The Alimentary Canal: GI Tract Overview

• Known as The Alimentary Canal.

Learning Outcomes

• Point-form lecture outcomes to check learning. Successfully completing the study of the GI System will allow you to:
• Outline anatomical features of the GI tract enabling it to function effectively.
• Discuss role/importance of GI secretions and control mechanisms.
• Discuss processes related to breaking down/absorbing food.
• Describe the movements taking place within the GI tract.
• Describe the events of defecation.

GI Tract Overview

• A "disassembly" line where food is digested and absorbed. As this happens nutrients become available to the body.
• Six essential activities occur: Ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.

GI Tract Consists of

• Primary digestive organs are: mouth, pharynx, oesophagus, stomach, small intestine, large intestine.
• Accessory digestive organs include: teeth, tongue, salivary glands (for lubrication and protection), gallbladder (to store bile), liver (to produce bile), and pancreas (to produce digestive fluids).

Primary Function

• The main function of the GI tract is to process food.
• It also handles six classes of nutrients.
• An adequate diet includes all six nutrient classes.
• Dietary requirements: Carbohydrates (50-60%), lipids (25-35%), proteins (15-25%), vitamins/minerals/water (0.5-2%).

Salivary Glands

• Three pairs of salivary glands exist.
• Parotid glands are the largest, anterior to ears.
• Submandibular glands are inferior to the jaw.
• Sublingual glands are inferior to tongue and produce mucus.
• Amylase is an important salivary enzyme.

Digestion Begins in Mouth

• Saliva is produced at 1-1.5 L/day and is mostly water (98-99%).
• Salivary amylase is a digestive enzyme that breaks down starches.
• Mucins are mucous that lubricates the mouth and food.
• It also includes ions, buffers, metabolites, antibodies, etc.
• Actions that occur here are moistening food and converting the resulting mixture into a bolus.
• A bolus is defined as a mass of moistened food that can be easily swallowed.

Histology of the GI Tract

• From the oesophagus to the anal canal, the walls of the GI tract have the same four layers (tunics).
• The layers from the lumen outward are: mucosa, submucosa, muscularis externa, and serosa.
• Mucosa is in contact with bolus.
• Submucosa includes blood vessels and lymph.
• Muscularis externa is divided into most inner and outer longitudinal layers.
• Serosa maintains intestine functions and prevents perforation.

The Stomach

• Contains lots of folds, useful for expanding to hold food.

Stomach Wall

• The muscularis layer is modified according to the stomach functions facilitating churning and mixing abilities for mechanical food breakdown.
• It has three layers: circular, longitudinal, and an additional innermost oblique layer.
• The stomach surface is lined with invaginations called gastric pits and produces mucus.

Mucus Cells

• Produce glycoprotein products throughout the GI tract.
• Primary function of the mucus is as a lubricant, as well as regionally specialized ones.
• Protect against some substances like bacteria.
• Bicarbonate generates a pH gradient to protect the stomach wall from digestion by gastric acid and enzymes.

Chief Cells

• Zymogen granules are created on stimulation to release pepsinogen.
• Pepsinogen is converted to pepsin in the presence of acid to assist with protein breakdown.
• Also contains protease to breakdown proteins.
• Gastric lipase is used for fat-digesting.

Released from Chief Cells

• Pepsinogen is converted to pepsin.
• Pepsin breaks down proteins into smaller polypeptides and amino acid fragments.
• It is involved in around 40% of preduodenal lipolysis.

Parietal Cells Secrete

• Secrete H+ and Cl- secretion against a concentration gradient (100,000:1).
• HCl, a strong acid, increases stomach acidity (pH 1.5 - 3.5) for food denaturation, pepsinogen activation, bone dissolving, and as a bacteriocide.
• Also secrete intrinsic factor required for vitamin B12 absorption in the small intestine
• Only essential stomach function for life

Endocrine Cells

• G cells secrete gastrin and stimulate acid release.
• D cells secrete somatostatin and inhibit gastrin secretion.
• Enterochromaffin-like cells secrete histamine and stimulate acid release from parietal cells.

Gastric Secretion Functions

• Mucus secreted by the mucous neck cells provides a physical barrier between the lumen and epithelium.
• Bicarbonate buffers gastric acid to prevent damage to epithelium.
• Gastric acid activates pepsin and kills bacteria.
• Intrinsic factor complexes with vitamin B12 to permit absorption.
• Histamine stimulates the secretion of gastric acid.
• Pepsinogen digests proteins.
• Gastric lipase digests fats.
• Somatostatin inhibits gastric acid secretion.
• Gastrin stimulates gastric acid secretion.

Stomach Wall Self-Protection

• Why doesn't gastric juice digest the stomach walls?
• Protection is guaranteed via mucous barrier, epithelial cells etc.
• A mucous barrier neutralizes acid on the stomach lining.
• Tight junctions prevent acid leaking to tissue.
• There is rapid turnover of epithelial cells with a replacement cycle of around three days.

Gastric Emptying Rate

• The rate of gastric emptying depends on the type of food ingested.
• Fluids pass through quickly in around 90 minutes.
• Solids remain until reduced to very small particles and are dissolved in gastric juices, typically taking 3 to 4 hours.
• Carbohydrates are emptied first, fats take the longest to leave the stomach.

Small Intestine

• The chyme enters the small intestine after it exist the stomach.
• It is divided into: duodenum, jejunum, and Ileum.

Intestine Absorption

• There is a large surface area to increase absorption.
• Finger-like projections in the muscosal layer are called villi.
• There are also invaginations called crypts.
• Increased surface area means increased absorption.
• Small intestine is the location where all nutrient absorption occurs.
• On entering the small intestine carbohydrates and proteins are partially digested.
• Fats are undigested.
• Bile from the liver and enzymes are added from the pancreas to the chyme.
• Enteroendocrine cells in the duodenum secrete peptides.

The Liver

• Yellow-green alkaline secretion containing water, bile, and salts.
• Aids in digestion by synthesizing and secreting bile.
• Synthesizes bile salts from cholesterol.
• Function of bile is enabling fat emulsification.
• Breaks down large fat globules into small droplets, creating a larger surface area for better lipase access.

The Gallbladder

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

The Pancreas - Exocrine Function

• The pancreas secretes pancreatic juice into the pancreatic duct from Acinar cells.
• The pancreatic juice has a watery alkaline fluid (pH = 8) and contains HCO3- and various digestive enzymes.
• Pancreatic juice contains around 99.5% water and 0.5% solids.
• The solubles consist of organic and inorganic substances.
• Enzymes include proteolytic and lipolytic enzymes.
• Bicarbonate content = 110 to 150 mEq/L (high)

Pancreatic Enzymes Secretion

• Lumen of small intestine.
• Alpha-Amylase has no activation needed.
• Trypsinogen is inactive until trypsin is formed.
• Trypsin then activates chymotrypsinogen, procarboxypeptidase, procolipase, and prophospholipase.

Intestinal Juice

• Watery secretion with neutral pH (7.0).
• Serves as a medium for digestion and absorption of nutrients.
• Glands secrete ~1-2 L/day.
• Contains digestive enzymes secreted by epithelial cells in the small intestine.
• Disaccharidases are involved in carbohydrate digestion and breakdown of disaccharides to monosaccharides.
• Peptidases are involved in protein digestion for polypeptides to amino acids.
• Lipases are involved in fat digestion for lipid to monoglyceride and fatty acids.

Carbohydrate Digestion

• Pancreatic amylase continues the hydrolysis of carbohydrates already initiated in the mouth.
• Enzyme action on the intestinal lumen's brush border completes the final stage of digestion.
• Simple sugars are absorbed across the intestinal mucosa.
• Glucose is absorbed by active transport.
• Amylase is responsible for the breakdown of starch into disaccharides: Maltose, Sucrose, and Lactose.
• Specific Enzymes convert these into the building blocks:
  • Maltase → 2 glucose
  • Sucrase → 1 glucose + 1 fructose
  • Lactase → 1 glucose + 1 galactose
• Cellulose cannot be digested by humans and is passed to the large intestine.

Protein Digestion

• Started in the stomach with pepsin and is continued in small intestine using pancreatic fluid (containing trypsin and chymotrypsin) that further dismantle into tripeptides, dipeptides, and single amino acids.
• Amino acids also join with Na+ for transport.

Lipid Digestion

• Triacylglycerides (TG) are the most abundant dietary fat.
• Major lipid breakdown occurs by the emulsifying action of bile and the hydrolytic action of pancreatic lipase.
• Bile increases the lipid droplets' solubility and digestibility through emulsification.
• Pancreatic lipase breaks down TGs into 2 free fatty acids (FFA) and 1 monoglyceride (glycerol).
• Free-fatty acids (with 10 or less C's) are rapidly absorbed into the portal vein.

Lipid Absorption Process

• Steps in the process are: mechanical digestion, emulsification, micellar formation, hydrolysis, absorption, re-esterification, and lipoprotein.
• Long-chain fatty acids are absorbed by the intestinal mucosa, reform into triglycerides, and then form chylomicrons.
• Chylomicrons move slowly through the lymphatic system and empty into the venous blood of the systemic circulation.

Vitamin Absorption in the Small Intestine

• 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.
• Afterwards Chylomicrons and lipoproteins transport these vitamins to the liver + fatty tissues
• Water-soluble vitamins diffuse into the blood, except for vitamin B12, which requires intrinsic factor.
• Deficiency of intrinsic factor results in lack of vitamin B12 absorption
• Water-soluble vitamins pass into the urine when their concentration in plasma exceeds the renal capacity for reabsorption.

Electrolytes in the Small Intestine

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

The Large Intestine

• The large intestine consists of: Colon and Rectum.
• The colon includes: Ascending, Transverse, and Descending. Terminates at rectum/anus.
• Divided into colon and rectum.

Colon's Regions

• Extends from the ileocecal valve to the rectum.
• Rectum includes the last 6" of large intestine and extends to the anal canal.
• Anal canal opens to the exterior of body at anus.
• It has internal + external sphincters.

Large Intestine, Anatomical Modifications

• Mucosa contains many goblet cells. Because there is no need for further absorption, no villi are present.
• The mucosa is alkaline (pH≈8).
• The longitudinal muscle layer is arranged in bands, producing out-pocketings of the wall that become haustra.

Intestine Function

• Proximal tubule is responsible for the absorption of water to dehydrate the faeces and removes it from most of the body (SI).
• Distal tubule provides the power propulsion involved in the defecation reflex.
• Within this is lumen you will find millions of bacteria that digest and metabolize remaining nutrients and Produce Vitamin
  • B1 vitamin = thiamine
  • K = clotting factors
  • B7 = Biotin
  • B12 = Cobalamin
• Byproducts of digestion include hydrogen, methane, CO2, and hydrogen sulfide.

Gut Motility

• Two main types: peristalsis (move material) and segmentation (mix material).
• Small intestine: 3 contractions/min occur.
• Inner circular muscles contract, thus tightening the tube and pushing the food forward.
• Outer longitudinal muscles contract and the intestinal tube loosens. This way the chyme moves ahead of the constriction.
• Segmentation: circular muscles squeeze contents, pushing contents back.

Large Intestine Motility

• Transit occurs approximately 36–48h.
• Has haustrations: bulging of portions + circular/longitudinal contractions + faecal matter dug +allows more water absorption.
• Mass movements.
• Includes the haustral contractions driven by circular and longitudinal muscles.
• Process is supported by the vagus and pelvic nerves
• Sympathetic action stops it.
• Sets next action around 1/2 to 1 day.

Control of Colonic Motility

• Intramural plexus (ACh) and Substance P occur on-site.
• Reflex actions include colonocolonic (sympathetic) and gastrocolic mechanisms.

Defecation

• Decision of the central nervous system (CNS).
• The brain commands the sacral cord to relax the external anal sphincter.
• Skeletal motor commands contract abdominal muscle and diaphragm.
• Straightening occurs of the anorectal angle aided by decent of the pelvic floor as well as finally opening of anus.
• Voluntary control is learned in childhood.