Digestive System: Structure, Function, and Disorders

The Digestive System

Overview and Function

The digestive system is responsible for the mechanical and chemical breakdown of food and the absorption of nutrients. It consists of the alimentary canal (gastrointestinal tract) and accessory organs that aid in digestion.

• Mechanical breakdown: Physical processes such as chewing and mixing.

• Chemical breakdown: Enzymatic reactions that convert food into absorbable molecules.

• Absorption: Movement of nutrients from the digestive tract into the bloodstream.

Major Organs and Regions

• Esophagus: Transports food from mouth to stomach.

• Stomach: Mixes and digests food; initiates protein breakdown.

• Small Intestine: Main site for nutrient absorption; divided into duodenum, jejunum, and ileum.

• Large Intestine: Absorbs water and forms feces; includes cecum, colon, rectum, and anus.

• Accessory Organs: Liver, gallbladder, pancreas, salivary glands.

Histology of the Alimentary Canal

Layers of the Canal

The wall of the alimentary canal consists of four main layers, each with distinct functions:

• Mucosa (innermost): Protects tissues and carries out absorption.

• Submucosa: Contains glands, blood vessels, and nerves.

• Muscular Layer: Smooth muscle responsible for peristalsis (involuntary contractions that move food).

• Serosa (outermost): Lubricates surfaces to reduce friction.

Movements in the Canal

• Mixing Movements: Combine food with digestive juices.

• Peristalsis: Wave-like muscle contractions that propel food through the digestive tract.

Neural and Hormonal Control

Enteric Nervous System

The GI tract has its own nervous system, the enteric nervous system, which responds to internal (e.g., pH, stretch) and external (e.g., smell, taste) stimuli via short and long reflex arcs.

Hormones of the Gastrointestinal Tract

Over 50 hormones regulate digestive activity. Key hormones and their sources are summarized below:

Hormone	Secreted From
Gastrin	Stomach-pyloric mucosa
CCK-Pz (Cholecystokinin-pancreozymin)	Upper small intestine
Secretin	Upper small intestine
Gut glucagon	Stomach and small intestine
Gastrone	Stomach
Willikins	Upper small intestine
Hepatocrinin and Enterocrinin	Intestine
GIP (Gastro inhibitory peptide)	Duodenum and jejunum
Enterogastrone	Duodenum
VIP (Vasoactive Intestinal Peptide)	Entire small intestine and colon
Motilin and chymodenin	Duodenum and jejunum
Enteroxytonin	Small intestine

Anatomy of the Mouth

Structures and Functions

• Mouth: Begins digestion by chewing and mixing food with saliva.

• Tongue: Moves food; connected to the floor of the mouth via the frenulum; contains papillae for taste.

• Palate: Forms the roof of the oral cavity (hard and soft); uvula is suspended at the back.

• Palatine Tonsils: Part of the immune system; crypts increase surface area for trapping pathogens.

Tonsil Disorders

• Tonsillitis: Inflammation of the tonsils, may cause sore throat and fever.

• Tonsil Stones (Tonsilloliths): Hard white formations caused by trapped debris and bacteria in tonsil crypts.

Teeth

Types and Anatomy

• Primary (baby) teeth vs Secondary (adult) teeth: Humans have two sets of teeth in their lifetime.

• Types of Teeth: Incisors, cuspids (canines), bicuspids (premolars), molars (1st, 2nd, 3rd).

Anatomy of a Tooth

• Crown: Visible part above the gum.

• Root: Embedded in the jawbone.

• Enamel: Hard outer layer.

• Dentin: Beneath enamel, supports structure.

• Pulp: Contains nerves and blood vessels.

• Gums: Surround and protect the teeth.

Salivary Glands

Types and Functions

• Parotid Gland

• Submandibular Gland

• Sublingual Gland

Saliva contains amylase, an enzyme that breaks down starch into sugars. Mucus cells produce mucus for lubrication during swallowing.

Esophagus

Structure and Function

• Esophageal hiatus: Opening in the diaphragm where the esophagus passes through.

• Cardiac (esophageal) sphincter: Controls flow of food into the stomach.

Stomach

Regions and Sphincters

• Cardiac

• Fundic

• Body (greater and lesser curvature)

• Pyloric

• Cardiac sphincter: Prevents reflux.

• Pyloric sphincter: Controls flow of chyme to small intestine.

Stomach Secretions

• Mucus: Protects stomach lining.

• Hydrochloric acid (HCl): Breaks down food, activates pepsin, kills pathogens.

• Pepsin: Digests proteins into short peptide chains.

• Gastric lipase: Digests a small amount of fat.

• Intrinsic factor: Binds vitamin B12 for absorption.

Chyme and Rugae

• Chyme: Semi-fluid paste of partially digested food.

• Rugae: Folds in the stomach wall that allow expansion.

Stomach Muscles

• Longitudinal, Circular, and Oblique muscle layers mix and propel food.

Pancreas

Structure and Function

The pancreas is a flattened, spongy gland located behind the stomach. It functions as both an exocrine gland (secreting digestive enzymes) and an endocrine gland (secreting insulin and glucagon).

• Insulin: Regulates blood sugar.

• Proteases: Digest proteins.

• Amylase: Digests carbohydrates.

• Lipase: Digests fats.

Pancreatic enzymes empty into the duodenum, the first part of the small intestine.

Liver and Biliary System

Liver Structure

• 1 large right lobe, 1 smaller left lobe.

Biliary System

• Consists of liver, gallbladder, and ducts.

• Cystic duct (gallbladder) and hepatic duct (liver) join to form the common bile duct, which empties into the duodenum.

Liver Functions

• Regulates blood glucose levels.

• Breaks down lipids and fats.

• Protein metabolism.

• Stores vitamins.

• Recycles red blood cells.

• Removes toxins.

• Secretes bile into the small intestine.

Bile: Yellowish-green liquid that aids in digestion and breakdown of fat.

Jaundice

• Common in newborns due to high red blood cell turnover and immature liver function.

• Treated with phototherapy (bili lights) to reduce bilirubin levels.

Small Intestine

Structure and Function

• Main function: Absorption of nutrients.

• Duodenum: First section.

• Jejunum: Second section.

• Ileum: Third section.

• Mesentery: Supports coils and contains blood vessels.

Intestinal Villi and Microvilli

• Villi: Finger-like projections that increase surface area for absorption.

• Microvilli: Smaller projections forming a brush border, further increasing surface area and containing enzymes.

Secretions

• Fluid contains water and mucus, but few enzymes.

• Most digestive enzymes come from pancreatic juice and bile from the liver.

Large Intestine

Structure

• Cecum: Start of large intestine, attached to appendix.

• Colon: Four sections—ascending, transverse, descending, sigmoid.

• Rectum: Stores waste before expulsion.

• Anus: Muscular sphincter controlling exit of waste.

Greater Omentum

• "Curtain-like" membrane covering intestines, stores fat, and lays like a drape.

Appendix

• Located in lower right abdomen.

• Function is uncertain; may store good bacteria or be vestigial.

Cecum

• Absorbs fluids and salts remaining after digestion.

• Mixes contents with mucus for lubrication.

Function of Large Intestine

• Secretes mucus, reabsorbs water, contains bacteria (intestinal flora) to aid digestion.

• Mass movements (defecation) remove undigested food.

• Main job: Water reabsorption.

Key Terms and Concepts

• Peristalsis: Involuntary muscle contractions moving food through the digestive tract.

• Chyme: Semi-fluid mass of partially digested food.

• Rugae: Folds in the stomach wall for expansion.

• Intrinsic factor: Glycoprotein required for vitamin B12 absorption.

• Intestinal flora: Beneficial bacteria aiding digestion in the large intestine.

Relevant Equations

• Body Mass Index (BMI): $BMI = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

Example

When food enters the stomach, it is mixed with gastric juices containing HCl and pepsin, forming chyme. The chyme is then released into the duodenum, where pancreatic enzymes and bile further digest nutrients for absorption in the small intestine.

Additional info: The notes above expand on the original content by providing definitions, context, and academic explanations for each anatomical structure and physiological process described.