The Gastrointestinal System

• The gastrointestinal (GI) system is also known as the Alimentary Canal
• The GI tract digests and absorbs food, making nutrients more available to the body with each step

Six Essential Activities of the GI Tract

• Ingestion: Taking food into the digestive tract
• Propulsion: Moving food through the digestive tract
• Mechanical Digestion: Physically breaking down food
• Chemical Digestion: Using enzymes to break down food
• Absorption: Transferring digested nutrients from the GI tract to the blood or lymph
• Defecation: Eliminating undigested material from the body

Primary Digestive Organs of the GI Tract

• Mouth
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine

Accessory Digestive Organs of the GI Tract

• Teeth
• Tongue
• Salivary glands which provide lubrication and protective effects
• Gallbladder: Stores Bile
• Liver: Produces Bile
• Pancreas: Produces digestive fluids

Main Function

• The GI tract mainly functions to process food

Six Classes of Nutrients.

• Carbohydrates: 50-60% required
• Lipids: 25-35% required, important for membranes
• Proteins: 15-25% required, important for synthesis
• Vitamins: 0.5-2% required
• Minerals: 0.5-2% required
• Water: 0.5-2% required

Salivary Glands

• There are three pairs of salivary glands
• Parotid glands: The largest salivary glands that are located anterior to the ears
• Submandibular glands: Located inferior to the jaw
• Sublingual glands: Located inferior to the tongue which produce lots of mucus containing the important enzyme amylase

Digestion and Saliva

• Digestion begins in the mouth with saliva
• 1 - 1.5 L of saliva is produced per day
• Saliva is comprised of 98-99% Water

Saliva Components

• Salivary Amylase: A digestive enzyme that breaks down starches
• Mucins: Mucous that lubricates the mouth and food
• Ions, buffers, metabolites, antibodies, etc
• It moistens foodstuff and converts it into a bolus
• Bolus: A mass of moistened food that is easily swallowed

Histology of the GI Tract

• From oesophagus to anal canal, walls have same four layers (tunics)
• Mucosa of GI Tract: In contact with bolus
• Submucosa of GI Tract: Contains blood vessels & lymphs
• Muscularis Externa of GI Tract: Innermost layer, outermost longitudinal layer
• Serosa of GI Tract function: Maintains intestine functions and prevents perforation

Stomach Modifications

• The Muscularis layer is modified based on stomach functions, such as greater churning and mixing along with mechanical breakdown of foodstuff into smaller pieces
• It is composed of 3 layers: Circular, Longitudinal and Oblique layer, an additional innermost layer.
• The epithelial surface is lined with invaginations called ''gastric pits'' which produce mucus,

Mucus Cells

• Glycoprotein products found throughout the entire GI tract
• Primary function as a lubricant
• Can have other regionally specialized functions like protecting against some substances (bacteria)
• Bicarbonates generate a pH gradient within the mucus to protect the stomach wall from digestion by gastric acid and enzymes

Chief Cells

• Zymogen granules on stimulation release pepsinogen which is inactive until converted by acid to pepsin (active) .

Chief Cells Production

• Pepsinogen: Converted to pepsin to break down proteins into smaller polypeptide and amino acid fragments
• Gastric lipase: A fat-digesting enzyme, roughly 40% of preduodenal lipolysis done by this.

Parietal Cells Secretion

• Parietal Cells have H/K-ATPase for Translocation on stimulation
• HCl secretion: A strong acid that increases acidity of the stomach (pH 1.5-3.5)
• HCl denatures food to breakdown proteins
• HCl activates pepsinogen to Pepsin
• HCl dissolves bone and is bacteriocidal
• Intrinsic factor required for vitamin B12 absorption in the small intestine

Unique Stomach Function

• Only stomach function essential for life is Intrinsic factor

Endocrine Cells

• G cells release Gastrin: Release Stimulates Acid
• D cells release Somatostatin: Release Inhibits Gastrin secretion
• Enterochromaffin-like cells: Secrete histamine in bloodstream and stimulate acid release from parietal cells

Why Gastric Juice does not digest the walls

• The Gastric Mucous barrier and Epithelial cells protect stomach walls so it doesn’t self-digest.
• Neutralizes alkaline acid on stomach lining
• Tight junctions between epithelial cells prevent acid from leaking to underlying tissue
• Rapid turnover, replaced every 3 days

Rate of Gastric Emptying

• Rate depends on the type of food ingested

• Fluids pass through quickly in 90 minutes

• Solids remain reduced to very small particles in dissolved gastric juices, taking 3-4 hours

• Gastric emptying of nutrients

• Carbohydrates are emptied first (fastest), followed by proteins, with Fats taking the longest to leave the stomach (slowest)

• the Stomach Wall contains three muscle layers: Mucosa, Submucosa and Subrosa.

• Pepsinogen is primarily produced by Chief cells

Chyme Enters Small intestine

• Small intestine has Duodenum, Jejunum, and Ileum regions

• Intestines have a large surface area for maximizing absorbtion

• Additional surface area created by Plica (folds) / (invaginations) in the Mucosa

• Further increase with Villi by finger-like projections and crypts and invaginations,

Small Intestine

• It digests Carbohydrates and proteins, which are partially digested to be absorbed
• However, Fats are partially undigested
• All nutrient absorption occurs here
• Duodenum contains enteroendocrine cells whose secretion of peptides that add bile from liver to chyme and enzymes to pancreas

The Liver

• aids in the synthesis and secretion of bile.
• The Liver contains a Yellow-green alkaline solution, Water, and Bile salts
• All are synthesized from cholesterol
• It Emulsificates fats globules by mechanically breaking globs into smaller droplets with larger surface to allow Enzyme Lipases to act effectively

Gallbladder

• Small green sac located on the inferior surface of the liver.
• Concentrates and stores bile
• Does have the ability to not synthesise bile
• Sphincter of Oddi helps keep bile out of the duodenum

The Pancreas

• Acinar cells:
  • Secrete Watery alkaline fluid (pH = 8), HCO3-, and juice into pancreatic duct
• It provides digestive enzymes

Pancreas Enzymes

• Provides Water (≈ 99.5%) and Solids (≈ 0.5 %) to digestive Juice
• With water: it has Proteolytic enzymes, Lipolytic enzymes,, and Amylolytic enzymes

Pancreatic Secretion/ Activation

• It sends secretion into small intestine where it is acted on by Trypsinogen an inactive Trypsin source.
• It's then activated by Enteropeptidase which is secreted by the duodenum epithelium which in turns creates other inactive enzymes which are subsequently activated such as:
  • Chymotrypsinogen -> Chymotrypsin
  • Procarboxypeptidase -> Carboxypeptidase
  • Procolipase -> Colipase
  • Prophospholipase -> Phospholipase

Intestinal Juice

• A watery secretion that functions with neutral pH (7.0)
• Serves as a digestion and absorption medium for all nutrients
• Normally, glands secrete ~1-2 L/day digestive enzymes by epithelial cells in the small intestine
• This includes - Carbohydrate digestion: disaccharides to monosaccharides
• Protein digestion of polypeptides to amino acids of
• The Fat digestion of Lipids, lipases to monoglyceride and fatty acids

Carbohydrate Digestion in the Lumen

• Pancreatic amylase in intestinal lumen hydrolicerbolus in intestinal lume brush border
• Enzymes in intestinal Lumen act to absorb Simple Sugars across intetsinal Mucosa.
• Glucose absorbed and by active transport action

Carbohydrates can be digested and produce

• Produce Maltose
• Act on Sucrose
• Act on Lactose
• However, cellulose cannot be digested, instead is passed to the large intestine

Protein digestion

• Starts in the stomach with Pepsin
• Continues in small intestine using Trypsin and chymotrypsin,
• Where smaller peptides fragments are further dismantled into tripeptides, dipeptides, and single amino acids.
• It occurs in intestinal and cellular levels

Lipid Digestion

Triacylglycerides (TG) is the top dietary fat

• Lipases causes breakdown due to bile and pancreatic acids
• Results in digested products called: Monoglyceride and Fatty Acids
• Emulsifies fats for enzymatic activity
• Allows for a water-based enzyme to emulsify fats and break globs down

Steps For Lipid Absorption

• Mechanical digestion breaks globs.
• Chylomicrons (long-chain fatty acids) help emulsify to act as Intestinal Mucosa reforming Triacylglycerols to form Chylomicrons
• Slow transport helps push from Lumen and to Capillary for eventual water absorption
• Fatty acids move between Lumen/ ER cell as transport chains

Vitamin Absorption

• In Small Intestines where passive diffusion occurs at Jejunum
• Fats transported with Chylomicrons (ADE,K)
• Requires transport to/through liver
• Intrinsic factors from B12 and stomach enable Chylomicrons movement
• Water can be absorbed by the body, anything in excess is disposed through urine
• B12 helps transport electrolytes for fluid movement

Large Intestine

• Large Intestine contains ascending+ descending colon
• Ileocecal helps with terminal part
• Lumen helps with nutrients transport

Large Intestine Functions

• Consists of rectum to store stool
• Sphincters help prevent stool leakage
• Absorbs electrolytes
• Protects lining and holds feces
• It produces gut vitamin bacteria for a foodbolus

Large Intestine

• It's a tubule that primarily helps dehydrate bowels
• Power of the bolus helps propel material for a complete release.
• Can generate several food byproducts
• Hydrogen that is a gas released
• CO2 that is released
• Help reuptake Vitamins and Bacteria

Water and waste reuptake amount

• It occurs around +9 to 12L, a signifiant reuptake quantity
• Normal faecal loss/day with waste product

Control of Colonic Motility

• ACH aids plexus to direct internal function for the bolus
• Relaxes colon, while stimulating bowel.

Primary function

• To aid voluntary/ automatic functions and enable elimination
• Brain directs cord which enable/inhibit release