Gastrointestinal System: An Overview

Gastrointestinal System Overview

• The Gastrointestinal (GI) system is also known as the Alimentary Canal.
• Digestion and absorption of food happens in the GI tract.
• Nutrients are more available to the body in each step.
• Six essential activities occur within the GI tract:
  • Ingestion
  • Propulsion
  • Mechanical digestion
  • Chemical digestion
  • Absorption
  • Defecation
• The primary digestive organs of the GI tract include:
  • Mouth
  • Pharynx
  • Oesophagus
  • Stomach
  • Small Intestine
  • Large Intestine
• Accessory digestive organs include:
  • Teeth
  • Tongue
  • Salivary Glands
  • Gallbladder
  • Liver
  • Pancreas
• The main function of the GI tract is to process food.
• There are six classes of nutrients.:
  • Carbohydrates (50-60% dietary requirement)
  • Lipids (25-35% dietary requirement)
  • Proteins (15-25% dietary requirement)
  • Vitamins (0.5-2% dietary requirement)
  • Minerals (0.5-2% dietary requirement)
  • Water (0.5-2% dietary requirement)
• An adequate diet includes all six classes of nutrients.

Salivary Glands and Digestion in the Mouth

• There are three pairs of salivary glands:
  • Parotid glands are the largest salivary glands and are anterior to the ears.
  • Submandibular glands are inferior to the jaw.
  • Sublingual glands are inferior to the tongue and produce lots of mucus.
• Digestion begins in the mouth.
• Saliva production is 1 - 1.5 L/day.
• Saliva's basic composition:
  • Water (98-99%)
  • Salivary Amylase (digestive enzyme that breaks down starches)
  • Mucins (mucus that lubricates the mouth and food)
  • Ions, buffers, metabolites, antibodies, etc.
• Saliva moistens foodstuff and converts it into a bolus.
• A bolus is a mass of moistened food that can be easily swallowed.

Histology of the GI Tract

• The walls of the GI tract from the oesophagus to the anal canal have four layers (tunics).
• From the lumen outward, the four layers are:
  • Mucosa (in contact with bolus)
  • Submucosa (blood vessels and lymphs)
  • Muscularis externa (inner most layer and outer most longitudinal layer)
  • Serosa (tissue maintains intestine functions and prevents perforation)

Stomach Modifications

• The muscularis layer is modified to suit the stomach's functions, like peristalsis.
• Modifications of the stomach wall facilitates greater churning/mixing ability.
• Foodstuffs are broken down into smaller pieces
• The stomach comprises of 3 layers:.
• Circular
• Longitudinal
• Oblique (additional innermost layer)
• The stomach is lined with invaginations called gastric pits.

Mucus Cells in the Stomach

• Glycoprotein products are found throughout the entire GI tract and primarily function as a lubricant.
• Mucus has regionally specialized functions such as protection against some substances like bacteria.
• Bicarbonate generates a pH gradient within the mucus to protect the stomach wall from digestion by gastric acid and enzymes.

Chief Cells and Stomach Secretions

• Zymogen granules on stimulation release pepsinogen
• Protein is acted on by pepsin, turning it into peptides
• Pepsinogen is converted (inactive) to pepsin (active) with acid
• Pepsin is an enzyme for protease that breaks down proteins
• Chief Cells produce pepsinogen, converted to pepsin
• Pepsin breaks down proteins into smaller polypeptide and amino acid fragments
• Chief Cells produce gastric lipase, a fat-digesting enzyme
• Approximately 40% of preduodenal lipolysis is performed by gastric lipase.

Parietal Cells

• Parietal cells secrete hydrogen (H+) and chloride (Cl-) ions forming hydrochloric acid against a concentration gradient of 100000:1.
• Hydrochloric Acid (HCl):
  • Increases the acidity of the stomach (pH = 1.5 - 3.5)
  • Denatures food
  • Activates pepsinogen that in turn creates pepsin
  • Dissolves bone
  • Bacteriocidal
• Parietal cells secrete Intrinsic Factor, which is required for Vitamin B12 absorption in the small intestine.
• Only the stomach function of secretions is essential for life.

Endocrine Cells

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

Gastric Secretions Summary

• Production of mucus from the mucous neck cells
• The Stimulus for Release is Tonic which has an increased interaction with the mucosa
• The gastric acid buffers to prevent damage to the epithelium
• Bicarbonate secreted with mucus buffers to prevent damage to the epithelium
• Gastric acids (HCI) are triggered by acetylcholine, gastrin and histamine
• This enables the activation of pepsin and destroys bacteria
• Intrinsic factor complexes with vitmain B12 and allows for permit absorption
• Acetylcholine and gastrin and Histamine stimualtes gastric and secretions of histamine
• Production of pepsin from secretions of pepsinogen and gastric lipases triggers release of acetylcholine and acid
• This then digests proteins
• Somatostatin inhibits gastrin secretions
• Acetylcholine, peptides and amino acids stimulates gastric acid secretions

Gastric Juice and Stomach Protection

• Gastric juice does not digest the walls of the stomach because of:
  • Mucous barrier
  • Epithelial cells
• The mucous barrier is alkaline:
  • It neutralizes acid on the stomach lining.
• Tight junctions between epithelial cells prevent acid from leaking to underlying tissue.
• Rapid turnover of epithelial cells occurs.
• Epithelial cells are replaced every 3 days.

Rate of Gastric Emptying

• Rate is dependent on the type of food ingested.
• Fluids pass through quickly in roughly 90 minutes.
• Solids remain in the stomach until reduced to very small particles.
• Solids remain in the stomach until dissolved in gastric juices for 3-4 hours.
• Gastric emptying of nutrients involves:
  • Carbohydrates emptying first (fastest)
  • Proteins (followed by)
  • Fats (slowest to leave the stomach)

Stomach Anatomy Questions

• Anatomical modifications that support function include:
• The existence of three muscle layers with Mucosa-Submucose-Surasa
• Chief cells produce pepsinogen

Small Intestine Overview

• The chyme enters the small intestine from the stomach.
• The subdivisions of the small intestine are:
  • Duodenum
  • Jejunum
  • Ileum
• Intestines have large surface area which increases area for absorption
• A number of ways to increase surface area:
• Plica - folds with invaginations in the intestines
• finger-like projections in the mucosa layer called the villi
• invaginations called crypts
• The increased surface area = increased absorption

Intestinal Villi details

• Intestinal contains microvilli, also contains absorptive cells, and also has goblet cells
  • Goblet cells can be identified by crypt lumen, secretory cells, endocrine cells and lamina propri
  • Also contain muscularis mucosae

Small Intestine Function

• On entering the small intestine:
  • Carbohydrates and proteins are only partially digested
  • They're still too large to be absorbed through the SI wall
  • Fats are undigested
• All nutrient absorption occurs in the small intestine
• In the duodenum:
  • enteroendocrine cells secrete peptides
  • bile from liver is added to chyme
  • enzymes from pancreas are added to the chyme

The Liver and Bile

• Aids in digestion by synthesizing and secreting bile
• Is a yellow-green alkaline solution
• The Composition:
  • Water
  • Bile salts -Synthesized in the liver from cholesterol. -Function: Emulsification of fat globules. -Mechanical break down of fat globules into small droplets. -Small droplets have larger surface area thus allowing lipases to act more effectively.

The Gallbladder

• The gallbladder's small green sac is located on the inferior surface of the liver.
• Concentrates and stores bile, but does not synthesize it.
• Sphincter of Oddi:
  • When closed, bile cannot enter duodenum.
  • Bile is then stored in the gallbladder.

Pancreas and Acinar Cells

• Acinar cells secrete digestive enzymes
• The Pancreas provides - Exocrine Function for digesting foods.
• Secrete pancreatic juice into the pancreatic duct
• Watery alkaline fluid (pH = 8)
• Contains HCO3- and various digestive enzymes

Pancreatic Juice Contents

• Consists predominantly of water (99.5%) and solids (0.5%)
• The solids include organic and inorganic substances
• Enzymes:
  • Proteolytic enzymes (Trypsin, Chymotrypsin, Carboxypeptidases, Nuclease, Elastase, Collagenase)
  • Lipolytic enzymes (Pancreatic lipase, Cholesterol ester hydrolase, Phospholipase A, Phospholipase B, Colipase, Bile salt-activated lipase)
  • Amylolytic enzyme (Pancreatic amylase)
• Other organic substances (Albumin, Globulin)
• The inorganic substances include:
  • Sodium
  • Calcium
  • Potassium
  • Magnesium
  • Bicarbonate
  • Chloride
  • Phosphate
  • Sulphate
• The bicarbonate content is 110 to 150 mEq/L

Pancreatic enzyme activation

• The inactive pancreatic secreations in the small intestine get triggered by Trypsinogen in inactive form, this activates the activation of Typsin in its active form
• The amylase does not need activation, but the few arbohydrates can become available and is safe for breakdown of the active compounds to breakdown carbohydrates, this is triggered with enteropeptidase and enterokinase

Intestinal Juice Functions

• Is a watery secretion with neutral pH (7.0)
• Serves as a medium for digestion and absorption of nutrients
• Glands normally secrete ~1-2 L/day
• Contains digestive enzymes secreted by epithelial cells in the small intestine
• Its composition allows for Disaccharidases
  • Carbohydrate digestion: disaccharides to monosaccharides
• Peptidases
  • Protein digestion: polypeptides to amino acids
• Lipases
  • Fat digestion: lipid to monoglyceride and fatty acids

Carbohydrate Digestion

• Pancreatic amylase continues carbohydrate hydrolysis.
• Enzyme action on the intestinal lumen's brush border completes its final stage of digestion.
• Simple sugars are absorbed across intestinal mucosa.
• Glucose is absorbed by active transport.
• Maltase, Surcrase, and Lactase assist in the break down of carbohydrates
• Maltase acts on maltose
• Sucrase acts on Sucrose
• Lactase acts on Lactose
• Cannot be digested by humans; passes to the large intestine
• Nondigestible carbohydrate is called fibre.

Protein Digestion

• Started in stomach with Pepsin and breakdown through trypsin
• Continued in small intestine using pancreatic fluid and further dismantled to tripeptides, dipeptides, and single amino acids.
• Amino acids also joining with Na+ for transport
• EndOpeptidase breaks the peptides up and removes peptides
• ("mostly” found on surface of intestinal absorptive cells)
  • (1)hemove amino acid
  • (2)Process absorption

Lipid Digestion

• Triacylglycerides (TG) are the most abundant dietary fat.
• The major lipid breakdown occurs emulsifying action ofbile and the hydrolytic action of pancreatic lipase.
• Bile increases the lipid droplets the' solubility and digestibility through emulsification.
• Pancreatic lipase breaks down TGs into: • 2 free fatty acids (FFA) • 1 monoglyceride (glycerol) •Free-fatty acids (with 10 or less C’s), rapidly absorb into the portal vein • 1: Emulsification • 2 Micellar formation • 3 Liped Droplets recycle from Bile • 4: Absorption • 5: Re-esterification • 6: Lipoprotein formation/transport

Long-Chain Lipids

• Long-chain fatty acids are absorbed by the intestinal mucosa by bile salts which recycle (FFA Glycerol), the triglycerides go through re-esterfication
• They reform into triacylglycerols (triglycerides) then form chylomicrons
• Chylomicrons move slowly through the lymphatic system and empty to lacteals
• They empty into the venous blood of the systemic circulation

Vitamin Absorption

• Vitamin absorption occurs mainly by the passive process of diffusion in the jejunum and ileum.
• Fat-soluble vitamins (A, D, E, K) are absorbed with dietary lipids.
  • Once absorbed, chylomicrons and lipoproteins transport these vitamins to the liver and fatty tissues.
• Water-soluble vitamins diffuse into the blood, except for vitamin B12 (needs intrinsic factor)
  • This vitamin combines with intrinsic factor produced by the stomach, which the intestine absorbs by endocytosis.
• Water-soluble vitamins pass into the urine when their concentration in plasma exceeds the renal capacity for reabsorption.

Electrolyte Absorption

• Sodium (Na+) and Chloride (Cl-) ions are absorbed via active transport.
• Potassium (K+) ions are absorbed via passive diffusion.
• Water (H2O) is:
• Absorbed via osmosis
• moved Freely across intestinal mucosa

Large Intestine Overview

• Two major Regions: the Colon and the Rectum
• Colon is the largest, Rectum is more distal
• Includes anal glands and sphincters
• Runs inline with the sphicters

Anatomical modifications

• Features are mainly microvilli to assist with water absorption
  • Mucosa contains goblet cells and there is no more need for absorption and further
  • Lining has a alkaline mucosa- PH8
  • Has Holds faeces togteher and protects the lining

Intestine Functions

• Proximal Section has absorbs water to dehydrate the wastes
• Distval has main defecation propulsion
• The intestines contains millions of bacteria
• This allow for metabolize remining nutrient
• Produce Vitamins B1, B7, B12, and K, these are absorbed

Secretion and Water Balance

• Water & 2.0L Food and drink: 1.5 L Saliva, 0.5 L Bile(liver), 2.0 Gastricsecretions, 1.5 LPancreatic secretions and 1.5 Lintestinal secretions
• Leads to absorption and water totals of 8.9L
• Totals in 9.0 L Total input into lumen and 1.4 L From lage intestine and excretion of 0.1 L in feces

Secretion Absorption and Water

• Fluid setion ranges from 1.5 ml per day to 3 and small intensities ahve 1.5ml compared to colon that do not send. dietary is at 1.5 day and total water is 9.5L day compared to feacal of 9.2L / day
• This creates a small feacal loss

Defecation

• Vaeces reach the reectom mass movements, which distends the section and triggers defecation'