The Digestive System: An Overview

• GDC Learning Outcomes include describing relevant anatomy and physiology and explaining their application to patient management related to dental, oral, craniofacial, and general health.

Intended Learning Outcomes

• Session goals include describing digestive organs, explaining the functions of major digestive organs, detailing tissue and lining types in digestive organs, listing gastric juice functions, and summarizing food's chemical digestion.

Curriculum Rationale

• Understanding the digestive system's structure and function is necessary for student dental hygienists and therapists.

Digestion Activities

• The five main activities in the digestive system are Ingestion, Propulsion, Digestion, Absorption, and Elimination (defaecation).

Alimentary Tract

• Extends from the mouth to the rectum.
• Food is ground and mixed with digestive juices as it moves through the alimentary canal.

Alimentary Tract Function

• Converts food to nutritional molecules for absorption and distribution
• Excretes unused residue.
• Part of the immune system.

Alimentary Tract Active Processes

• Ingestion
• Propulsion
• Digestion
• Absorption
• Elimination (defaecation)

Alimentary Tract Immune System

• Is part of the immune system via low pH, mucus (IgA), lysozyme, and enzymes (saliva, bile).
• Saliva is poisonous in some animals to quieten down living prey.

Digestive System components

• Consists of the mouth, oesophagus, stomach, small and large intestines, and rectum.

Accessory organs

• Include the salivary glands, liver, pancreas, and gall bladder.

Alimentary Tract - four layers

• Adventitia/Serosa: Outer connective tissue covering.
• Muscularis: Smooth muscle for peristalsis, with longitudinal and inner-circular layers.
• Submucous: Loose connective tissue containing blood vessels, lymphatic vessels, nerves, and glands.
• Mucosa: Lining epithelial layer coated with mucous.

Mouth - Mechanical

• Teeth: Physically break down food into smaller pieces, producing a bolus.
• Uvula: Prevents food or liquid from entering the nasal cavity.
• Tongue: Voluntary muscular structure crucial for mastication and deglutition (swallowing).

Mouth - Chemical

• Salivary amylase (ptyalin) breaks down polysaccharides/starch in the bolus into maltose.
• Starch particles are too large to be absorbed in the small intestine.
• Taste buds allow appreciation, and sample potential hazards.

Bolus Formation

• A round or oval mass of food formed from chewing.
• Food is crushed and ground by the teeth through mastication.
• Salivary glands secrete saliva, which moistens the food.

Salivary Glands

• Submandibular glands at the angle of the jaw produce 70% of saliva.
• Sublingual glands in front of the submandibular glands produce 5% of saliva.
• Parotid glands below the external acoustic meatus produce 25% of saliva.
• Saliva is mostly water (99.5%) and contains mucus and salivary amylase to begin carbohydrate digestion.

Swallowing Phases

• Involves the tongue, soft palate, pharynx, and oesophagus.
• The voluntary phase forces food into the pharynx.
• Swallowing becomes involuntary/reflex.
• The tongue blocks the mouth, the soft palate closes off the nose, and the larynx rises, closing off the trachea with the epiglottis.
• Food moves into the pharynx and onwards via peristalsis.

Oesophagus

• The swallowed bolus travels from mouth to stomach via the oesophagus, a muscular tube.
• Is 25cm long.
• Lined with a mucus membrane which secretes fluid to lubricate food movement downwards to stomach.
• Smooth muscle wall to squeeze food.

Peristalsis

• Contents are propelled through the digestive tract via rhythmic contractions of smooth muscle.
• Circular contraction occurs behind the bolus to prevent backwards movement.
• Longitudinal smooth muscles contraction pushes bolus forwards.
• Bolus turns into chyme when it enters the stomach.

Digestive Tract Movements

• Segmentation mixes food in the small intestine.
• Periodic constriction of segments of intestine moves food without backwards movement.
• Vomiting propels food upwards via abdominal muscles and does not reverse peristalsis in the oesophagus.

Stomach

• Hollow, J-shaped muscular organ located on the left side of the abdominal cavity.
• The fundus, body, and pylorus are distinct regions.
• A pyloric sphincter separates the stomach from the small intestine.
• A highly acidic environment.
• An oesophageal sphincter separates stomach from oesophagus.

Stomach Structure

• The lining has folds called rugae when empty.
• Gastric glands secrete "gastric juice".
• Parietal cells produce HCL acid to denature proteins.
• Chief cells produce pepsin for protein digestion.
• Mucus cells produce mucus to protect against hydrochloric acid.

Stomach Lining

• Forms deep pits and contains glands.
• Absorbs some water, lipids, caffeine, alcohol.
• Epithelium forms deep pits which can contain glands
• Glands secrete enzymes, hormones, and mucus.
• Hormones (e.g., gastrin) increase HCL secretion and stomach motility.
• Enzymes (e.g., pepsinogen to pepsin) are necessary for protein digestion.

Stomach Functions

• Temporarily stores food for 3-4 hours; churned by muscular layers to form chyme (creamy substance) that is then voided via the pyloric sphincter into the duodenum.
• Muscles relax and contract to aid mechanical breakdown.
• Rugae increase surface area.
• Pyloric sphincter holds food in stomach, but allows for emptying of chyme into small intestine.

Small Intestine

• Comprises the duodenum, jejunum, and ileum.
• Site of the majority of digestion.
• Longest part of the digestive system.

Duodenum

• Is 25 cm long.
• Curves around the head of the pancreas.
• Functions at the entry of common bile duct.
• Chemical degradation of food controlled by pyloric sphincter begins here.
• Enzymes secreted by the pancreas and duodenum aid in fats emulsification.

Duodenum Ducts

• Two ducts which enter are the bile duct draining gall bladder from liver, and the pancreatic duct draining pancreas.

Jejunum and Ileum

• Continue the absorption of carbohydrates, proteins, and fats.
• Surface area increased by fold/wrinkles, with villi microscopic projections.
• Lined with simple columnar epithelium called microvilli.

Villi

• Each villus has a capillary and a lacteal.
• Lacteals absorb fat into the lymphatic system.
• Capillaries absorb everything else.

Liver

• Largest solid organ and largest gland in the body.
• Multifunctional.
• Secretes bile in gall bladder between meals.
• Small intestine capillaries drain fat and nutrient-rich lymph into it via hepatic portal system.

Gall Bladder

• Small pear-shaped organ on the liver's underside.
• Stores and concentrates bile during the fasting state.
• Mucosal cells release the peptide hormone cholecystokinin when fat enters the duodenum, and this stimulates the gall bladder to contract and discharge bile.
• Bile is alkaline so it makes the food alkaline enough for pancreatic enzymes to act on it.
• Gallstones are usually cholesterol-based and may block the hepatic or common bile ducts causing pain and jaundice.

Bile

• A greenish fluid produced by the liver and secreted to gall bladder for storage.
• Released on demand via the common bile duct.
• Contains bile salts/acids, bile pigments (mainly bilirubin), cholesterol, and phospholipids.
• Bile salts and phospholipids emulsify fats.
• Large amounts are secreted daily, 95% are reabsorbed in the ileum, returned to the liver, and reused.

Pancreas

• Is both an endocrine and exocrine gland.
• The exocrine part produces many enzymes which enter the duodenum via the pancreatic duct.
• The endocrine part produces insulin and glucagon.
• The exocrine pancreas' digestive functions are secreting digestive enzymes in response to cholecystokinin to help with digestion of fats, starches, and proteins, and secreting aqueous bicarbonate to help neutralise stomach acid.

Pancreatic Fluid

• Contains Sodium bicarbonate to neutralise acidity of the contents arriving from the stomach.
• Pancreatic amylase breaks carbohydrates into simple sugars.
• Pancreatic lipase breaks lipids into fatty acids and glycerol.
• Trypsin & chymotrypsin break protein into amino acids.
• Nucleases hydrolyse ingested nucleic acids into component nucleotides.

Large Intestine (colon)

• No food broken down.
• Absorption of remaining water from indigestible food matter.
• Some electrolytes and indigestible food left.
• Stores and eliminates waste.
• Absorbs vitamins produced by gut bacteria, especially vitamin K and certain B vitamins.

Large Intestine Details

• Less muscular.
• More goblet cells secrete mucus for lubrication.
• No villi are present.
• Takes about 16 hours to complete remaining processes.
• Gas is due to bacterial fermentation of undigested material.
• Chyme is mixed with mucus and bacteria, resulting in Faeces.
• Gut bacteria synthesize vitamins, digest polysaccharides for which there are no enzymes, and supply 10% of calories.

Rectum

• Is the last 13 cms of the large intestine.
• Stores solid waste until it exits via the anus
• Peristaltic waves move content.
• Sphincter relaxation is controlled both voluntarily and involuntarily.

Digestion and the Gut

• Involves the mixing of food, the movement of food, and the chemical breakdown of larger food molecules into smaller ones (catabolism).
• The gut physically and chemically modifies food and disposes of unusable waste.
• This depends on exocrine and endocrine secretions and controlled movement.

Digestive Juices

• Stomach and small intestine each produce a digestive juice
• Other digestive juices empty into the alimentary canal from salivary glands, gall bladder and pancreas
• Most juices contain digestive enzymes which speed up the reactions involved in the breakdown of food

Digestive Enzymes

• Amylase: starch to maltose to glucose.
• Protease: protein to amino acids.
• Lipase: lipids to fatty acids and glycerol.

Carbohydrate Digestion

• Begins in the mouth (salivary amylase).
• Salivary and pancreatic amylase converts disaccharides (double sugars).
• Intestinal enzymes (sucrase, maltase, lactase) break down into disaccharides and monosaccharides.
• Polysaccharide + salivary amylase -> maltose & small polysaccharides
• Undigested polysaccharides + pancreatic amylase -> maltose, disaccharides
• Maltase, sucrase & lactase -> monosaccharides

Protein Digestion

• Starts in the stomach and ends in the small intestine.
• Pepsin in stomach, trypsin and chymotrypsin in pancreatic juice digest protein.
• Aminopeptidase present in intestinal secretion finally digest protein into amino acids.
• Protein + pepsin -> short polypeptides
• Short polypeptides + trypsin, chymotrypsin -> small polypeptides & peptides
• Short polypeptides + carboxypeptidase, peptidases, dipeptidases -> amino acids

Lipid Digestion

• Bile produced by the liver is poured into the duodenum and brings about emulsification.
• Pancreatic lipase splits the lipid molecules into fatty acids and glycerol.
• Fats + bile salts -> emulsified fat droplets
• Emulsified fat droplets + pancreatic lipase -> fatty acids & glycerol

Absorption Locations

• Occurs by combination of simple diffusion, facilitated diffusion and active transport
• The main absorption occurs in small intestine.
• Only a few substances are absorbed in the stomach.
• Amino acids and glucose are transported to the liver via the hepatic portal vein.majority occurs in the jejunum
• Fatty acids and glycerol are absorbed by intestinal cells
• Iron is absorbed in the duodenum
• Vitamin B12 / bile salts are absorbed in the terminal ileum
• Water and lipids is absorbed throughout