Gastrointestinal System Overview

Overview of the Gastrointestinal System

• The gastrointestinal (GI) system, also known as the alimentary canal, facilitates digestion and absorption of food.
• Nutrients become more accessible to the body through each step of the digestive process.

Key Activities of the GI Tract

• Ingestion starts the digestive process.
• Propulsion moves food through the digestive tract.
• The food is physically broken down during mechanical digestion
• Chemical digestion uses enzymes to break down food.
• Absorption involves the transfer of nutrients into the body.
• Defecation eliminates undigested material.

Components of the GI Tract

• Primary digestive organs include the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine.
• Accessory digestive organs include the teeth, tongue, salivary glands, gallbladder, liver, and pancreas.
• Salivary glands aid in lubrication and have a protective effect on the GI tract.
• The gallbladder stores bile.
• The liver produces bile.
• The pancreas produces digestive fluids.

Main Function and Nutrients

• The main function of the GI tract is to process food, breaking it down into nutrients.
• Key nutrient classes include carbohydrates, lipids, proteins, vitamins, minerals, and water.
• An adequate diet should contain 50-60% carbohydrates, 25-35% lipids, and 15-25% proteins, with vitamins, minerals, and water making up the remainder.

Salivary Glands

• Salivary glands include the parotid, submandibular, and sublingual glands.
• Parotid glands are the largest salivary glands and are located anterior to the ears.
• Submandibular glands are inferior to the jaw.
• Sublingual glands are inferior to the tongue and produce mucus.
• Amylase is an important enzyme produced inside the mouth.

Saliva

• Saliva production ranges from 1 to 1.5 liters per day.
• 98–99% of saliva is water
• It also contains salivary amylase, which breaks down starches.
• Mucins in saliva lubricate the mouth and food, and saliva also contains ions, buffers, metabolites, and antibodies.
• Saliva moistens foodstuff, converting it into a bolus that is easily swallowed.

Histology of the GI Tract

• From the oesophagus to the anal canal the walls of the Gl tract have the same four layers (tunics)
• The four layers of the GI tract, from the lumen outward, are the mucosa, submucosa, muscularis externa, and serosa.
• The mucosa is in contact with the bolus.
• Submucosa contains blood vessels and lymph vessels.
• The muscularis externa consists of most layers of the GI tract including the outer longitudinal layer.
• The serosa provides structure, helps maintain intestinal functions, and prevents perforation.

Stomach Anatomy

• Key features of the stomach.
• The oesophagus connects to the stomach, which includes the funds, body, antrum, and pyloric valve.
• Rugae are folds inside the stomach that expand to hold food.

Modifications of the Stomach Wall

• The muscularis layer of the stomach is modified for functions like churning and mixing, facilitating mechanical breakdown of food.
• The stomach wall consists of three layers: circular, longitudinal, and an additional innermost oblique layer.

Stomach Lining and Secretions

• The surface of stomach walls has invaginations known as gastric pits.
• Mucus cells produce glycoprotein for the Gl tract to act as a lubricant and serve specialized functions in the region, and secrete neutral which produces a pH gradient wall, preventing autodigestion.
• Bicarbonate is created to to neutralize the acid.

Chief Cells

• Stimulation of zymogen granules in chief cells leads to the release of pepsinogen.
• Pepsinogen turns into Pepsin using Acid.

Pepsinogen and Gastric Lipase

• Pepsinogen is converted to pepsin to break down proteins and form smaller peptide and amino acid fragments.
• Gastric lipase is for fat-digesting and is approximately 40% of preduodenal lipolysis.

Parietal Cell Translocation

• Parietal cells secrete H+ and Cl- to aid digestive funtions.
• The secretion happens against concentration gradient.

Parietal Cells Secretions

• Parietal cells secrete HCl, a strong acid that increases the acidity of the stomach to a pH of 1.5–3.5.
• HCl helps denature food, activates pepsinogen, dissolves bone, and acts as a bactericidal agent.
• Parietal cells also secrete intrinsic factor, which is required for vitamin B12 absorption in the small intestine.

Endocrine Cells

• G cells secrete gastrin, stimulating acid release.
• D cells secrete somatostatin, inhibiting gastrin secretion.
• Enterochromaffin-like cells secrete histamine, stimulating acid release from parietal cells.

Gastric Secretions

• Mucous neck cells secrete mucus, providing a physical barrier between the lumen and epithelium.
• Gastric acid activates pepsin and kills bacteria.
• Parietal cells secrete Intrinsic factor helps complex the vitamin, which permits absorbtion.
• pepsin(ogen) gastric lipase in Chief cells digest Protiens.
• D cells in Somatostatin helps inhibit gastric acid secretion.
• G cells in Gastrin stimulates gastric acid secretion

Protection of Stomach Walls

• The stomach walls are protected from gastric juices by a mucous barrier and epithelial cells.
• The mucous barrier neutralizes acid, and tight junctions between epithelial cells prevent acid leakage.
• Rapid cell turnover replaces epithelial cells every 3 days, and the alkaline mucous protects from the stomach walls

Gastric Emptying

• The rate of gastric emptying depends on the type of food ingested.
• Fluids pass through quickly in about 90 minutes.
• Solids remain in the stomach until reduced to small particles, usually taking 3–4 hours.
• Carbohydrates are emptied first, followed by proteins, with fats taking the longest to leave the stomach.

Stomach Anatomy Modifications

• Modifications that support its function include that it contains three muscle layers.
• Pepsinogen is produced by Chief cells.

Small Intestine

• Leaving the stomach contains chyme, which then enters the small intestine.
• The small intestine are the duodenum, jejunum, and ileum.
• Intestines have a large surface area to increase absorption of food.

Absorption in the Small Intestine

• Intestinal mucosa shows mucosal layer, or circular folds inside of the intestines that is used to increase the amount of absorption of nutrients
• Mucosal layer has projections called villi.
• Small intestine has crypts and villi that increases absorption

Small Intestine Function

• Carbohydrates and proteins are partially digested.
• As fats are undigested nutrient absorption occurs in the walls of the small intestine.
• The enteroendocrine hormone is secreted in the duodenum, with bile (created in the liver and stored in the gall bladder) and enzymes (created in the pancreas ) added to the Chyme during absorption process in the small intestine.

Liver Function

• The liver aids in digestion through bile secretion and synthesis.
• Bile is a yellow-green alkaline solution that emulsifies fats, breaking them down into smaller droplets for easier digestion by lipases.

Gallbladder stores Bile

• The gallbladder is a small green sac on the inferior surface of the liver that concentrated bile
• Doesn't synthesize bile
• Sphincter is stored in the Oddi, when bile cannot enter the duodenum, it is then stored in the gall bladder.

Pancreas

• The pancreas functions in the body to produce acinar cells which secrete pancreatic juice in order to regulate pancreatic function.
• Also secretes Watery Alkaline fluid with a pH = 8
• Contains HCO3 and other digestive enzymes.

Pancreatic Juice

• Pancreatic juice is mostly water, but also contains solids.
• It has organic and inorganic substances
• Made up of Proteolytic, Lipolytic and Amylolytic enzymes.
• Made up of electrolytes and is high in Bicarbonate content.

Activation of Pancreatic Enzymes

• Amylase requires no activation needed
• Activated pancreatic enzymes needs Trypsin, Chymotrypsin and Carboxypeptidase to secrete by the duodenal the lumen of the small intestine.

Intestinal Juice

• Intestinal juice is a watery secretion.
• Serves as a medium that digest and absorbs nutrients
• Helps secrete digrestive enzymes
• Secretes Disaccharidases that break down Carbohydrates
• Protein digestion is aided by the secretion of Petidases.
• Fat is digested by lipases

Carbohydrate Digestion

• Pancreatic amylase continues carbohydrate hydrolysis.
• Action on the Intestinal Lumen's brush continues the final stage of digestion.
• Simple sugars get absorbed and the glucose is absorbed across intestinal mucosa.

Carbohydrate Digestion continued

• Process starts with starch and glycogen, they form polysaccharides.
• Next form into act on with Maltase, Sucrase, Lactase, which becomes the specific disaccharides that convert to mono saccharides.

Can all Carbohydrates be Absorbed?

• No
• Conserve water.
• Cannot be digested by the body, and helps move through the large intestine

Protein Digestion

• Starts within the stomach with Pepsin to continue with the process in the intestine using pancreatic fluids.
• Further Dismantling into Amino Acids also has Na for transport.

Protein Digestion further explained

• The digestive track then secretes enzymes to complete the whole digestion process, in which it is found on the "surface" side of the cells.
• It also helps in blood transport.

Lipid Digestion in the SI

• TGs are fats that gets reduced and hydrolyzed through digestion.
• The lipids also help breakdown emulsifying and other pancreatic lipase
• Bile increases solubility and digestibility through emulsification.
• Pancreatic lipase is broken down into smaller acids and fatty acids, with the small fatty acids absorping into the portal vein.

Lipid Absorption

• They have to first have pancreatic lipase and colipase
• The smaller fatty acids and cholesterol can get into the wall of the membrane through emulsion due to the smaller size through hydrolysis
• Next can transform into chylomicrons for transport.

Lipid Absorption continued

• Long fattty acids are absorbed by the Lumen intestine mucosa
• The chylomicrons slowly transports through the body.

Vitamin absorption

• Absorption of vitamins uses vitamin diffusion.
• Dietary lipids help absorp vitamins.
• The vitamin B12 is not absorbed in the lining, they need other intrinsic factors in that specific structure as they enter the bloodstream
• It dissolves and then travels to the liver.
• After being absorbed into the blood, it dissolves into water, passes in plasma, then to the kidneys to absorb.

Absorption of major nutrients

• Vitamins also get absorbed into their various forms after digestion.
• Chylomicrons bind to the small fat vitamins to help it absorb into the lymph system, however water is absorbed via active transport.

Electrolyte Nutrients

• Na+ & Cl- both need to be absorbed through active nutrient absorption.
• The small intestine absorbs and distributes everything into the lymphatic and then vascular system.

Large Intestine

• Ascending colon
• Ileocecal valve
• Hepatic vein
• Inferior Vena Cava
• Aorta
• Transverse Colon
• Intestinal Crypts
• Cecum
• Appendix
• Teniae Coli
• Ileum
• Lymphord nodule
• Muscularis
• Rectum
• Sigmoid Colon
• Anus

Large Intestine Regions

• Colon
  • Largest Portion of Large Intestine
  • Illeocecal valve
  • Rectum
• Rectum
  • Extends to Anal Canal
    • Opens outside the body

• • Sphincters

    • Intenal Sphincter
    • External Sphincter

Large Intestine Modifications

• Mucosa contains many Goblet

• • No villi

  pH ~ 8%

  • Holds feces & protects lining

• Longitudinal Muscle Layer

  • 3 bands
  • Called haustra

Intestine Functions

• **Proximent Dehydrates Faces
• **Distal
• Power Propultion

Functions

• Digestion
  • Millions of Bacteria
    • Metabolize -Produce
• Vitamin
  • Bi
    • Produce
• • K
• B7
• By 12

G.I Section and Absortion

• Balances Water Input Food and Drink
• 1.5 L Saliva
• 0.5 L-Bile -L
• 1. 5 Intestonial
• 9 L Total -1

Flux Section And Absorption

• Salivary Gladnds 1.5l DAY
• Stomach = 2L/Day
• Pancratice 3L/Day
• Smell intestine. 5L/Daily
• Very Little

Digestive Function

• Power Propolution for Movement

Digestion and Absorption

• Ingestion
• Propolution
• Digestion
• Absorbtion
• Defication

Points to Remeber!

• *Important Digestive
• Transport to help liver!