W16N GI System

Functions of the GI System

• The gastrointestinal (GI) system, or digestive system, is a group of organs that work together
• It breaks down ingested food into smaller, absorbable nutrient molecules
• These molecules are absorbed into the blood
• The blood distributes them to all the cells in the body
• The GI system digests ingested food into simple nutrient molecules
• It absorbs and transfers nutrient molecules, electrolytes, and water from ingested food into systemic circulation
• The GI system makes preformed organic nutrient molecules in ingested food available as fuel sources or building blocks for body cells
• The five basic digestive processes are motility, secretion, digestion, absorption, and storage/elimination

Motility

• Motility refers to the muscular contractions that mix and move contents in the digestive tract
• Propulsive movements propel contents forward
• Mixing movements mix food with digestive juices, promote digestion, and facilitate absorption

Secretion

• Secretion plays a vital role in the digestive process
• Several digestive juices are secreted in the GI tract in response to neural or hormonal stimulation
• Secretions consist of water, electrolytes, and organic constituents like enzymes and mucus
• Exocrine secretions are secreted into the GI tract lumen
• These include H2O, HCl, HCO3-, bile, lipase, pepsin, amylase, trypsin, and histamine
• Endocrine secretions are secreted into the bloodstream, such as gastrin, secretin, CCK, VIP, and somatostatin

Digestion and Absorption

• Digestion is the biochemical breakdown of complex foodstuffs into smaller, absorbable units
• This is known as chemical digestion which is accomplished by enzyme-mediated hydrolysis
• Carbohydrates (starch), proteins, and fats are broken down into monosaccharides, amino acids and glycerol & fatty acids, respectively
• Absorption involves the transfer of small digested units, water, vitamins, and electrolytes from the GI tract into blood or lymph
• Absorption occurs largely and most completely in the small intestine

Gastro-intestinal Anatomy

• The GI Tract: a long, continuous hollow tube, spanning about 30 feet from mouth to anus
• Accessory Organs: The salvary glands, liver and pancreas

Alimentary Layers

• Mucosa
  • Innermost, moist, folded lining
  • Increases surface area for absorption of nutrients, water, and electrolytes
  • Lines the luminal surface, divided into mucous membrane, lamina propria and muscularis mucosa
  • The mucous membrane features:
  • An inner epithelial layer for protection
  • Modifications for secretion and absorption
  • Exocrine gland cells: Secrete digestive juices
  • Endocrine gland cells: Secrete blood-borne GI hormones
  • Specialized epithelial cells: Aids absorption of digested nutrients
  • Lamina Propria: A thin, connective tissue layer where epithelium rests
  • GALT: Gut-associated lymphoid tissue is housed here and is very important for fighting intestinal bacteria
• Muscularis Mucosa: thinly dispersed outermost mucosal layer of smooth muscle
• Functions of the Mucosa:
  • Protects against pathogens
  • Secretes mucus, enzymes, and hormones
  • Absorbs nutrients from the blood
  • Rapid cell regeneration in a very acidic environment
• Submucosa: Dense connective tissue with larger blood and lymphatic vessels, lymphoid follicles, and nerve fibers
  • Surrounds the mucosal layer; contains the submucosal nerve plexus, ENS, and digestive function
  • Distensibility and elasticity in digestive tract
  • Glands secrete digestive enzymes
  • Nerve plexus supports regulating gut motility
• Muscularis Externa:
  • Inner layer: circular smooth muscle contraction restricts the tube diameter
  • Outer layer: longitudinal smooth muscle contraction shortens the tube
  • Auerbach's myenteric plexus lies in between the muscles of the enteric plexus
  • Facilitates mixing and movement of food (peristalsis and segmentation), as well as help gut mobility
• Serosa:
  • Outer connective tissue and epithelial covering; anchors digestive organs to abdominal wall
  • Secretes serous fluid for lubrication; reduces friction

Digestive Regulation

• ICCs: Interstitial cells of Cajal (pacemaker cells located throughout the external muscular layers), cause spontaneous, rhythmic electrical activity also described as slow-wave potentials The ileum contracts as result of the cells
• ENS: Intrinsic nerve plexuses, supports reaction to GI contraction
• ENS: Extrinsic nerves, influence GI contraction or secretion
• Gastrointestinal Hormones: Affect GI contraction or secretion

Digestive System Functions

• Oral Cavity
  • Responsible for, ingestion, which is the intake of nutrients/foods through the mouth to stimulate taste buds on tongue
  • Initiates mastication movement (food is broken down and formed into a bolus) and mixed wit h saliva. Amylase is secreted, which begins carbohydrate (starch) digestion Protection is accomplished with mucous and lysozymes
• Pharynx
  • Promotes swallowing, and uses mucus for protection
• Oesophagus
  • Transports ingested food (bolus) to the stomach via peristaltic contractions
• Stomach
  • Stores foods via expanding rugae; allows protein digestions to begin via hydrochloric acid and pepsin
• Small intestine
  • Accomplishes digestion via enzymes (pancreatic and bile), and neutralizes stomach acids with bicarbonate
  • Water in digestive secretions is absorbed and most nutrients via circular folds
• Large Intestine
  • Where absorptions of remaining water and elimination of waste with faeces is carried out

Propulsion Types

• Swallowing: Movement of digested material from mouth to oesophagus
• Peristalsis: The methodical movement of food from one section of digestive tract to another
• Includes mouth and stomach as well
• Mass Movement: Muscular movement that pushes digested ingredients along the digestive tract

Mastication, Deglutition

• Mastication (chewing): Aids in the breakdown of larger foods, is mixed with salivary amylase and lubricates for antibacterial properties
• Deglutition (swallowing): Includes both the oropharyngeal and later oesophageal stages

The Stomach Key Aspects

• The stomach is J-shaped sac-like between the oesophagus, small intestine and is distensible, supporting up to 15ml large meals
• Absorbs nutrients from digestion, in the small intestine
• Gastric secretions are digestive, beginning with HCL protein digestion
• Contains, Pepsin, salivary amylase
• The stomach helps kill bacteria, digests protein and fat with Gastric secretions
• Chyme is formed to neutralise partially digested food

Stomach Structure, Sphincters and Components

The stomach is divided into the fundus, body and antrum sections

• Each section plays a different role in stomach activity. The Muscle layer is also different thicknesses
• Pyloric Sphincter: A barrier, in the small intestine, helping regulate stomach, duodenum and small intestines

Stomach Gland Types and Regulation

• The gastric gland, including HCl and intrinsic factor, has diverse exocrine and cell functions
• Gastric exocrine secretory cells:
  • Mucous cells secrete mucus lining gastric entrance
  • Parietal Oxyntic cells produces HCL
  • Chief cells secrete pepsinogen

Gastric Products

• Parietal cells produce hydrochloric acid for hydrolysis and to sterilize the meal) and intrinsic factor (for Vitamin 12 absoprtion)
• Chief cells releases pepsinogen for protein digestion
• Other secretants include peptide, mucus, gastrin and histamine
• Mucous cells release mucus to protect stomach lining

How Digestion Works

• The stomach parietal cells work to actively secrete hydrochloric acid and chlorine within the stomach for digestion
• Pepsinogen converts into the active enzyme, pepsin, during protein digestion

Hormones

• Gastrin, produced by G-cells and responds to stimuli such as acetylcholine for acid production
• Histamine (paracrine): An ECL secretion that responds to acetycholine and gastrin for greater acid function
• Histamine (paracrine): A D cell release that helps prevent the secretion of acid and regulates HCL production

Stages of gastric Secretions

• Cephalic- Occurs when thinking, smelling and tasting Releases peptide chain and histamine
• Gastric- Stomach activated with proteins to accelerate digestion
• Intestinal- Chyme enters in the gastric juices. Flow is then turned off

The Stomach Protection

• The walls protect damage with various protective actions and the gastric wall is replaced every 3 days via cell division
• This process is then followed as Prostaglandins inhibits acid secretion & promote alkaline mucus secretion

Small Intestine Functions

• Digestion and absorption of ingested food
• It is involved with secretions, uses segmentation, Bile and pancreatic, Intestinal (Brush- border enzymes)
• Is long but largely depends upon the duodenum and jejunum

Duodenum and Regions

• Contains common bile from various parts
• With food entering the duodenum it triggers the release of the bile and secretion needed

Pancreas and Exocrine

• The cells in the pancreas helps release sodium to break down acidity for regulation

Proteins, Amylase, and Lipase

Proteins: Secretions such as trypsin prevent digesting from the cells Amylase- Secreted in active manner to support digestion of starches, The lipids do not trigger any digestion on their part They depend entirely on triglycerides to do so Aqueous- Are actively produced to reduce any damage

Hormone Control and Liver

• Increased secretion of the intestine, neutralises acid and has enzymatic controls
• The liver is the largest gland in the body and the main digestive liver function and produce bile and is recreted

Enterohepatic Circulation

• Bile acids are secreted and recycled and synthesized within the liver
• These acids assist with with digestion, absorption and aid in fat and absorption

Breaking Down Fats and Cholesterols

• Hydrophobic functions break large fat components with the assistance of Lipase
• Transport fat soluble substances from within the body

Protein and Carbs

These digested matter is moved using Symporters and Glucose

Large Intestine Function and Composition

• Regulated to draw water, salt and electrolytes from waste products as well
• Includes fluids, electrolytes and B12 with help with bacteria Includes fermentation and acetic/fatty acid support Haustra: Supports shape of colon
• Taeniae: Supports motility