Gastrointestinal System and Salivary Glands

Gastrointestinal System Overview

• Also known as the Alimentary Canal.
• The GI tract digests and absorbs food, making nutrients more available.
• Six essential activities are associated with the digestive system: ingestion; propulsion; mechanical digestion; chemical digestion; absorption; and defecation.
• Primary organs consist of the mouth, pharynx, oesophagus, stomach, small intestine and large intestine.
• Accessory digestive organs consist of the teeth, tongue, salivary glands gallbladder, liver, and pancreas.
• The function is processing and absorbing nutrients from food.
• There are six classes of nutrients including: carbohydrates, lipids, proteins, vitamins, minerals, and water.
• An adequate diet will include all six classes of nutrients.

Salivary Glands

• There are three pairs of salivary glands.
• Parotid glands are the largest salivary glands located anterior to the ears.
• Submandibular glands are located inferior to the jaw.
• Sublingual glands are located inferior to the tongue, produce mucus, and are an important source of amylase.
• Produce 1-1.5 L of saliva each day.
• Consists of 98-99% water.
• Salivary amylase breaks down starches.
• Mucins lubricate the mouth and food.
• Ions, buffers, metabolites, and antibodies are also present.
• Helps moisten the food and turns it into a bolus, and can be easily swallowed.

GI Tract Histology

• From the oesophagus to the anal canal, the walls of the GI tract share the same four layers (tunics) including; the mucosa in contact with the bolus, the submucosa (contains blood vessels and lymphatics), the muscularis externa innermost/outermost layer and the serosa which helps maintain intestine function and perforation prevention.

Stomach Modifications

• Stomach muscularis layer modifications accommodate its functions, including peristalsis, churning and mixing, and mechanical breakdown of food.
• Stomach walls contains folds, and is lined with invaginations called gastric pits useful for expanding to hold food
• Mucus cells produce glycoprotein products, functioning primarily as lubricants throughout the GI tract, and protecting against harmful sustances
• Bicarbonate generates a pH gradient within the mucus to protect the stomach wall.
• Chief cells house zymogen granules that stimulate the release of pepsinogen.
• Pepsinogen then becomes pepsin with introduction of acid, that further breaks down proteins.
• Chief cells also produce gastric lipase.
• Approx. 40% of preduodenal lipolysis breaks down fats.
• Parietal cells secrete H+ and Cl- against concentration gradient; 100000:1.
• Parietal cells secrete hydrochloric acid, which denatures food, activates pepsinogen, dissolves bone, and is bactericidal.
• Parietal cells secrete intrinsic factor required for vitamin B12 absorption.
• Mucous neck cells secrete mucus and bicarbonate.
• G cells secrete gastrin which stimulates acid release.
• D cells secrete somatostatin, which inhibits gastrin secretion.
• Enterochromaffin-like cells secrete histamine to stimulate acid release.
• Tight junctions between epithelial cells and alkaline mucus barrier both help prevent digestion of the stomach walls.

Gastric Emptying Rate

• Depends on the type of food ingested.
• Fluids pass through quickly in about 90 minutes.
• Solids remain until reduced, dissolving in 3-4 hours.
• Carbohydrates are emptied first.
• Proteins follow carbohydrates.
• Fats take the longest to leave the stomach.

Small Intestine

• On leaving the stomach, chyme enters the small intestine via the Duodenum, jejunum, and Ileum.
• The intestinal lining has a large surface area as well as circular folds and invaginations to increase area for absorption
• Finger-like projections called villi increase surface area in the mucosal layer.
• Invaginations called crypts further increase surface area.
• Nutrients are then absorbed from the small intestine.
• Bile from the liver and enzymes from the pancreas are added to chyme in the duodenum.
• Carbohydrates and proteins are partially digested and fats are undigested upon entering the small intestine.

Liver

• Aids in synthesis of bile, secretion of bile aids in digestion.
• Bile is a yellow-green alkaline solution.
• Bile consists of water and bile salts which emulsify fat globules.
• Transports vitamins to the liver and fatty tissues, and is necessary for emulsification of fats to allow lipases to act more effectively

Gallbladder

• The gallbladder is a small green sac located on the inferior surface of the liver that concentrates and stores bile.
• The gallbladder does not synthesize bile.
• When closed, the sphincter of Oddi prevents bile to enter the duodenum.
• Bile is then stored in the gallbladder.

Pancreas

• Acinar cells in the pancreas secrete pancreatic juice containing digestive enzymes into the pancreatic duct.
• This juice contains a watery alkaline fluid (pH 8) and various digestive enzymes.
• Pancreatic juice consists of mostly water, solids and the bicarbonates enzymes along with proteolytic substances.
• If few carbs are available, amylase is produced, but safe to be active to break down carbs
• Proteolytic enzymes are secreted in an inactive form to prevent self-digestion.

Intestinal Juice

• Intestinal juice is a watery secretion with neutral pH (7.0).
• It serves as a medium for digestion and absorption of nutrients and normally secretes ~1-2 L/day.
• It contains digestive enzymes secreted by epithelial cells in the small intestine.
• Disaccharidases break down disaccharides to monosaccharides which can then be used by the body
• Peptidases break down polypeptides into amino acids.
• Lipases break down lipids to monoglycerides and fatty acids.

Carbohydrate Digestion

• Pancreatic amylase continues carbohydrate hydrolysis.
• Enzyme action on the intestinal lumen's brush border completes the final stage of digestion.
• Simple sugars are absorbed across intestinal mucosa.
• Glucose is absorbed by active transport.
• Starch and glycogen are broken down by amylase into the disaccharides maltose, sucrose, and lactose.
• Specific disaccharidases then breakdown to be monosaccharides
• The human body cannot digest cellulose.
• Nondigestible carbohydrate is called fibre.

Protein Digestion

• It begins in the stomach with pepsin usage.
• It continues in the small intestine using pancreatic fluids containing trypsin and chymotrypsin.
• Peptide fragments dismantle into tripeptides, dipeptides, and single amino acids.
• Amino acids join with Na+ for transport.

Lipid Digestion

• Major lipid breakdown occurs by the emulsifying action.
• Bile increases solubility and digestibility through emulsification, and contains water, bilirubin, bile salts, cholesterol, neutral fats, phospholipids, and electrolytes.
• Free-fatty acids (with 10 or less C's), rapidly absorb into the portal vein.
• Steps include: mechanical digestion triglycerides and cholesterol, step of bile emulsification, micellar formation, hydrolysis and absorption, re-esterification, and process formation.
• Bile creates an emulsion.
• Pancreatic lipase and colipase further the digestion to micelles and a mixture of glycerol and fatty acids
• The glycerol absorbs, but the remainder of the products are encased in chylomicrons, that exocytose though they are not big enough to be in the Lacteal
• Long-chain fatty acids are absorbed by the intestinal mucosa, reform into triglycerides, then form chylomicrons.
• Chylomicrons move slowly through the lymphatic system and empty into the venous blood.
• Absorbed by chylomicrons and lipoprotiens, and later transported into the liver

Vitamin Absorption

• Vitamin absorption occurs mainly by diffusion in the jejunum and ileum.
• Fat-soluble vitamins are absorbed with dietary lipids transported into the liver and fatty tissues, A, D, E, and K and bind to ingested lipids from lacteals
• Water-soluble vitamins diffuse into the blood, where some bind and are broken down by the liver, except for vitamin B12.
• Excess Water-soluble vitamins pass into the urine when plasma concentration exceeds renal capacity, can only be absorbed when first binding with the intrinsic factor
• Electrolytes are absorbed.
• Na+ and Cl- are absorbed via active transport.
• K+ is absorbed via passive diffusion.
  • H2O is absorbed via osmosis.

Large Intestine

• Has two major regions - the colon and the rectum.
• The Colon begins at the ileocecal valve to the rectum
• The distal portion forms the distal portion, and has an anal canal
• The lumen contains millions of bacteria that metabolize remaining nutrients and produce other vitamins.
• Large intestine’s mucosa contains many goblet cells, alkaline mucosa.
• Longitudinal muscle layer is arranged in 3 bands and produces out-pocketings called haustra.
• Proximal responsible for absorption of water, while distal responsible for the power involved in defection

Gut motility

• Affected by absorption, and are two types include peristalsis move the material and emulsification to Mix it

Small Intestinal Motility

• Peristaltic contractions are responsible for forward movement, at 3 contractions a minute.
• Longitudinal contractions tighten the muscular contractions, that moves food forward

Segmentation

• Periodically squeezing the contents to push the intestinal contacts back and to promote mixer with water in cells
• 12-16duodenal duodenum times a minute, and 9 in the Illeum

Large Intestine Function

• Large intestine moves slower to allow the haustration and longitudinal contractions, in order to dig deep.
• Normal flow is mass movements during defecation to create propulsion if any
• Colononic control include both sympethetic and gastroenteritic communication
• When body reaches defecation it’s the desicion of the nervous system, when voluntary control has been learned.
• Body commands from Sacral will close external sphincters
• The skeletal structure will contract to engage abdominal and diagphram support, that all work together for movement