Digestion and the GI System

Introduction to the Gastrointestinal System

• The gastrointestinal (GI) system is responsible for breaking down and absorbing foods and liquids.
• The main GIT is made up of the alimentary canal, which runs from the mouth to the anus.
• Organs include the mouth, oesophagus, stomach, small and large intestines.

Sphincters of the GI Tract

• Sphincters in the GI tract are specialized circular muscles.
• They separate two adjacent organs.
• They act as one-way valves to prevent backward movement of intraluminal contents.
• Exceptions to this one way valve function exist with the internal and external anal sphincters.
• Sphincters have ongoing contractile tone, contracting or relaxing as needed.

Accessory Digestive Organs

• Pancreas
  • The pancreas has both endocrine, such as insulin and glucagon, and exocrine functions, such as digestive enzymes and buffers.
• Gall Bladder
  • The gall bladder concentrates and stores bile for lipid digestion.
• Liver
  • The liver receives nutrient-laden blood from the portal vein.
  • It is involved in reabsorption and secretion of nutrients.
  • It also produces bile and participates in carbohydrate metabolism by processing glycogen and glucose.

General Structure of the GIT

• Mucosa
  • The mucosa consists of an epithelial layer (villi or crypts), lamina propria, and muscularis mucosae.
• Submucosa
  • The submucosa contains connective tissue, blood vessels, and secretory glands.
• Muscle Layers
  • The GIT musculature has inner circular and outer longitudinal layers.
• Enteric Nervous System
  • The enteric nervous system helps to regulate and coordinate digestive processes.
• Serosa
  • The serosa is the outermost layer of the GIT.

Basic Mucosal Types

• Squamous mucosa is protective.
• Gastric mucosa is secretory.
• Intestinal mucosa is absorptive.
• Colorectal mucosa is absorptive and protective.

Musculature of the GIT

• Skeletal Muscle
  • Found in the mouth, pharynx, upper third of the oesophagus, and external anal sphincter.
• Smooth Muscle
  • Located everywhere else in the GIT, featuring specialized pacemaker cells for motility.

Patterns of GI Motility

• Propulsion
  • Propulsion involves controlled movement of foods, liquids, secretions, and sloughed cells.
• Tituration
  • Tituration involves the crushing and grinding of food by the stomach.
• Mixing Function
  • Mixing includes the blending of GI secretions.
• Reservoir
  • Holding of luminal content via sphincters

Primary Modes of Contractility

• Tone
  • Maintains steady pressure on the contents of the digestive system.
  • Prevents walls from remaining permanently stretched after distension.
• Muscular contractions
  • Move contents along the tract (peristalsis)
  • Mix the contents in the digestive tract

Peristalsis

• Peristalsis is the organized propulsion of material within the intestinal lumen.

Segmentation

• Segmentation mixes and propels chyme in the small intestine.
• Smooth muscle contracts, relaxes, then contracts in a different place (oscillating).
• Large intestine exhibits haustral contractions.

Enteric Nervous System (ENS)

• The GI system differs from other peripheral organs due to the ENS.
• It has an extensive intrinsic nervous system.
• It is vast, consisting of 200-600 million neurons.

Distribution of Enteric Ganglia

• Myenteric: mainly controls motility (except in the stomach, where it also controls secretion)
• Submucosal: mainly controls secretomotor and vasomotor functions

Mouth

• Digestion begins in the mouth; the first step can begin even before food is present.
• The cephalic phase of digestion starts when you see or smell food.
  • This leads to salivation, gastric relaxation, and gastric acid secretion.
• Chewing depends on a motor pattern generator located in the brain stem.
• The tongue is a voluntary set of muscles controlled via the motor cortex.
• Swallowing is another voluntary process.

Pharynx

• The pharynx has three sections: the nasopharynx, oropharynx, and laryngopharynx.
• It moves the bolus of food towards the oesophagus via muscular contractions.

Esophagus

• The esophagus is a muscular tube about 25 cm long.
• It serves as a conduit to pass food from the mouth to the stomach.
• Its digestive function is food propulsion.
• Primary peristalsis is controlled by the CNS via the vagus nerve.
• The enteric nervous system also plays a role via secondary peristalsis.

Stomach

• The stomach is a thick-walled organ between the oesophagus and the first part of the small intestine.
• It facilitates mechanical digestion and propulsion.
• The stomach also handles chemical digestion, through the digestion of proteins by pepsin.
• Absorption of a few fat-soluble substances, like aspirin and alcohol, occurs here.

Small Intestine (~6m)

• Duodenum
  • This is the shortest part of the small intestine.
  • It receives secretions from the pancreas and gall bladder.
• Jejunum and ileum
  • Begins at duodenaljejunal flexure and ends at ileocaecal junction.
• Assimilation of nutrients
  • Secretions for digestion and absorption.
  • Motility mixes contents and increases surface area.
  • It controls delivery of chyme from the stomach.
  • Regulates secretion of biliary and pancreatic secretions.

Large Intestine (~1m)

• There is chemical digestion, where some remaining food residues are digested by enteric bacteria.

• Absorption primarily involves water, electrolytes, and bacteria-produced vitamins.

• Propulsion is important for movement.

• Defecation

Secretion in the GI Tract

• A number of secretions are produced along the GI tract.
• Digestive secretions consist of water, electrolytes, and organic constituents such as enzymes, bile salts, or mucus.
• Secretion demands energy, involving active transport of raw materials into the cell, sometimes with passive diffusion.

Digestion

• Humans consume three classes of food: carbohydrates, proteins, and fats.
• Carbohydrates are digested into monosaccharides, mostly glucose.
• Proteins are digested into amino acids and a few small polypeptides.
• Fats (triglycerides) digested into monoglycerides and free fatty acids.
• These molecules cannot be absorbed directly as they are too large so must be chemically degraded to smaller absorbable units by enzymes.

Absorption

• Most absorption occurs in the small intestine.
• Small absorbable molecules, H2O, vitamins, electrolytes, are transferred from the digestive tract lumen to the plasma or lymph.
• Some molecules have to be moved by active transport, like CHO and AAs.
• Lipids can cross the cell membrane freely.

Other Functions of the GIT

• Fluid and electrolyte balance with 9 L into GIT and 0.1L lost.
• It plays a role in immune defence (GALT) with lymphoid tissue aggregates and diffuse cells.
• It is a site of uptake for medicines.
• It is a barrier.
• The gut microbiota plays a role.

GI Conditions

• Stomach and duodenal ulcers
• Coeliac's disease
• IBD
• Dysphagia
• Conditions of the ENS
• Cancers
• Gall bladder stones
• Pancreatic insufficiency
• Liver disease
• Parasites, bacteria & viruses

Key points

• The GIT involves multiple organs and "accessory” organs
• The structure and function of the GIT differs along its length
• The primary function of the GIT is to digest and absorb nutrients for us to assimilate
• This differs for each food class; fat, carbohydrate and protein
• There are multiple levels of regulation for the GIT: hormones, CNS, ENS, humoral factors