Muscularis – three layers, longitudinal layer, circular layer, and an inner oblique layer. Rugae – large folds of submucosa and mucosa, allows stomach to stretch. Stomach is lined with simple columnar cells, with tube like gastric pits, which are openings for

the gastric glands. Surface mucous cells – produce mucus on the surface and lines the gastric pit.

Gastric gland cells Mucous neck cells – produce mucus.

Parietal cells – produce hydrochloric acid and intrinsic factor. Chief cells – produce pepsinogen. Endocrine cells –produce regulatory hormones.

  SECRETIONS OF THE STOMACH Chyme – food and stomach secretions mixed.

SECRETION SOURCE FUNCTION Mucus Surface and neck mucus cells . Lubricates, protectsIntrinsic factor Parietal cells in gastric glands of the

pyloric region.Binds with and makes more readily absorbed in the ilium VITAMIN B12

Hydrochloric acid Parietal cells Low pH bactericidal, denature proteins, provides proper pH for pepsin.

Pepsinogen Chief cells Converts to pepsin, catalyzes the cleavage of peptide bonds in proteins.

 

Functions of Gastrointestinal Hormones

Site of Production Method of Stimulation Secretory Effects Motility Effects

Gastrin      

Stomach & duodenum

Distention, partially digested proteins, autonomic stim., alcohol & caffeine

Increases gastric secretions

Increased gastric emptying by increasing motility and relaxing pyloric sphincter.

Secretin      

Duodenum Acidity of chyme

Inhibits gastric secretions, stimulates pancreatic secretions, increases the rate of bile and increases intestinal secretions; mucus secretion

Decreases gastric motility

Cholycystokinin       

IntestinesFatty acids and other lipids

Slightly inhibits gastric secretions, stimulates pancreatic secretions, contraction of gall bladder, relaxation of hepatopancreatic ampular sphincter.

Decreases gastric motility

Gastric Inhibitory Polypeptide

     

Duodenum & JejunumFatty acids and other lipids

Inhibits gastric secretions Decreases gastric motility

GASTRIC MOVEMENTSMixing of Stomach Contents

Gentle mixing waves which are peristalsis like contractions proceed from the body toward the pyloric sphincter.  80%

Peristaltic waves occur less frequently, significantly more powerful, force chyme near the periphery of the stomach toward the pyloric sphincter.  20%

Stomach Emptying Fluids remain in the stomach approx. 1.5 to 2.5 hours, food approx. 3 to 4 hours. Each peristaltic contraction is strong enough to force a small amount of chyme through the

pyloric sphincter and into the duodenum, this is referred to as Pyloric Pump.

Regulation of stomach emptying Hormonal - Gastrin CNS - distention of stomach wall and local reflexes All promote increased in stomach emptying.

SMALL INTESTINEConsists of three parts; the duodenum, the jejunum, and the ilium.

Length approx. 4.6 to 9 meters (14 to 29 feet). Site of greatest amount of digestion and absorption. 8 to 8.5 liters of water enter the small intestine per day.

Duodenum Begins with a short superior end, which is where it exits the pylorus of the stomach, and ends in a

sharp bend, where it joins the jejunum. Major duodenal papilla & lesser duodenal papilla - two small mounds 2/3 of the down the

descending part.  At the major papilla the    common bile duct  and pancreatic duct join to form the  hepatopancreatic ampular sphincter  which empties into the duodenum.

Hepatopancreatic sphincter - regulates the opening of the ampular. Internal surface has modifications which increase surface area about 600 fold. Circular folds - mucosa and submucosa form a series of folds. Villi- tiny fingerlike projections of the mucosa, simple columnar epithelium, contains blood

capillary network and lacteals. Microvilli - most of the cells of the surface of the villi have these numerous cytoplasmic

projections to further increase surface area. Brush border - the combined microvilli on the entire epithelial surface.

Cells of Duodenum Cells FunctionAbsorptive Microvilli, produce digestive enzymes, absorb digested foodGoblet Produce a protective mucusGranular cells Protect the intestinal epithelium from bacteriaEndocrine Produce regulatory hormones

  JEJUNUM & ILEUMSimilar in structure to duodenum, EXCEPT

1.      Except a gradual decrease in the size of the lumen.2.      Decrease in thickness of the intestinal wall3.      Number of circular folds decrease4. Decrease in number of microvilli.

The duodenum and jejunum are major sites of nutrient absorption. Lymph nodes called Peyer’s Patches are numerous in the mucosa and submucosa of the ileum.

o Ileocecal junction – junction between the ileum and the large intestine.o Ileocecal sphincter – ring of smooth muscle.o Ileocecal valve – one-way flow.

  SECRETIONS

Duodenal glands, intestinal glands, and goblet cells secrete large amounts of mucus.o Mucus provides most of the wall with protection from the acidic chyme, and digestive enzymes.o Secretin & Cholycystokinin are released from the intestinal mucosa and stimulate pancreatic and

hepatic secretions.o Enzymes of the intestinal mucosa are bound to the membranes of the absorptive cell microvilli.

1.      Disaccharidase – breaks down disaccharides to monosaccharides.2.      Peptidase – hydrolyze the peptide bonds of amino acid chains.3.      Nucleases – break down nucleic acids.

  MOVEMENT IN THE SMALL INTESTINES

o Mixing and propulsion of chyme are the primary mechanical events.o Mechanical movement occurs due to segmental or peristaltic contractions of the smooth muscle

wall, only propagated small distances.o Segmental contractions – mix intestinal contents.o Peristaltic contractions – propel the intestinal contents along the entire length of the intestine.o Wave of contraction can begin in the stomach.o Rate of movement 1cm/min

Regulation1.      Distention of intestinal wall2.      Hypertonic or hypotonic solutions3.      pH of contents

Ileocecal sphincter – the juncture between the ileum and the large intestine, remains mildly contracted, peristaltic wave will relax and allow movement of chyme from the small intestine to the large intestine.  LARGE INTESTINES

o Extends from the ileocecal junction to the anus.o Consists of; cecum, colon, rectum, anal canal.

  Cecum

o Proximal end of the large intestine.o Extends approx. 6cm. Inferiorly past the ileocecal junction to form a blind sac.o Vermis Appendix – a small blind tube extending approx. 9cm from the blind sac. Contains many

lymph nodes.  Colon Consists of four part;

1.    Ascending colon2.    Transverse colon3.    Descending colon4. Sigmoid colon

o The circular muscular layer of the colon is complete, but the longitudinal layer of muscle is incomplete

o The mucosal lining consists of simple columnar epithelium.o Lining is not folded or villi, but has numerous straight tubules glands called crypts.o Crypts are similar to the glands of the small intestine, with absorptive, goblet, and

granular cells.o The major difference is goblet cells are the predominate cell type while the others

decrease in number.

Rectum

o Straight muscular tube that begins at the termination of the sigmoid colon and ends at the anal canal

o Simple columnar epithelium.o The muscular tunic is relatively thick compared to the rest of the digestive tract.

  Anal Canal

o Last 2 to 3cm. Of the digestive tract.o Begins at the inferior end of the rectum an ends at the anus.o The smooth muscle layer is even thicker than that of the rectum.o Internal anal sphincter – superior end of anal canal.o External anal sphincter – skeletal muscle, inferior end of the canal.o Simple columnar to stratified squamous from superior to inferior anal canal.

  Secretions of Large Intestines

o Major secretory product MUCUS.o Mucus lubricates and helps fecal matter to stick together.o Numerous microorganisms inhabit the colon and constitute approx. 30% of the dry weight of

feces.o These microorganisms break down vitamin K for absorption and breakdown a small amount of

cellulose to glucose.o Flatus – bacterial action produce gases, which are released. The amount of flatulence depends on

the number of microorganisms and the type of food.  Movement in the Large Intestines

o Peristaltic waves are largely responsible for moving chyme along the ascending colon.o Mass Movements – several strong contractions in the transverse and descending colon. This

large contraction propels the colon contents a considerable distance toward the anus.o Defecation Reflex – local reflex distention of the rectal wall by feces, weak contractions of the

rectum and relaxation of internal and external anal sphincters. Parasympathetic reflexes cause strong contractions of the rectum, normally responsible for most of defecation.

 LIVER

o Production of bile.ANATOMY

o Largest gland in the bodyo Occupies most of the right hypochondriac and epigastric regions. Inferior to diaphragm,

protected by ribs.o Four lobes:Right lobe – largest; Left lobe; Caudate lobe – posterior; Quadrate lobe – inferior to

left lobeo Falciform Ligament separates right and left lobes, suspends liver the liver from the diaphragm.o Porta hepatis – entrance for the hepatic artery and hepatic portal vein, and common hepatic duct.

HISTOLOGYo Each functional unit is called a liver lobule. Roughly hexagonal in shape.o Hepatocytes – liver cells, plates of hepatocytes are arranged around a central vein. o Portal triad – located at each of the six corners, three basic structures are always present, Hepatic

artery, Hepatic portal vein, and bile duct.

BILE o Fat emulsifier o Yellow green alkaline solution o Contains

Bile salts, Bile pigments, Cholesterol neutral fats , Phospholipids, Electrolytes 

GALL BLADDER o Storage of bileo Concentration of bile.o Thin walled, green, muscular sac.o Located in a shallow fossa on the ventral surface of the liver.o Secretes bile by muscular wall contracting, bile travels out the Cystic duct and then flows to the

bile duct.  PANCREAS

o Lies deep to the stomach.o Produces a broad spectrum of digestive enzymes.o Pancreatic juice- exocrine product, drains via the Main pancreatic duct.o Acini – small clusters of secretory cells.

SecretionsPancreatic juice, Trypsinogen, Carboxypeptidase, Chymotrypsin, Amylases, Lipases,

Nucleases

CHEMICAL DIGESTIONCarbohydrates

Salivary amylase, Pancreatic amylase, Dextrinase, Glucoamylase, Maltase, Sucrase, Lactase

ProteinsPepsin, Trypsin, Chymotrypsin, Carboxypeptidase, Aminopeptidase, Dipeptidase

LipidsLipases

Nucleic acidsPancreatic nucleases, Nucleosidase, Phosphatases.

ABSORPTIONCarbohydrates

Monosaccharides are transported thought out the epithelial cell walls with protein carriers.

Proteins Amino acids are transported using several types of carrier molecules, before entering the blood

via diffusion.Lipids

Micelles – collection of fatty elements clustered together with bile salts. Easily diffuse through microvilli. Fat digestion completed in the ileum.

  Nucleic acids

Transported actively across epithelium by special carriers.