Digestive System: Structure, Function, and Histology (Unit 3 Test Review, Chapters 22–24)

Introduction to the Digestive System

Overview

The digestive system is a set of organs that breaks down food into nutrients, which are then delivered by the bloodstream to cells throughout the body. The organs of the digestive system are located from the head to the abdominopelvic cavity.

• Alimentary canal (GI tract or digestive tract): A continuous tube through which food passes directly. Includes the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory organs: Not part of the alimentary canal but assist in digestion. Includes teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Food generally does not come into direct contact with accessory organs (except teeth and tongue).

Basic Digestive Functions and Processes

Major Processes

• Ingestion: Bringing food and water into the digestive system, typically via the mouth.

• Secretion: Digestive organs secrete substances (mucus, enzymes, acids, hormones) to aid digestion.

• Propulsion: Movement of food and liquids through the digestive tract, mainly by peristalsis (rhythmic contractions of smooth muscle), aided by mucus.

• Digestion: Breakdown of food by two mechanisms:

  • Mechanical digestion: Physical breakdown into smaller pieces (chewing, mixing, churning).

  • Chemical digestion: Enzymatic breakdown of food into small molecules.

• Absorption: Movement of digested food, water, electrolytes, and vitamins into blood or lymph.

• Defecation: Elimination of indigestible substances and metabolic waste as feces.

Histology of the Alimentary Canal

General Structure

Most regions of the alimentary canal share a four-layered structure surrounding the lumen:

• Mucosa: Inner epithelium, lamina propria (loose connective tissue), and muscularis mucosae (thin smooth muscle).

• Submucosa: Dense irregular connective tissue with blood/lymphatic vessels and submucosal glands. Contains the submucosal plexus (Meissner's plexus) for regulating secretion and blood flow.

• Muscularis externa: Two layers of smooth muscle (inner circular, outer longitudinal). Motility is regulated by the myenteric plexus (Auerbach's plexus).

• Serosa (visceral peritoneum): Simple squamous epithelium and loose connective tissue (in organs within the peritoneal cavity). Adventitia (dense irregular connective tissue) is found in organs outside the cavity.

Mucosa Details

• Epithelium from stomach to large intestine is simple columnar with goblet cells (secrete alkaline mucus).

• Mucosa protects underlying tissues and houses regenerative epithelial cells, blood/lymphatic vessels, and mucosa-associated lymphoid tissue (MALT).

• Muscularis mucosae: Inner circular and outer longitudinal layers for motility.

Oral Cavity and Associated Structures

Oral Cavity (Mouth)

• Site of ingestion, secretion, chemical and mechanical digestion, and propulsion.

• Accessory organs: teeth and tongue.

• Three pairs of salivary glands secrete saliva into the oral cavity.

• Chewed food forms a bolus, which is swallowed (propelled to the stomach).

The Teeth and Mastication

• Teeth are located in bony sockets (alveoli) within the mandible and maxilla, held by the periodontal ligament.

• Teeth masticate (chew) food, increasing surface area for enzyme action.

Salivary Glands

• Parotid glands: Anterior to ear, secrete via parotid duct.

• Submandibular glands: Medial to mandible, secrete via submandibular duct.

• Sublingual glands: Inferior to tongue, secrete via sublingual ducts.

Saliva contains water, electrolytes, mucus, enzymes, and other solutes:

• Salivary amylase: Begins carbohydrate digestion.

• Lysozyme: Antibacterial enzyme.

• Secretory IgA: Immune defense.

• Bicarbonate ions: Neutralize acid.

Pharynx and Esophagus

Pharynx

• Divided into nasopharynx, oropharynx, and laryngopharynx (only the last two are part of the alimentary canal).

• Contains palatine and lingual tonsils for immune defense.

• Propels food via pharyngeal constrictor muscles during swallowing.

Esophagus

• Muscular tube (~25 cm) posterior to trachea, transports bolus to stomach.

• Lined with stratified squamous nonkeratinized epithelium.

• Contains upper esophageal sphincter (skeletal/smooth muscle mix) and gastroesophageal sphincter (prevents reflux).

Stomach

Anatomy

• J-shaped organ in left upper quadrant, divided into cardia, fundus, body, pyloric antrum, and pylorus (with pyloric sphincter).

• Interior contains folds (rugae) for expansion.

Histology

• Mucosa forms gastric pits with columnar and goblet cells (secrete mucus).

• Gastric glands contain:

  • Mucous neck cells: Secrete acidic mucus.

  • Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor (for vitamin B12 absorption).

  • Chief cells: Secrete pepsinogen (becomes pepsin for protein digestion).

  • DNES (enteroendocrine) cells: Secrete hormones (e.g., gastrin).

Functions

• Secretion: HCl, pepsinogen, mucus, intrinsic factor.

• Motility: Receives food, churns to form chyme, controls emptying into small intestine.

• Receptive relaxation: Allows stomach to fill without increasing pressure.

Small Intestine

Overview

• Longest portion of alimentary canal (~6 meters).

• Main site for secretion, digestion, absorption, and propulsion.

• Cells (enterocytes) produce digestive enzymes, hormones, and mucus.

Divisions

• Duodenum: Initial segment, receives secretions from gallbladder and pancreas, contains Brunner's glands (alkaline mucus).

• Jejunum: Middle segment, most active site for digestion and absorption.

• Ileum: Final segment, ends at ileocecal valve (prevents backflow from large intestine).

Structure and Function

• Internal surface has folds to increase absorption area (400–600x):

  • Circular folds (plicae circulares): Largest, visible ridges.

  • Villi: Projections with blood/lymphatic capillaries (lacteals).

  • Microvilli: Smallest, form brush border with digestive enzymes.

• Motility includes peristalsis (propulsion) and segmentation (mixing/churning).

Large Intestine

Overview

• Runs along abdominal cavity border, ~1.5 meters long, larger diameter than small intestine.

• Receives undigested material, absorbs water/electrolytes, forms feces.

• Secretes mucus, propels, and defecates waste.

• Houses bacteria that synthesize vitamins.

Anatomy

• Includes cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal.

• Rectum contains rectal valves (passage of gas without feces).

• Anal canal has internal (involuntary) and external (voluntary) anal sphincters.

Accessory Digestive Organs

The Pancreas

• Located in left upper abdomen, retroperitoneal; divided into head, body, and tail.

• Main pancreatic duct merges with bile duct near duodenum.

• Acinar cells secrete digestive enzymes; duct cells secrete bicarbonate ions (alkaline).

• Pancreatic juice: Contains water, enzymes, and bicarbonate; neutralizes acidic chyme and digests macronutrients.

• Secretion regulated by hormones:

  • Cholecystokinin (CCK): Stimulates enzyme secretion in response to lipids/proteins.

  • Secretin: Stimulates bicarbonate secretion in response to acid.

The Liver and Gallbladder

• Liver: Largest gland, right upper quadrant, four lobes, covered by capsule and peritoneum.

• Gallbladder: Small sac on posterior liver, stores and concentrates bile.

• Bile: Produced by liver, contains water, electrolytes, bile salts, and organic compounds.

• Bile salts: Amphiphilic, emulsify lipids for digestion and absorption.

• Liver functions: Metabolism, detoxification, synthesis of plasma proteins, storage of nutrients, and excretion of waste.

Organ	Main Function(s)
Pancreas	Secretes digestive enzymes and bicarbonate; regulates blood glucose (endocrine)
Liver	Produces bile, metabolizes nutrients, detoxifies chemicals, stores vitamins
Gallbladder	Stores and concentrates bile, releases it into duodenum

Additional info: The notes above are based on the provided slides and expanded with standard academic context for clarity and completeness.