Digestive System: Structure, Function, and Physiology (Chapter 24 Study Guide)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Digestive System Overview

Functions of the Digestive System

The digestive system is responsible for processing food, extracting nutrients, and eliminating waste. Its main functions include:

Ingestion: Taking in food and liquids through the mouth.
Mechanical Processing: Physical breakdown of food (chewing, churning).
Digestion: Chemical breakdown of food into smaller molecules by enzymes and acids.
Secretion: Release of water, acids, enzymes, and buffers to aid digestion.
Absorption: Uptake of nutrients, water, and vitamins into the bloodstream or lymph.
Excretion: Elimination of indigestible substances and waste products.

Anatomy of the Digestive System

Organs of the Digestive Tract and Accessory Organs

Digestive Tract (GI Tract): Mouth, pharynx, esophagus, stomach, small intestine, large intestine.
Accessory Organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Example: The pancreas secretes digestive enzymes into the small intestine, aiding in the breakdown of carbohydrates, proteins, and fats.

Epithelium of Digestive Tract Regions

Oral cavity, pharynx, esophagus: Stratified squamous epithelium (protects against abrasion).
Stomach, small intestine, large intestine: Simple columnar epithelium (specialized for secretion and absorption).
Anal canal: Stratified squamous epithelium (protects during defecation).

Four Layers of the Digestive Tract

Mucosa: Innermost layer; includes epithelium, lamina propria, muscularis mucosae.
Submucosa: Connective tissue with blood vessels, lymphatics, and submucosal plexus.
Muscularis externa: Circular and longitudinal smooth muscle layers; contains myenteric plexus for motility control.
Serosa: Outermost layer (visceral peritoneum); in the esophagus, called adventitia.

Peristalsis vs. Segmentation

Peristalsis: Wave-like contractions that move food (bolus) forward through the tract.
Segmentation: Churning and mixing contractions that do not produce net movement; mix contents for digestion and absorption.
Both processes involve smooth muscle and are regulated by neural mechanisms.

Oral Cavity and Teeth

Anatomy of the Oral Cavity

Structures: Lips, cheeks, hard and soft palate, tongue.
Salivary glands: Parotid, submandibular, sublingual glands.
Functions: Mechanical processing, lubrication, limited chemical digestion (e.g., salivary amylase).

Anatomy of a Tooth

Crown, neck, root: Main regions of a tooth.
Enamel: Hard, outer covering.
Dentin: Bone-like tissue beneath enamel.
Pulp cavity: Contains nerves and blood vessels.
Periodontal ligament, cementum: Anchor tooth to jawbone.

Types of Teeth and Dental Succession

Incisors: Cutting teeth.
Cuspids (canines): Tearing teeth.
Bicuspids (premolars): Crushing and grinding.
Molars: Grinding teeth.
Deciduous teeth: 20 primary (baby) teeth.
Permanent teeth: 32 adult teeth.

Pharynx and Esophagus

Pharynx Structure

Regions: Nasopharynx, oropharynx, laryngopharynx.
Shared by both respiratory and digestive systems.

Anatomy of the Esophagus

Muscular tube connecting pharynx to stomach.
Muscle composition: Upper third: skeletal muscle; middle: mixed; lower third: smooth muscle.
Lower esophageal sphincter: Prevents reflux of stomach contents.

Phases of Swallowing (Deglutition)

Buccal phase: Voluntary; food is pushed into the oropharynx.
Pharyngeal phase: Involuntary reflex; food passes through pharynx.
Esophageal phase: Peristalsis moves bolus to stomach.

Stomach

Major Functions of the Stomach

Food storage.
Mechanical breakdown (churning).
Chemical digestion (acid and enzymes).
Production of intrinsic factor (needed for vitamin B12 absorption).

Anatomy of the Stomach

Regions: Cardia, fundus, body, pylorus.
Rugae: Folds that allow expansion.
Pyloric sphincter: Controls passage to duodenum.
Lesser and greater curvature: Borders of the stomach.

Gastric and Pyloric Glands

Gastric glands (fundus/body): Parietal cells (secrete HCl and intrinsic factor), chief cells (secrete pepsinogen).
Pyloric glands: Produce mucus, gastrin (stimulates gastric activity), somatostatin (inhibits gastric activity).

Phases of Gastric Activity

Cephalic phase: Triggered by sight, smell, or thought of food; prepares stomach for arrival of food.
Gastric phase: Food in stomach stimulates secretion and motility.
Intestinal phase: Chyme enters duodenum; slows gastric activity.

Digestion and Absorption in the Stomach

Protein digestion begins (pepsin).
Limited absorption (alcohol, some drugs).

Small Intestine

Major Parts of the Small Intestine

Duodenum: First segment; receives chyme, bile, and pancreatic juice.
Jejunum: Middle segment; primary site of nutrient absorption.
Ileum: Last segment; connects to large intestine.

Histology of the Small Intestine

Circular folds (plicae): Increase surface area.
Villi and microvilli: Further increase absorptive surface.
Intestinal glands (crypts): Secrete intestinal juices.

Intestinal Glands and Secretions

Goblet cells: Secrete mucus.
Enteroendocrine cells: Secrete hormones (CCK, secretin, GIP).
Paneth cells: Secrete antimicrobial enzymes.

Intestinal Movements

Segmentation: Mixes chyme.
Peristalsis: Moves chyme forward.
Gastroenteric and gastroileal reflexes: Coordinate movement and emptying.

Accessory Organs: Pancreas, Liver, Gallbladder

Anatomy of the Pancreas

Regions: Head, body, tail.
Pancreatic duct: Joins bile duct before entering duodenum.

Exocrine vs. Endocrine Pancreas

Exocrine: Acinar cells secrete digestive enzymes.
Endocrine: Islets of Langerhans secrete insulin and glucagon.

Four Classes of Pancreatic Enzymes

Proteases: Digest proteins.
Lipases: Digest lipids.
Amylase: Digests carbohydrates.
Nucleases: Digest nucleic acids.

Anatomy of the Liver

Lobes: Right, left, caudate, quadrate.
Lobules: Functional units with central vein.
Hepatic portal triad: Branches of hepatic artery, portal vein, and bile duct.

Bile Duct System

Right/left hepatic ducts → common hepatic duct.
Cystic duct → gallbladder.
Common bile duct → hepatopancreatic ampulla → duodenum.
Sphincter of hepatopancreatic ampulla regulates bile flow.

Liver Functions

Metabolic regulation: Carbohydrate, lipid, amino acid metabolism; detoxification.
Hematological regulation: Plasma protein synthesis, removal of old RBCs, iron storage.
Bile production: Bile salts emulsify fats for digestion.

Gallbladder Anatomy and Physiology

Stores and concentrates bile.
Releases bile in response to cholecystokinin (CCK).

Intestinal Hormones

Secretin: Stimulates bicarbonate release from pancreas.
CCK (cholecystokinin): Stimulates gallbladder contraction and pancreatic enzyme secretion.
GIP (gastric inhibitory peptide): Inhibits gastric activity.
VIP (vasoactive intestinal peptide): Dilates intestinal capillaries.

Large Intestine

Anatomy of the Large Intestine

Regions: Cecum, colon (ascending, transverse, descending, sigmoid), rectum.
Haustra: Pouches for expansion and elongation.
Taeniae coli: Longitudinal muscle bands.

Physiology of the Large Intestine

Absorbs water and electrolytes.
Produces vitamins (K, biotin, B5) via bacterial action.
Forms and stores feces.
Mass movements and defecation reflex expel waste.

Digestion and Absorption of Nutrients

Carbohydrates

Broken down by amylase to monosaccharides.
Absorbed by cotransport mechanisms in the small intestine.

Lipids

Emulsified by bile salts.
Digested by lipase to fatty acids and monoglycerides.
Form micelles, absorbed, then packaged as chylomicrons for transport.

Proteins

Digested by pepsin (stomach) and pancreatic proteases.
Absorbed as amino acids by active transport.

Absorption of Water, Ions, and Vitamins

Water: Absorbed by osmosis.
Ions: Absorbed by active transport (e.g., Na+, Ca2+).
Vitamins: Fat-soluble (A, D, E, K) absorbed with micelles; water-soluble absorbed by diffusion or transporters.

Summary Table: Digestive Tract Layers

Layer	Main Components	Function
Mucosa	Epithelium, lamina propria, muscularis mucosae	Secretion, absorption, protection
Submucosa	Connective tissue, blood/lymph vessels, submucosal plexus	Support, nutrient supply
Muscularis externa	Circular & longitudinal muscle, myenteric plexus	Motility (peristalsis, segmentation)
Serosa	Visceral peritoneum (or adventitia)	Protection, structural support

Summary Table: Major Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Amylase	Salivary glands, pancreas	Starch	Maltose, glucose
Pepsin	Stomach (chief cells)	Proteins	Peptides
Trypsin, chymotrypsin	Pancreas	Proteins	Peptides, amino acids
Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
Nucleases	Pancreas	Nucleic acids	Nucleotides

Key Equations

Osmosis (water absorption):

Active transport (e.g., sodium absorption):

Additional info: The study guide covers all major aspects of the digestive system, including structure, function, histology, and physiology, as well as the roles of accessory organs and the processes of digestion and absorption. These notes are suitable for exam preparation in a college-level Anatomy and Physiology course.