Overview of the Digestive System

Functions and Major Organs

The digestive system is essential for converting food into nutrients that fuel and build the body. It consists of the alimentary canal (gastrointestinal tract) and accessory organs. The alimentary canal is a continuous muscular tube from mouth to anus, while accessory organs aid digestion by secreting fluids and enzymes.

• Alimentary canal organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

• Accessory organs: Teeth, tongue, gallbladder, salivary glands, liver, pancreas

Digestive Processes

Six essential activities occur during digestion:

• Ingestion: Intake of food

• Propulsion: Movement of food via swallowing and peristalsis

• Mechanical breakdown: Chewing, mixing, churning, and segmentation to increase surface area

• Digestion: Enzymatic breakdown of food into absorbable molecules

• Absorption: Passage of nutrients into blood or lymph

• Defecation: Elimination of indigestible residues

Motility: Peristalsis and Segmentation

Peristalsis vs. Segmentation

Motility in the GI tract is achieved by two main mechanisms:

• Peristalsis: Alternating waves of contraction and relaxation propel food distally.

• Segmentation: Nonadjacent segments contract and relax, mixing food and breaking it down mechanically.

Relationship to the Peritoneum

Peritoneal Structures

The peritoneum is a serous membrane lining the abdominopelvic cavity. It consists of:

• Visceral peritoneum: Covers digestive organs

• Parietal peritoneum: Lines the body wall

• Peritoneal cavity: Space between membranes, lubricated by serous fluid

• Mesentery: Double layer of peritoneum anchoring organs and providing routes for vessels and nerves

Organs are classified as intraperitoneal (suspended by mesentery) or retroperitoneal (behind the peritoneum).

Histology of the Alimentary Canal

Four Basic Layers (Tunics)

The alimentary canal wall consists of four layers:

• Mucosa: Innermost layer; secretes mucus, enzymes, hormones; absorbs nutrients; protects against pathogens

• Submucosa: Areolar connective tissue with blood vessels, lymphatics, nerve plexus, and elastic fibers

• Muscularis externa: Smooth muscle responsible for peristalsis and segmentation; forms sphincters

• Serosa: Outermost layer; visceral peritoneum; replaced by adventitia in retroperitoneal organs

The Mouth (Oral Cavity)

Anatomy and Functions

The mouth is the site of ingestion, mechanical breakdown, and the beginning of digestion. It is bounded by lips, cheeks, palate, and tongue, and lined with stratified squamous epithelium for protection.

• Lips and cheeks: Aid in holding and manipulating food

• Palate: Hard palate forms the roof; soft palate closes off nasopharynx during swallowing

• Tongue: Mixes food, forms bolus, initiates swallowing, speech, and taste

The Tongue and Papillae

The tongue contains intrinsic and extrinsic muscles, and is attached by the lingual frenulum. Its surface bears papillae:

• Filiform papillae: Provide friction, no taste buds

• Fungiform papillae: Mushroom-shaped, contain taste buds

• Vallate papillae: V-shaped row at back, contain taste buds

• Foliate papillae: Lateral aspects, contain taste buds

The Salivary Glands

Types and Functions

Salivary glands secrete saliva, which cleanses the mouth, dissolves food, moistens it, and begins starch digestion via amylase. Major glands include parotid, submandibular, and sublingual.

• Serous cells: Produce watery, enzyme-rich secretion

• Mucous cells: Produce mucus

The Teeth

Types and Structure

Teeth are essential for mastication (chewing), breaking food into smaller pieces. Humans have two sets:

• Deciduous (primary) teeth: 20 baby teeth

• Permanent teeth: 32 adult teeth, including wisdom teeth

Tooth Structure

Each tooth has:

• Crown: Exposed part, covered by enamel

• Neck: Constricted region connecting crown and root

• Root: Embedded in jawbone, covered by cementum

• Dentin: Shock-absorbing, surrounds pulp cavity

• Pulp cavity: Contains blood vessels and nerves

• Periodontal ligament: Anchors tooth in socket

The Pharynx and Esophagus

Transport of Food

The pharynx and esophagus move food from the mouth to the stomach. The esophagus is a muscular tube with four tunics, including a mucosa of stratified squamous epithelium and an adventitia.

Swallowing (Deglutition)

Swallowing involves coordinated muscle activity in two phases:

• Buccal phase: Voluntary, tongue pushes bolus into oropharynx

• Pharyngeal-esophageal phase: Involuntary, controlled by vagus nerve and swallowing center in brainstem

The Stomach

Gross Anatomy

The stomach is a temporary storage tank, continuing physical and chemical breakdown of food. It has four main regions: cardia, fundus, body, and pyloric part. The stomach wall contains rugae (folds) for expansion.

Microscopic Anatomy

The stomach wall has three muscle layers and a mucosa with gastric pits leading to glands. Gland cells include:

• Mucous neck cells: Secrete acidic mucus

• Parietal cells: Secrete HCl and intrinsic factor

• Chief cells: Secrete pepsinogen and gastric lipases

• Enteroendocrine cells: Secrete hormones and paracrines

Clinical Note: Gastric Ulcers

Most gastric ulcers are caused by Helicobacter pylori bacteria, which damage the mucosal barrier. Treatment includes antibiotics and acid-reducing drugs.

Regulation of Gastric Secretion and Motility

Phases of Gastric Secretion

Gastric secretion is regulated by neural and hormonal mechanisms, divided into three phases:

• Cephalic phase: Triggered by sight, smell, taste, or thought of food

• Gastric phase: Triggered by food entry, distension, and chemical stimuli

• Intestinal phase: Brief stimulation followed by inhibition as chyme enters duodenum

Hormones and Paracrines in Digestion

Several hormones regulate digestive activities. The following table summarizes their sites of production, stimuli, target organs, and activities:

Mechanism of HCl Secretion

Parietal cells pump H+ into the stomach lumen via H+/K+ ATPases. Cl- diffuses in to form HCl. The process creates an alkaline tide as HCO3- is exported to blood.

The Liver, Gallbladder, and Pancreas

Liver Anatomy and Functions

The liver is the largest gland, producing bile for fat emulsification. It consists of four lobes and is anchored by ligaments. Blood enters via hepatic artery and portal vein; bile leaves via hepatic ducts.

Microscopic Anatomy of the Liver

Liver lobules are hexagonal units with plates of hepatocytes radiating from a central vein. Portal triads contain branches of hepatic artery, portal vein, and bile duct. Sinusoids are leaky capillaries lined with macrophages.

Bile Composition and Circulation

Bile contains bile salts, pigments (bilirubin), cholesterol, and phospholipids. Bile salts are recycled via enterohepatic circulation.

Gallbladder and Pancreas

The gallbladder stores and concentrates bile. The pancreas produces digestive enzymes and bicarbonate-rich juice, with both exocrine and endocrine functions.

Activation of Pancreatic Proteases

Proteases are secreted inactive and activated in the duodenum by enteropeptidase, which converts trypsinogen to trypsin.

Relationship to Duodenum

Bile and pancreatic juice enter the duodenum via regulated ducts.

The Small Intestine

Structure and Function

The small intestine is the major site for digestion and absorption. Its length and structural modifications (circular folds, villi, microvilli) greatly increase surface area.

Histology and Cell Types

• Enterocytes: Absorptive cells with microvilli

• Goblet cells: Secrete mucus

• Enteroendocrine cells: Secrete hormones

• Paneth cells: Secrete antimicrobial agents

• Stem cells: Continuously divide and replace epithelium

The Large Intestine

Structure and Function

The large intestine absorbs water and eliminates feces. It includes the cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal. Unique features include teniae coli, haustra, and epiploic appendages.

Microscopic Anatomy

The large intestine is lined with simple columnar epithelium (except anal canal, which is stratified squamous). It lacks villi and brush border, but has deep crypts with many goblet cells for mucus secretion.

Digestive Processes

• Haustral contractions: Slow segmenting movements for mixing

• Mass movements: Powerful waves moving feces toward rectum

• Defecation: Reflex triggered by rectal distension, involving voluntary and involuntary sphincters

Physiology of Digestion and Absorption

Enzymatic Hydrolysis

Digestion is a catabolic process breaking macromolecules into monomers via hydrolysis, primarily in the small intestine.

Mechanisms of Absorption

Most nutrients are absorbed through enterocytes by active or passive transport. Lipids are absorbed via simple diffusion and transported in chylomicrons.

Processing of Nutrients

Carbohydrates

• Digestion begins in mouth (salivary amylase), continues in small intestine (pancreatic amylase, brush border enzymes)

• Monosaccharides absorbed via secondary active transport with Na+

Proteins

• Digestion begins in stomach (pepsin), continues in small intestine (pancreatic proteases, brush border enzymes)

• Amino acids absorbed via secondary active transport with Na+

Lipids

• Emulsified by bile salts, digested by pancreatic lipase

• Absorbed as micelles, reassembled into triglycerides, transported as chylomicrons

Nucleic Acids

• Digested by pancreatic nucleases and brush border enzymes

• Absorbed as nitrogenous bases, pentose sugars, and phosphate ions

Absorption of Vitamins, Electrolytes, and Water

• Fat-soluble vitamins absorbed with lipids; water-soluble vitamins by passive or active transport

• Electrolytes actively transported; water absorbed by osmosis

Summary Table: Functions of GI Organs

Clinical Notes

• Peritonitis: Inflammation of peritoneum, often from ruptured appendix

• Xerostomia: Dry mouth due to reduced saliva

• Dental caries: Cavities from bacterial action

• Heartburn: Acid reflux into esophagus

• Gastric ulcers: Often caused by H. pylori

• Hepatitis and cirrhosis: Liver diseases

• Gallstones: Crystallized cholesterol in gallbladder

• Appendicitis: Inflammation of appendix

• Diverticulosis: Herniations in colon wall

• Malabsorption: Impaired nutrient absorption, e.g., celiac disease