Gastrointestinal System and Digestion

Gastrointestinal System (Alimentary Canal)

• Digests and absorbs food, increasing nutrient availability at each step.
• There are six essential activities: ingestion, propulsion, mechanical digestion, chemical digestion, absorption and defecation.

GI Tract Components

• Primary digestive organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory digestive organs include teeth, tongue, salivary glands, gallbladder (stores bile), liver (produces bile), and pancreas (produces digestive fluids).

Function

• The primary function is to process food.

Six Nutrient Classes

• Carbohydrates comprise 50-60% of dietary needs.
• Lipids make up 25-35% of dietary consumption, which are important for membranes.
• Proteins consist of 15-25% of the dietary requirements, and are important for synthesis.
• Vitamins, minerals, and water constitute 0.5-2% of dietary needs.

Salivary Glands

• There are 3 pairs of salivary glands: parotid, submandibular, and sublingual.
• Parotid glands are the largest and located anterior to the ears.
• Submandibular glands are inferior to the jaw.
• Sublingual glands sit inferior to the tongue, producing mucus and amylase.

Saliva

• Production is 1-1.5 L/day.
• Water content stands at 98-99%.
• Amylase is present, which is a digestive enzyme that breaks down starches.
• Mucins lubricate the mouth and food.
• Ions, buffers, metabolites, and antibodies are also present.
• Saliva helps moisten food and converts it into a bolus for easy swallowing.

GI Tract Histology

• From the esophagus to the anal canal, the GI tract walls consist of four layers or tunics.
• The layers, from the lumen outward, include the mucosa (in contact with bolus), submucosa (contains blood vessels and lymph), muscularis externa (inner is the most layer, outermost is the longitudinal layer), and serosa.

Stomach Wall Modifications

• The muscularis layer is modified based on stomach functions like greater churning and mixing.
• It supports mechanical breakdown of food into smaller pieces.
• It consists of three layers: circular, longitudinal, and an additional innermost oblique layer.
• The surface is lined with invaginations called gastric pits.

Mucus Cells

• Glycoprotein products found throughout the GI tract offer a primary function as a lubricant.
• There's a neutral pH.

Bicarbonate Function

• Generates a pH gradient within the mucus to protect the stomach wall.
• Protects against digestion from gastric acid and enzymes.

Chief Cells

• Zymogen granules are present
• Release pepsinogen upon stimulation.
• Pepsinogen converts to pepsin in the presence of acid.
• Protease helps breakdown proteins.

Chief Cells Produce

• Pepsinogen, which converts to pepsin.
  • Breaks down proteins into smaller polypeptide and amino acid fragments.
• Gastric lipase, a fat-digesting enzyme that is about 40% of preduodenal lipolysis.

Parietal Cells

• Secrete H+ and Cl- at a concentration gradient of 100000:1, and against the gradient.
• They secrete HCl, a strong acid that increases stomach acidity (pH 1.5-3.5).
  • This denatures food, activates pepsinogen to pepsin, dissolves bone, and is bacteriocidal.
• They secrete intrinsic factor, which is required for vitamin B12 absorption in the small intestine, and is essential for life.

Endocrine Cells

• G cells secrete gastrin, which stimulates acid release.
• D cells secrete somatostatin and inhibits gastrin secretion.
• Enterochromaffin-like cells secrete histamine and help stimulate acid release from parietal cells.

Gastric Secretions Overview

• Mucous neck cells secrete mucus for physical barrier between lumen and epithelium.
• Parietal cells secrete gastric acid (HCl) which activates pepsin, and kills bacteria.
  • They secrete intrinsic factor, which complexes with vitamin B12 to permit absorption.
• Enterochromaffin-like cells secrete histamine to stimulate gastric acid secretion.
• Chief cells produce pepsin(ogen) and gastric lipase which digests proteins and fats.
• D cells secrete somatostatin to inhibit gastric acid secretion.
• G cells secrete gastrin, which stimulates gastric acid secretion.

Protection Against Gastric Juice

• Mucous barrier and epithelial cells protect walls of stomach from digestion.
• The mucous barrier is alkaline and neutralizes acid on the stomach lining.
• Tight junctions between epithelial cells prevent acid leakage.
• There is high turnover of epithelial cells, with replacement every 3 days.

Gastric Emptying Rates

• Depends on the type of food ingested.
• Fluids pass through quickly in 90 minutes.
• Solids remain until reduced to small particles, dissolving in gastric juices after 3-4 hours.
• Carbohydrates are emptied first, followed by proteins.
• Fats take the longest to leave the stomach.

Small Intestine (SI)

• Chyme enters the small intestine when it leaves the stomach.
• Subdivisions of the small intestine are the Duodenum, jejunum, and ileum.
• The wall contains mucosa submucosa serosa.

Large Surface Area

• Intestines have a large surface area, which increases area for absorption.
• This is achieve by plica folds and invaginations.
• There are finger-like projections called villi in the mucosal layer.
• Invaginations called crypts.
• Increased surface area translates to increased absorption.
• Intestinal cells contain projections on cells called microvilli

Small Intestine Processes

• When chyme enters the small intestine carbohydrates and proteins are partially digested, but still too large to be absorbed through the SI wall, while fats are undigested.
• All nutrient absorption occurs in the small intestine.
• In the duodenum, enteroendocrine cells secrete peptides.
• Bile from the liver and enzymes from the pancreas are added to chyme.

Liver

• Aids in digestion via bile synthesis and secretion, offering mechanical digestion during the emulsification of fat droplets.
• Yellow-green alkaline solution
• Cholesterol is present in the bile salts.

Gallbladder

• This small, green sac stores and concentrates bile.
• Does not synthesize bile.
• When the sphincter of Oddi is closed, bile is stored in the gallbladder, instead of entering the duodenum.

Pancreas (Exocrine Function)

• Acinar cells secrete pancreatic juice into the pancreatic duct.
• Watery alkaline fluid, with a pH of 8.
• Contains HCO3- and various digestive enzymes.

Pancreatic Juice Composition

• Water constitutes approximately 99.5%.
• Solids make up about 0.5%.
• Proteolytic enzymes present include trypsin, chymotrypsin, and carboxypeptidases.
• Lipolytic enzymes include pancreatic lipase, cholesterol ester hydrolase, and phospholipase
• The amylolytic enzyme is pancreatic amylase.
• Contains a high Bicarbonate content = 110 to 150 mEq/L

Pancreatic Enzyme Secretion/Activation

• Trypsinogen is inactive
• Enteropeptidase (Enterokinase)
  • Secreted by duodenal epithelium.
  • Is a small amount.

Intestinal Juice

• A watery secretion with neutral pH.
• Secretes ~1-2 L/day; it serves as digestion and absorption medium of nutrients.
• It contains digestive enzymes from epithelial cells in the small intestine.
• Disaccharidases are present, which does carbohydrate digestion by disaccharides to monosaccharides.
• Performs protein digestion by polypeptides to amino acids via Peptidases
• It performs lipid digestion from lipid to monoglyceride and fatty acids with the assistances of Lipases

Carbohydrate Digestion

• Pancreatic amylase continues carbohydrate hydrolysis.
• Enzyme action on the intestinal lumen's brush border completes the final digestion stage.
• Simple sugars are absorbed across the intestinal mucosa.

Carbohydrate digestion simplified

• Starch becomes amylase through secretions
• Specific Disaccharides assist in absorptive cells
• Carbohydrate Digestion also acts as a Conserve water for the body.
• Nondigestible carbohydrate is known as fibre.

Protein Digestion

• Protein digestion begins in the stomach with pepsin and continues in the small intestine using pancreatic fluid that assists Trypsin and chymotrypsin.
• The peptide fragments further dismantle into tripeptides, dipeptides, and single amino acids.
• Amino acids join with Na+ for transport.

Lipid Digestion in SI and SL

• Triacylglycerides (TG) are the major diet components.
• Major lipid breakdown occurs by bile and the hydrolytic action of pancreatic lipase.
• Bile increases the lipid droplets' solubility and digestibility through emulsification.
• It breaks down TGs into 2 free fatty acids (FFA) and 1 monoglyceride (glycerol).
• Free-fatty acids with 10 or less carbons rapidly absorb into the portal vein.

Steps for Fat Absorption

• Digestion
  • triglycerides and cholesterol.
• Large Fat Digestion
  • Mechanical (Digestion occurs)
• Liver Bile
  • Recycles Salts.
• Enzymes and lipase breakdown
  • forms emulsion
  • micellar breakdown
• Cells can now transport protein.

Lipid Absorption in System

• Long-chain fatty acids are absorbed by the intestinal mucosa, reforming into triglycerides and then chylomicrons.
• Chylomicrons move slowly through the lymphatic system and empty into the venous blood for systemic circulation.

Small Intestine Vitamin Absorption

• Vitamin absorption occurs mainly by the passive diffusion in the jejunum and ileum.
• Fat-soluble vitamins are absorbed with dietary lipids.
  • Chylomicrons and lipoproteins transport these vitamins to the liver and fatty tissues.
• Water-soluble vitamins diffuse into the blood, except for vitamin B12.
  • It combines with intrinsic factor produced by the stomach, absorbed by the intestine via endocytosis.
• Water-soluble vitamins pass into the urine when their plasma concentration exceeds the renal capacity for reabsorption.

Vitamin Function

• Fat-soluble vitamins (A, D, E, K) bind to ingested lipids, and are absorbed with lipids.
• Water-soluble vitamins - Mostly absorbed with water. Exception - vitamin B12 requiring bind with intrinsic factor produced by parietal cells.
  • B12 is absorbed via endocytosis.

Absorption of Minerals (Electrolytes)

• Na+ and Cl- are absorbed via active transport.
• K+ absorbed via passive diffusion.
• H2O is absorbed via osmosis.
  • It also Moves freely across intestinal mucosa

Large Intestine (LI) Anatomy

• Extends from the ileocecal valve to the rectum.

LI Anatomical Modifications

• The mucosa contains many goblet cells
  • This provides microvilli
• Contains an alkaline mucosa
  • aids with Faeces handling.
• Holds faeces together.
  • It protects the lining.
• A Longitudinal muscle layer which is arranged in 3 bands.
  • and are known as the out-pocketings haulstra
  • Haustra help provide out-pocketings in the lining.
  • also have Teniae coli aids in longitudinal support.

Large intestine (LI) Function

• The proximal tubule absorbs water.
  • Dehydrating the faeces.
• The distal tubule removes faeces from body.
• The power propulsion is involved in the defecation reflex.

The large intestine (LI) Digestion

• Contains Millions of types of bacteria inside.
• Metabolizes remaining.
• Also metabolizes Vitamin -B1 -K -B7 -B12

Fluid Composition of Digested Water

• Consists if 8.9L of total absorbed and 9.OL consisting of input

Fluid Composition for Secretion

• 1.5L consisting of the salivary glands
• Bile is about 0.5L
• Gastricsecretions consist of 2.0L
• The small intestine consist of 1.5L

Gut Motility

• Important for absorption

Intestinal Motility

• Peristaltic helps in forward moving of materials.
  • Typically around 3 contractions / min.

Function of the Intestine

 - it connects when circular muscles are around it,
 - it pushes the Food the inside it,
 When Circular contract outer muscle goes is constricts it.
  - Chyme comes to ahead and gets worked.

Segmentation Functions

• When Muscular contractions are mixing contents
  • No Net movement is shown.
• This allows the digestive tract to Push contents a towards digestive breakdown.

Segmentation Overview

• The Small intestine has this digestion, however no movement is performed
• Alternating contractions of duodenum is about 12-16 /min