The Digestive System

Secretion

Release of enzymes, mucus, and ions from GI epithelial cells → lumen or ECF to support digestion and regulation 

Digestion

Mechanical and chemical breakdown of food into small molecules that can be absorbed

Absorption

Transfer of nutrients, ions, and water from the GI lumen into the ECF for entry into blood or lymph

Motility

Movement of GI contents caused by smooth muscle contractions ( peristalsis + segmentation)

Peritoneum

A double-layered serous membrane of the abdominopelvic cavity that supports, protects, and lubricates digestive organs. It includes the visceral peritoneum, parietal peritoneum, and the peritoneal cavity between them

Visceral Peritoneum

Serous membrane that covers the external surfaces of digestive organs 

Parietal Peritoneum

Serous membrane that lines the inner surface of the abdominal body wall

Peritoneal Cavity

The fluid-filled space between the visceral and parietal peritoneum; contains serous fluid that reduces friction and lubricates mobile organs during digestion

Peritonitis

Inflammation of the peritoneum caused by abdominal trauma, ulcers, or a ruptured appendix;treated with debridement and high-dose antiboitics

Major GI Tract Organs

Mouth → Pharynx → Esophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

Accessory Digestive Organs

Salivary glands, liver, gallblader, pancreas; provide enzymes, bile, and secretions for digestion

Mesentary

A double layer of peritoneum that anchors organs to body wall, provides routes for blood vessels, nerves, lymphatics, and stores fat

Omentum

Double-layered fold of peritoneum that holds stomach in place; includes the greater and lesser omentum

Four Tunics of GI Tract

Mucosa, Submucosa, Muscularis externa, Serosa (visceral peritoneum)

Mucosa

Innermost layer; functions to secrete mucus/enzymes/hormones, absorb nutrients, and protect from pathogens

Submucosa

Layer of elastic connective tissue with blood vessels, lymphatics, lymphoid tissues, and the submucosal nerve plexus

Muscularis Externa

Two smooth muscle layers: inner circular (forms sphincters) + outer longitudinal; responsible for segmentation and peristalsis

Serosa

Outermost layer; visceral peritoneum that may form mesentary

Splanchnic Circulation

Blood flow to digestive organs via arteries ( celiac, superior/inferior mesenteric) and return through portal vein → liver before entering systemic circulation

Enteric Nervous System (ENS)

The “gut brain” — a network of neurons running from esophagus to anus; contains more neurons than spinal cord

Myenteric Plexus

ENS plexus located between muscle layers; controls GI motility 

Submucosal Plexus

ENS plexus in the submucosa; regulates glands and smooth muscle of mucosa

Short Reflex

Digestive reflex involving the enteric nervous system only; triggered by GI tract stimuliL

Long Reflex

Digestive reflex involving CNS + a autonomic nervous system; triggered by Gi or external stimuli

Parasympathetic Influence on GI Tract

Enhances digestion by increasing secretion and motility

Sympathetic Influence on GI Tract

Inhibits digestion by reducing secretion and motility

Mouth (Oral Cavity)

Begins mechanical and chemical digestion; food is chewed and mixed with saliva to form a bolus

Tongue

Skeletal muscle that grips, repositions, and mixes food during chewing; forms bolus; essential for swallowing, speech, and taste

Functions of Saliva 

Cleanses mouth, dissolves food for taste, moistens food to form bolus, and begins starch digestion via amylase

Major (extrinsic) Salivary Glands

Outside oral cavity; produce most saliva

Minor Salivary Glands

Throughout mucosa; keep mouth moist

Teeth

Located in mandible and maxilla; perform mastication (mechanical breakdown) of food

Dental Caries 

Cavities caused by demineralization of enamel and dentin; prevented by brushing and flossing 

Gingivitis

Inflammation of gums caused by plaque calcifying into tartar, which disrupts the seal and alllows anaerobic bacterial infection

Pharynx

Passageway for food and air; food travels from mouth → oropharynx → laryngopharynx

Esophagus

Flat muscular tube from laryngopharynx to stomach; enters abdomen via esophageal hiatus and joins stomach at cardial orifice

Gastroesophageal (Cardiac) Sphincter

Prevents reflux by closing after food enters stomach; mucus cells protect esophagus from acid

Two Phases of Deglutition

1. Buccal phase: Voluntary; tongue pushes bolus into oropharynx

2. Pharyngeal-esophageal phase: Involuntary; controlled by vagus nerve and ENS

Buccal Phase

Tongue presses against hard palate; upper esophageal sphincter closes; bolus pushed into oropharynx

Pharyngeal-Esophageal Phase (Initiation)

Tongue blocks mouth, soft palate/uvula close nasopharynx, larynx rises; epiglottis closes trachea, upper esophageal sphincter relaxes

Pharyngeal-Esophageal Phase (Continuation)

Pharynx constrictor muscles contract, bolus forced into esophagus, peristalsis moves bolus downward

Gastroesophageal Sphincter Opening

Opens to allow food into stomach, then closes to prevent regurgitation

Functions of the Stomach

Mechanical breakdown, holding area, delivers chyme to SI, HCL denatures proteins, Pepsin digests proteins, Absorbs alcohol & aspirin, and secretes intrinsic factor (B12 absorption, RBC maturation or pernicious anemia)

Intrinsic Factor

Stomach secretion required for vitamin B12 absorption → RBC maturation; lack causes pernicious anemia

Stomach Muscularis Externa

Contains three layers — longitudinal, circular, and inner oblique layer for powerful churning. (Unique to stomach)

Gastric Pits & Glands

Invaginations in mucosa containing cells that produce gastric juice; lined with simple columnar epithelium and alkaline mucus

Gastric Gland Cells

1. Mucous neck cells: Acidic mucus

2. Parietal cells: HCL + intrinsic factor

3. Chief cells: Pepsinogen + lipases

   1. Enteroendocrine cells: Serotonin, histamine, somatostatin, gastrin

Three Phases of Gastric Secretion

1. Cephalic Phase

2. Gastric Phase

3. Intestinal Phase

Cephalic Phase

Triggered by aroma, taste, sight, or thought of food; CNS → vagus nerve stimulates gastric glands

Gastric Phase

Lasts 3-4 hours; provides 2/3 of gastric juice; stimulated by distension (reflexes), peptides, and low acidity

Intestinal Phase (Inhibitory)

Duodenum inhibits gastric secretion when filled with chyme; prevents overfilling; stimulated by acid, partially digested food

Enterogastrones

Hormones from the small intestine that inhibit gastric function: Secretin, Cholecystokinin (CCK), and Gastric Inhibitory Hormone (GIH / GIP)

Enterogastric Reflex

Neural reflex (short + long pathways) that slows stomach activity when duodenum detects fatty, acidic, or hypertonic chyme

Stimulatory Events (Summary Chart)

• Sight/thought of food → vagus nerve

• Stomach distention → stretch receptors

• Peptides & rising pH → G cells release gastrin

Inhibitory Events (Summary Chart)

• Loss of appetite

• Excess acidity

• Emotional stress

• Duodenal distension

• Enterogastrones (CCK, secretin)

Liver — Digestive Function

Produces bile, a fat emulsifier needed for digestion and absorption of lipids 

Other Functions of the Liver

• Converts glucose → glycogen

• Synthesizes proteins from amino acids

• Stores fat-soluble vitamins

• Detoxifies ammonia → urea

Bile

Alkaline solution containing bile salts (fat emulsifiers) and bilirubin (heme breakdown pigment)

Gallbladder

Thin-walled muscular sac that stores and concentrates bile; releases bile via cystic duct → common bile duct

Gallstones (Biliary Calculi)

Hardened cholesterol crystals that can block bile flow; treatment includes dissolving drugs, ultrasound lithotripsy, laser vaporization, or surgery

Pancreas

Most enzymes to digest chime and bicarbonate

Pancreas - Exocrine Function

Produces pancreatic juice containing digestive enzymes and bicarbonate

Pancreas - Endocrine Function 

Secretes insulin and glucagon to regulate blood glucose 

Digestive Enzymes in Pancreatic Juice

1. Proteases (inactive forms) - protein digestion

2. Amylase - carbohydrate digestion

3. Lipases - lipid digestion

4. Nucleases - nucleic acid digestion

Clinical - Pancreatitis

Inflammation of the pancreas; causes nausea, vomiting, and pale, greasy stool due to poor fat digestion

Small Intestine - Structure

Major organ of digestion and absorption; extends from pyloric sphincter → ileocecal valve; consists of duodenum, jejunum, ileum

Small Intestine Surface Area Adaptations

Increased 600x (size of tennis court) via: circular folds, villi, Microvilli

Regions of the Large Intestine 

1. Cecum - first part of large intestine

2. Appendix (lymphoid tissue)

3. Colon - ascending, transverse, descending, sigmoid 

4. Rectum - 3 rectal valves (stop feces passage with flatus)

5. Anal canal - internal (involuntary) & external (voluntary) sphincters

Large Intestine - Functions

1. Absorption of water & electrolytes

2. Control of defecation

Appendicitis

Acute inflammation of the appendix due to blockage by feces → bacterial growth → risk of rupture & peritonitis. Symptoms: pain (umbilical → RLQ), nausea, vomiting. Tx: appendectomy

Defecation Reflex - Step 1 (Involuntary)

Stretch receptors signal → parasympathetic output causes sigmoid colon & rectum contraction and internal anal sphincter relaxation

Defecation Reflex - Step 2 (Voluntary)

External anal sphincter (skeletal muscle) consciously relaxes to allow defecation

Diarrhea

Watery stools from insufficient water absorption; prolonged cases cause dehydration & electrolyte imbalance

Constipation

Hard stools from excessive water absorption due to slow transit; caused by low fiber/fluid, poor habits, inactivity, or laxative abuse

Oral Cavity & Esophagus - Functions

• Secretion: Saliva 

• Digestion: Carbohydrates 

• Absorption: none

• Motility: chewing, swallowing

Stomach - Functions

• Secretion: HCL, enzymes, intrinsic factor

• Digestion: Proteins

• Absorption: alcohol, aspirin

• Motility: peristalsis, mixing

Small Intestine - Functions

• Secretion: Enzymes, mucus, hormones

• Digestion: Carbs, proteins, fats, nucleic acids

• Absorption: nutrients, ions, water

• Motility: segmentation + peristalsis

Large Intestines - Detailed Functions

• Secretion: Mucus

• Digestion: Bacterial only

• Absorption: Vitamins, water, minerals

• Motility: Segmental mixing & mass movement

Purpose of Digestion 

Breaks down ingested food into chemical building blocks so that only small molecules can cross the small-intestine wall and be absorbed 

Absorbable Form of Carbohydrates

Only monosaccharides (glucose, galactose, fructose) can be absorbed

Enzymes in Carbohydrate Digestion

• Salivary amylase → starch → smaller polysaccharides

• Pancreatic amylase → oligosaccharides → disaccharides

• Brush border enzymes (lactase, maltase, sucrase, etc.) → disaccharides → monosaccharides

Carbohydrate Absorption

• Glucose & galactose: cotransported with Na⁺ (secondary active transport)

• Fructose: facilitated diffusion

• All monosaccharides leave epithelial cells by facilitated diffusion into capillaries → hepatic portal vein

Na⁺-K⁺ ATPase Role in Carb Absorption 

Basolateral pump maintains the Na⁺ gradient required for Na⁺ - glucose cotransport

Absorbable Form of Proteins

Proteins are broken down into amino acids, dipeptides, and tripeptides. Only amino acids fully enter the bloodstream

Stomach Protein Digestion

Pepsin (from pepsinogen + HCl) begins protein digestion; becomes inactive in high pH of duodenum

Enzymes in Protein Digestion

• Pancreatic proteases: trypsin, chymotrypsin, carboxypeptidases

• Brush border enzymes: aminopeptidase, dipeptidase

Protein Absorption 

• Amino acids cotransported with Na⁺

• Dipeptides/tripeptides cotransported with H⁺ and broken down inside cells 

• Exit via facilitated diffusion into capillaries → hepatic portal vein

Emulsification

Bile salts break large fat globules into small droplets, increasing surface area for lipase action, occurs in duodenum

Enzymes in Fat Digestion

• Lingual lipase (minor)

• Gastric lipase (minor)

• Pancreatic lipases (major enzyme) → triglycerides → monoglycerides + free fatty acids

Micelle Formation

Bile salts form micelles containing fatty acids, monoglycerides, and cholesterol for transport to epithelial cells

Lipid Absorption 

Fatty acids & monoglycerides diffuse into epithelial cells from micelles 

Chylomicron Formation

Inside epithelial cells, fats are recombined and packaged with proteins into chylomicrons

Chylomicron Transport

Chylomicrons are exocytosed into lacteals and carried through the lymphatic system before entering the bloodstream

Digestion of Nucleic Acids

1. Pancreatic nucleases → nucleotides

2. Brush border enzymes → nitrogenous base, pentose sugars, phosphate ions (actively transported)

Malabsorption Causes 

Interference with bile or pancreatic enzymes, or damaged intestinal mucosa (celiac, Crohn’s, chronic pancreatitis, enteropathy)

Malabsorption Symptoms

• Fats: pale, foul-smelling, greasy stools

• Protein: hair loss, dry hair, fluid retention

• Sugars: bloating, gas, explosive diarrhea

• Vitamins: anemia, malnutrition, weight loss, muscle wasting

Mouth - Major Functions

• Ingestion

• Mastication

• Salivary amylase begins carbohydrate digestion

• Forms bolus

Pharynx & Esophagus - Major Functions

• Peristalsis moves food into stomach

• Mucus lubricates and protects

Stomach - Major Functions 

• Mechanical breakdown, churning 

• HCl denatures proteins 

• Pepsin digests proteins 

• Absorbs alcohol & aspirin 

• Produces intrinsic factor 

Small Intestine - Major Functions

• Chemical digestion (enzymes + bile + pancreatic juice)

• Absorption of nutrients, water, vitamins

• Segmentation + peristalsis