GI Tract Anatomy and Function

• The gastrointestinal (GI) system is also known as the alimentary canal.
• During these lectures, the anatomical features, secretions, digestion, absorption, movements, and defecation within the GI tract will be discussed

Essential Activities of the GI Tract

• The GI tract functions as a "disassembly" line, digesting and absorbing food
• Nutrients become more accessible to the body through these processes
• There are six essential activities including ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation

GI Tract Organs

• The primary digestive organs include the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine
• The accessory digestive organs include teeth, tongue, salivary glands, gallbladder, liver, and pancreas

Main Function and Requirements

• The primary function of the GI tract is to process food
• There are six classes of nutrients: carbohydrates, lipids, proteins, vitamins, minerals, and water
• A balanced diet incorporates all these nutrients
• Daily dietary requirements should consist of 50-60% carbohydrates, 25-35% lipids, and 15-25% proteins

Salivary Glands

• There are three pairs of salivary glands
• The Parotid glands are the largest, positioned anterior to the ears
• Submandibular glands are inferior to the jaw
• Sublingual glands are inferior to the tongue, produces mucus and amylase enzyme

Saliva

• Saliva production ranges from 1 to 1.5 liters per day and primarily consists of 98-99% water
• Saliva contains salivary amylase to break down starches, mucins to lubricate the mouth and food
• It also consists of ions, buffers, metabolites, and antibodies
• Saliva moistens food, converting it into a bolus that can be easily swallowed

Histology of the GI Tract

• From the oesophagus to the anal canal, the GI tract walls consist of four layers.
• These layers, moving outward from the lumen, include the mucosa, submucosa, muscularis externa, and serosa

Stomach Anatomy

• The Stomach consists of the Oesophagus which moves into the Diaphragm
• Food gathers in the Fundus and Body before reaching the Antrum then leaves via the Pyloric Valve
• The lining of the Stomach consists of many folds of tissue called Rugae which expand to hold food

Modifications of the Stomach Wall

• The muscularis layer of the stomach is modified for churning/mixing ability known as peristalsis
• Churning is the act of Mechanically breaking down food into smaller pieces
• Consisting of three layers: circular, longitudinal, and an additional innermost oblique layer

Structure of Stomach Wall

• The stomach wall is lined with invaginations called gastric pits
• Gastric pits produce mucus

Mucus Cells

• Glycoprotein products are lubricant for the entire GI tract
• Other regionally specialized functions protect against substances
• Generate a pH gradient within the mucus to protect stomach wall
• Protects from digestion from gastric acid and enzymes

Chief Cells

• Zymogen granules are stimulated to release pepsinogen
• Pepsinogen is an inactive enzyme, and becomes pepsin when in contact with acid
• Pepsin breaks down protiens
• Enzyme involved is called a protease

Pepsinogen and Gastric Lipase

• Chief cells produce pepsinogen which is converted into pepsin
• Pepsin breaks down proteins into smaller polypeptide and amino acid fragments
• They also produce gastric lipase
• Gastric Lipase is a fat-digesting enzyme
• Gastric Lipase accounts for about 40% of preduodenal lipolysis

Parietal Cells and Acid Secretion

• Parietal cells secrete H+ and Cl-, enabling the secretion of Hydrochloric acid
• They also secrete against a concentration gradient.
• This is at a ration of 100 000:1

Parietal Cells and HCl

• Parietal cells secrete HCl (hydrochloric acid)
• HCl increases the acidity of the stomach to a pH of 1.5 - 3.5
• HCl denatures food and activates pepsinogen
• It also dissolves bone and is bacteriocidal
• Parietal cells secrete Intrinsic Factor which is Required for vitamin B12 absorption in the small intestine
• Only stomach function essential for life

Endocrine Cells

• The Endocrine cells release into the bloodstream
• 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 and Functions

• Mucous neck cells secrete mucus which provides physical barrier between lumen and epithelium
• Parietal cells secrete gastric acid (HCl) activating pepsin and killing bacteria/protection and Intrinsic Factor
• Chief cells secrete pepsin(ogen) digesting proteins and Gastric lipase digesting fats
• G cells secrete gastrin which stimulates gastric acid secretion

Protection of the Stomach Walls

• The stomach walls aren't digested due to the mucous barrier and epithelial cells
• The mucous barrier consists of alkaline secretions that neutralize acid on the stomach lining
• Tight junctions between epithelial cells prevent acid from leaking to underlying tissue
• Epithelial cells are replaced every 3 days

Gastric Emptying Rate

• Gastric emptying rate depends on the type of food
• Fluids empties quickly in around 90 minutes
• Solids remain until reduced to small particles, emptied in 3-4 horus
• Carbohydrates empty first followed by protins, Fats take the longest for emptying

Stomach Wall Modifications

• Large Surface for absorption
• Stomach walls contains three muscle layers: mucosa, submucosa and serosa

Pepsinogen Production

• Pepsinogen is produced by the chief cells

Small Intestine Divisions

• On exiting the stomach, the chyme enters the small intestine
• The small intestine has three divisions: Duodenum, Jejunum and Ileum

Factors Increasing Intestinal Surface Area

• Large Surface area to enhance absorption
• The use of finger-like projections known as villi
• Invaginations called crypts that further enhance the surface area

Intestinal Villi and Microvilli

• The instestines contain absorptive cells and goblet cells
• Cells secrete into the Crypt Lumen

Small Intestine Function

• As contents enter the small intestine, carbohydrates and proteins have been partially digested and fats are undigested
• Nutrient absorption primarily takes place in the small intestine
• In the duodenum, enteroendocrine cells secrete peptides
• Bile from the liver and enzymes from the pancreas are added in the small intestine

Liver Function and Bile

• The liver aids in digestion by synthesizing and secreting bile
• Bile is a yellow-green alkaline solution composed of water and bile salts
• Bile salts made from cholesterol
• Bile emulsifies fat globules by mechanically breaking them down into smaller droplets
• Smaller droplets have larger surface areas for lipase act more effectively

Gallbladder

• The gallbladder is a small green sac on the inferior surface of the liver that concentrates and stores bile
• Bile is not synthesized by the gallbladder
• Sphincter of Oddi prevents Bile from entering the Duodenum when closed
• Bile is then stored in the gallbladder

Pancreas and Exocrine Function

• The exocrine function consists of Acinar cells which secrete digestive enzymes
• Secrete pancreatic juice through a pancreatic duct
• Pancreatic juice contains Watery alkaline fluid which has pH of 8
• Pancreatic juice consists of bicarbonate and digestive enzymes

Pancreatic Juice

• Pancreatic juice is made up of ≈99.5% water and ≈0.5% solids
• Pancreatic juice contains organic substances and inorganic substances
• Pancreatic juice contains proteolytic enzymes and lipolytic enzymes

Pancreatic Enzymes

• (α-Amylase) has no activation needed
• Small amount enteropeptidase has to be active

Intestinal Juice

• Intestinal juice is a watery secretion with neutral pH (7.0), serves as medium of digestion
• It contains digestive enzymes and secretes about 1-2 L/day
• Disaccharidases break disaccharides to monosaccharides
• Peptidases which protein digestion polypeptides to amino acids
• Lipases which 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 the final digestion stage
• Absorbed across intestinal, Simple sugars and glucose
• Glucose absorbed by transporting through active transport

Carbohydrate Digestion Enzymes

• Disaccharides are acted on by specific disaccharidases

Absorption of Carbohydrates

• Cellulose not digested by humans, and is passed to the large intestine
• Nondigestible carbohydrate is called fibre

Protein Digestion

• Protein digestion starts in the stomach with pepsin
• Protein digestion continues in small intestine using pancreatic fluids
• Pancreatic fluids contain Trypsin and chymotrypsin
• The peptide fragments further dismantle into tripeptides, dipeptides, and single amino acids
• Single amino acids are joined with Na+ for transport

Lipid Digestion and Emulsification

• Mechanical digestion, absorption, transport
• Bile increases solubility and digestibility through emulsification
• Pancreatic lipase breaks down TGs into 2 free fatty acids (FFA)
• Results one monoglyceride (glycerol)

Steps in Lipid Absorption

• Steps of absorption: Emulsification, micellar formation, hydrolysis, absorption
• Absorbed cells re-esterificated
• Exocytosis
• Lipoprotein formation

Transport of Lipids

• Chain of fatty acids are absorbed by intestinal mucosa
• They reform into triacylglycerols (triglycerides)
• They chylomicrons are move through lymphatic system
• Chylomicrons empty into the blood stream , that circulates systemically

Vitamin Absorption

• Vitamin absorption mainly passive
• Jejunum and ileum are the site of diffusion
• Fat-soluble vitamins Absorbed with dietary lipids (A, D, E, K)
• Chylomicrons and lipoproteins transport, vitamins to liver and fatty tissue
• Water soluble, vitamins diffuse into blood, except vitamin B12
• Combines w/ intrinsic factor created stomach
• Reabsorbed into the intestine
• Vitamin B12 helps pass the urine, concentration of the plasma
• Exceeds renal capacity to be reabsorbed

Absorption of Fat-Soluble Vitamins

• Fat-soluble vitamins (A, D, E, K) bind to ingested lipids and are absorbed with lipids via lacteals
• Water-soluble vitamins are mostly absorbed with water, except for vitamin B12, which must bind with intrinsic factor
• Parietal cells produce Intrinsic factor, and are located in stomach
• Absorbed via endocytosis

Absorption Some Major Nutrients

• Sodium and Chloride is absorbed via active transport
• Potassium is absorbed via passive diffusion
• Water is absorbed via osmosis
• Move freely across intestinal mucosa

Large Intestine Anatomy

• LI: Ascending colon, Ileocecal valve, Ileum, Cecum, Appendix
• LI: Hepatic portal vein, inferior vena cava, Transverse colon
• LI: Aorta, tenia coli. Intestinal crypts, Submucosa
• LI: Lymphoid nodule,Longitudinal layer
• LI: Rectum
• LI: Anus

Large Intestine

• The large intestine has two major regions: colon and rectum
• Distal portion of GI tract, include large intestine
• Colon valve , to rectum
• Anal canal, at anus
• Internal and external sphincters

Modifications of the Large Intestine

• Contain Mucosa that has Goblet cells
• Does not contain Villi
• Is an Alkaline mucosa
• Aids holding feces together and protects the lining
• Has longitudinal Muscle layer arranged into 3 bands
• Haustra has out pocketings of the wall

Function of Large Intestine

• Proximal tubule responsible of what absorption
• Distal tubule the power propulsion

Role of Bacteria in the Large Intestine

• Consists of Bacteria
• Metabolize remaining nutrients
• K;Produce Vitamin: B7;K;B12
• Fermentation

GI Secretion & Absorption: Water

• Total, water consumed is. 9.0
• 8.9 from small intestine
• Secretion excretion 0.1

Fluid Secretion & Absorption in the Gut

• Salivary Glands secretion 1.5 L/day. Stomach Secrete 2 L/day
• Small intestine secretion 1.5 absorbed ≈ 8.2. colon absorb≈ 1l day
• Total ≈ 10 L/day absorbed 9L/day

Defecation Step

• Forced into the rectum. Which is a masse movement
• Is the distension of bowel wall. That triggers in the end

Integrated Digestion

• Digestion consists of five intergrated steps: Ingestion, Propulsion, Digestion, Absoption and Defection

Importance of the Liver

• Anything absorbed from GI tract to liver

Control of Gut Motility

• There are two types of motility: peristalsis - Move material, Mix material

Peristalsis in Intestinal Motility

• Peristaltic contractions Move material through digestive tract
• The circular muscles contracting and tightening to move forward, is and pushing it to the intestine

Segmentation in Intestinal Motility

• Peristaltic contractions move contents
• circular muscles contract,
• The chyme segments, mixes the digestive juices
• Mixes the chyme and bring the nutrients into contact absorption

Normal Movement

Intestine Normal Movement

• transit to large, for to digest and absorb everything

Intestine Normal Movement Function

• Unstimulated is haustration that is sacs
• Food has to be able to move slowly
• Allow for over absorption

Process of Defecation

• Occurs only if not occurs

Process of Anatomic Reflex

Anatomic Reflex

• is voluntary only

Anatomic Reflex Part

• CNS, sacral chord to anal chord and muscle

Sequence of Defecation

• Decision, to signal for a bowel movement