The Digestive System

The Digestive Tract (GI Tract / Alimentary Canal)

A continuous muscular tube from mouth to anus. The organs, in order:

Accessory Organs

Not part of the continuous tube, but assist digestion:

• Teeth — mechanical processing (mastication)
• Tongue — manipulation, sensory analysis, assists swallowing
• Salivary glands (parotid, sublingual, submandibular) — saliva production
• Liver — bile production, metabolic regulation, hematological regulation
• Gallbladder — bile storage and concentration
• Pancreas — digestive enzymes and alkaline buffers (exocrine); insulin and glucagon (endocrine)

Six Primary Functions

Defensive Role

The digestive lining protects surrounding tissues from corrosive acids and enzymes and provides nonspecific defense against swallowed and resident bacteria. When bacteria reach underlying tissues, macrophages and immune cells attack them.

Four Major Wall Layers (Lumen → Outside)

Epithelial Variation by Region

Mesenteries

Mesenteries are double sheets of serous membrane (parietal + visceral peritoneum) that suspend portions of the digestive tract within the peritoneal cavity. They stabilize organ positions, prevent intestinal entanglement, and provide pathways for blood vessels, nerves, and lymphatic vessels.

Peristalsis — Directional Propulsion

Coordinated waves of muscular contraction that propel materials along the tract. Triggered by pacesetter (pacemaker) cells in the smooth muscle.

Segmentation — Mixing Without Propulsion

Churning and fragmenting movements in the small intestine that mix chyme with digestive secretions. Segmentation does NOT follow a set pattern and does NOT propel material in any particular direction. It increases the thoroughness of digestion and absorption.

Four Functions of the Oral Cavity

Oral Cavity Structures

Hard palate (palatine + maxillary bones) and soft palate form the roof. The tongue dominates the floor; the lingual frenulum connects it to the floor. The uvula prevents food from entering the pharynx prematurely. The vestibule is the space between the cheeks/lips and teeth. Gums (gingivae) surround the bases of the teeth.

Three Pairs of Salivary Glands

Total daily saliva production: 1.0–1.5 liters. Saliva is 99.4% water. Contains mucins (form mucus), IgA antibodies, and lysozyme for bacterial control. pH shifts from 6.7 (resting) to 7.5 (eating). Salivary secretion is controlled by the ANS.

Teeth

Tooth structure: Crown (covered by enamel — hardest biological substance), neck, root (covered by cementum). Bulk = dentin (mineralized matrix without cells). Pulp cavity receives blood/nerves via root canals. Periodontal ligament anchors root in bony socket (alveolus).

Deciduous teeth: 20 (children). Permanent teeth: 32 (adults, including 4 wisdom teeth — third molars, appearing ages 17–21).

Swallowing (Deglutition) — Three Phases

Four Regions (Esophagus → Small Intestine)

Four Primary Functions

• Temporary storage of ingested food
• Mechanical breakdown via churning and mixing (three muscle layers)
• Chemical breakdown by HCl and pepsin
• Intrinsic factor production (needed for vitamin B₁₂ absorption in the ileum)

Gastric Wall Features

Rugae — prominent folds when empty; flatten as stomach expands to hold 1–1.5 liters. Gastric pits open onto the surface, leading to gastric glands deep in the lamina propria. The greater omentum hangs from the greater curvature like a fatty apron, protecting abdominal viscera. The lesser omentum connects the lesser curvature to the liver.

Gastric Gland Cells

Three Phases of Gastric Secretion

Three Segments (Proximal → Distal)

90% of nutrient absorption occurs in the small intestine. Transit time averages about 5 hours.

Surface Area Amplification

Three levels increase absorptive area from ~3,300 cm² to ~2 million cm²:

Intestinal Secretions and Glands

Intestinal juice: ~1.8 L/day. Moistens contents, buffers acids, dissolves enzymes and digestion products. Duodenal glands (submucosal glands) secrete alkaline mucus to protect the duodenal lining from gastric acid. Parasympathetic stimulation activates them; sympathetic stimulation inhibits them (which is why chronic stress causes duodenal ulcers — the protective secretions are suppressed).

Four Intestinal Hormones

Key Intestinal Reflexes

The Pancreas

Elongate organ (~15 cm) behind the stomach, extending from duodenum to spleen. Retroperitoneal. ~99% exocrine cells, ~1% endocrine (pancreatic islets).

Exocrine function: Produces pancreatic juice (~1000 mL/day) — a mixture of digestive enzymes and alkaline buffers. Secreted by acinar cells into the pancreatic duct, which enters the duodenum at the duodenal papilla alongside the common bile duct.

Regulation: Secretin triggers buffer secretion (sodium bicarbonate, pH 7.5–8.8). CCK triggers enzyme secretion.

The Liver

Largest visceral organ (~1.5 kg, 2.5% body weight). Four lobes: large left and right (separated by falciform ligament), smaller caudate and quadrate. The thickened posterior margin of the falciform ligament is the round ligament (remnant of fetal umbilical vein).

Blood supply: ~1/3 arterial (hepatic artery proper), ~2/3 venous (hepatic portal vein carrying absorbed nutrients from the GI tract). The liver receives ~25% of cardiac output. Blood leaves via hepatic veins → inferior vena cava.

Liver Lobule — Functional Unit

~100,000 lobules. Hepatocytes (liver cells) arranged like wheel spokes. Blood from hepatic portal vein and hepatic artery branches enters at portal areas (portal triads) at the six corners of each lobule → flows through sinusoids (highly permeable capillaries) between hepatocyte plates → drains into the central vein. Kupffer cells line the sinusoids — phagocytes that engulf pathogens, debris, and damaged blood cells.

Bile flows the opposite direction: secreted by hepatocytes into bile canaliculi → bile ducts → common hepatic duct.

Three General Liver Functions

The Gallbladder

Hollow, pear-shaped sac in a recess under the liver’s right lobe. Cystic duct extends from gallbladder → unites with common hepatic duct → forms the common bile duct. Common bile duct + pancreatic duct enter the duodenum at the duodenal papilla, surrounded by the hepatopancreatic sphincter.

Functions: (1) Bile storage (40–70 mL capacity). (2) Bile modification (water absorbed, bile salts concentrated). Bile is released ONLY when CCK relaxes the hepatopancreatic sphincter and contracts the gallbladder wall. More CCK is secreted when chyme contains more fat.

Three Main Parts

Three Main Functions

• Water reabsorption and fecal compaction — of ~1500 mL entering daily, only ~200 mL exits as feces (75% water, 5% bacteria, rest = indigestible material/cell debris)
• Vitamin absorption from bacteria — Vitamin K (clotting factors — bacteria supply ~50% of daily requirement), biotin (glucose metabolism), vitamin B₅ (pantothenic acid — steroid hormones and neurotransmitters)
• Fecal storage prior to defecation

Movements and Defecation

Mass movements — powerful peristaltic contractions a few times daily, triggered by distension of the stomach and duodenum. Force fecal material into the rectum.

Defecation reflex: Rectal wall distension → stretch receptors fire → two positive feedback loops: (1) local peristaltic contractions increase, (2) parasympathetic neurons stimulate mass movements in descending/sigmoid colon. The internal anal sphincter relaxes, but the external sphincter must be voluntarily relaxed for feces to pass.

Carbohydrate Digestion and Absorption

Lipid Digestion and Absorption

Protein Digestion and Absorption

Water, Electrolytes, and Vitamins

Water: ~2000 mL ingested + ~7000 mL from secretions = 9000 mL/day. Only ~150 mL lost in feces. Water follows solutes by osmosis. Na+ and Cl- absorption drives most water movement.

Fat-soluble vitamins (A, D, E, K) travel in micelles and are absorbed with lipid digestion products. Water-soluble vitamins (B vitamins, vitamin C) diffuse easily — except vitamin B₁₂, which requires intrinsic factor from gastric parietal cells for absorption in the ileum.

Six Age-Related Changes

The digestive system provides the same fundamental service to every other body system: absorbing organic substrates, vitamins, ions, and water required by all cells.

Four Abdominal Quadrants

Divided by two imaginary lines crossing at the umbilicus (one vertical, one horizontal).