Digestive System: Anatomy, Physiology, and Chemistry of Digestion Study Guide

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Digestive System Anatomy and Physiology

Functions of the Digestive System

The digestive system is responsible for the breakdown and absorption of nutrients, as well as the elimination of waste. Its six main functions are:

Ingestion: Taking in food and liquids.
Mechanical Processing: Physical breakdown of food (chewing, mixing, churning).
Digestion: Chemical breakdown of food into smaller molecules.
Secretion: Release of water, acids, enzymes, buffers, and salts.
Absorption: Movement of nutrients, water, and electrolytes into the blood or lymph.
Excretion: Removal of indigestible substances and waste products.

Major Layers of the Digestive Tract

The digestive tract consists of four major layers, each with distinct histological features:

Mucosa: Innermost layer; consists of epithelium, lamina propria, and muscularis mucosae.
Submucosa: Dense connective tissue containing blood vessels, lymphatics, and nerves.
Muscularis externa: Two or three layers of smooth muscle responsible for peristalsis and segmentation.
Serosa (or adventitia): Outermost layer; serosa is a serous membrane, adventitia is connective tissue (in esophagus).

Sublayers of the Mucosa

Epithelium: Varies by region (stratified squamous in mouth, pharynx, esophagus; simple columnar in stomach, intestines).
Lamina propria: Loose connective tissue with blood vessels and lymphatics.
Muscularis mucosae: Thin layer of smooth muscle that alters the shape of the mucosa.

Muscle Layers of the Digestive Tract

Circular muscle layer: Constricts the lumen.
Longitudinal muscle layer: Shortens the tract.
Oblique muscle layer: Present in the stomach for additional mixing.

Peristalsis in Smooth Muscle

Peristalsis is a coordinated, rhythmic contraction of the muscularis externa that propels food through the digestive tract.

Involves alternating contraction of circular and longitudinal muscles.
Moves bolus forward by squeezing and shortening the tract.

Salivary Glands

Parotid glands: Largest; secrete serous, enzyme-rich saliva via Stensen's duct.
Submandibular glands: Secrete mixed (serous and mucous) saliva via Wharton's duct.
Sublingual glands: Secrete mucous saliva via multiple small ducts.

Teeth

Primary (deciduous) teeth: 20 teeth; erupt in infancy and are replaced.
Secondary (permanent) teeth: 32 teeth; include incisors, canines, premolars, molars.

Feature	Primary Teeth	Secondary Teeth
Number	20	32
Eruption Age	6 months - 2 years	6 years - adulthood
Types	Incisors, Canines, Molars	Incisors, Canines, Premolars, Molars

Tissue Lining the Mucosa of Pharynx and Esophagus

Stratified squamous epithelium lines the mucosa, providing protection against abrasion.

Gross Anatomy of the Stomach

Regions: Cardia, fundus, body, pylorus.
Features: Greater and lesser curvature, rugae (folds), pyloric sphincter.

Function of Rugae

Rugae are folds in the stomach lining that allow expansion after eating.

Gastric Pits and Gastric Glands

Gastric pits: Indentations in the mucosa leading to glands.
Gastric glands: Secrete gastric juice (HCl, pepsinogen, mucus).

Formation of Gastric Acid

Parietal cells secrete HCl via active transport of H+ and Cl-.

is pumped into the lumen; follows, forming HCl.

Alkaline Tide

After a meal, increased bicarbonate () enters the blood from parietal cells, causing temporary alkalinity.

Phases of Gastric Activity

Cephalic phase: Initiated by sight, smell, taste, or thought of food; stimulates gastric secretion.
Gastric phase: Begins with food in the stomach; increases secretion and motility.
Intestinal phase: Begins when chyme enters the duodenum; inhibits gastric activity.

Folds of the Small Intestine

Plicae circulares: Permanent folds that increase surface area.
Villi: Finger-like projections covered with absorptive cells.
Microvilli: Extensions of cell membranes, forming the brush border.

Wall of the Colon vs. Rest of Digestive Tract

Colon has taeniae coli (longitudinal muscle bands) and haustra (pouches).
Mucosa lacks villi; more goblet cells for mucus secretion.

Defecation Process

Mass movements push feces into rectum.
Stretch receptors trigger reflex relaxation of internal anal sphincter.
External anal sphincter is voluntarily controlled.

Pancreas: Endocrine and Exocrine Functions

Endocrine: Islets of Langerhans secrete insulin and glucagon.
Exocrine: Acinar cells produce digestive enzymes released into the duodenum.

Location and Gross Anatomy of the Pancreas

Located posterior to the stomach; extends from duodenum to spleen.
Divided into head, body, and tail.

Gross Anatomy of the Liver

Largest internal organ; located in right upper quadrant.
Divided into right, left, caudate, and quadrate lobes.
Contains lobules with central veins and portal triads.

Bile Ducts Pathway

Bile flows from hepatocytes into bile canaliculi, then to bile ducts, hepatic ducts, common hepatic duct, cystic duct (to gallbladder), and common bile duct (to duodenum).

Bile Entry into Gallbladder

Bile enters the gallbladder via the cystic duct for storage and concentration.

Major Functions of the Liver

Metabolism of carbohydrates, lipids, proteins.
Detoxification of blood.
Synthesis of plasma proteins.
Storage of vitamins and minerals.
Production of bile.

Chemistry of Digestion

Enzyme-Substrate Interaction

Enzymes are biological catalysts that bind specific substrates at their active site, forming an enzyme-substrate complex.
Enzymes lower activation energy, speeding up reactions.

Chemical Composition of Carbohydrates

Monosaccharides: Simple sugars (glucose, fructose, galactose).
Disaccharides: Two monosaccharides linked (sucrose, lactose, maltose).
Polysaccharides: Long chains (starch, glycogen, cellulose).

Enzymes for Carbohydrate Digestion

Enzyme	Source	Substrate	Product
Salivary amylase	Salivary glands	Starch	Maltose
Pancreatic amylase	Pancreas	Starch	Maltose
Maltase, sucrase, lactase	Intestinal brush border	Maltose, sucrose, lactose	Glucose, fructose, galactose

Digestive Hormones: Secretin, CCK, Enterocrinin

Secretin: Stimulates bicarbonate secretion from pancreas; inhibits gastric acid.
Cholecystokinin (CCK): Stimulates enzyme secretion from pancreas and bile release from gallbladder.
Enterocrinin: Stimulates mucus secretion in intestines.

Carbohydrate Absorption

Monosaccharides are absorbed by active transport and facilitated diffusion into capillaries of villi.

Chemical Composition of Lipids

Triglycerides: Glycerol + 3 fatty acids.
Phospholipids: Glycerol + 2 fatty acids + phosphate group.
Steroids: Four fused carbon rings (e.g., cholesterol).

Enzymes for Lipid Digestion

Enzyme	Source	Substrate	Product
Lingual lipase	Salivary glands	Triglycerides	Fatty acids, monoglycerides
Gastric lipase	Stomach	Triglycerides	Fatty acids, monoglycerides
Pancreatic lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides

Bile: Emulsifies fats, increasing surface area for enzyme action; secreted by liver, stored in gallbladder.

Digestive Hormones for Lipid Digestion

Secretin and CCK stimulate bile and enzyme secretion for lipid digestion.

Lipid Absorption: Micelles and Chylomicrons

Micelles: Small lipid droplets surrounded by bile salts; transport lipids to enterocytes.
Chylomicrons: Lipoprotein particles formed in enterocytes; transport absorbed lipids into lacteals (lymphatic vessels).

Chemical Composition of Proteins

Proteins are polymers of amino acids linked by peptide bonds.

Enzymes for Protein Digestion

Enzyme	Source	Substrate	Product
Pepsin (from pepsinogen)	Stomach	Proteins	Polypeptides
Trypsin (from trypsinogen)	Pancreas	Polypeptides	Peptides
Chymotrypsin, carboxypeptidase	Pancreas	Polypeptides	Peptides, amino acids
Peptidases	Intestinal brush border	Peptides	Amino acids

Proenzymes (zymogens) are activated in the digestive tract (e.g., trypsinogen to trypsin).

Protein Absorption

Amino acids are absorbed by active transport into capillaries of villi.

Additional info: Some details, such as specific diagrams and figures, were inferred or expanded for completeness and clarity.