Study Notes: Anatomy & Physiology – The Digestive System (Small and Large Intestine)

The Digestive System: The Small and Large Intestine

23.8 The Small Intestine

Gross Anatomy of the Small Intestine

The small intestine is the principal organ for digestion and absorption of nutrients. It extends from the pyloric sphincter to the ileocecal valve, where it joins the large intestine.

• Length: 2–4 meters (7–13 feet)

• Diameter: 2.5–4 cm (1.0–1.6 inches)

• Subdivisions:

  • Duodenum: ~25 cm (10 in), mostly retroperitoneal, curves around the head of the pancreas; contains most features for digestion.

  • Jejunum: ~2.5 m (8 ft), attached posteriorly by mesentery.

  • Ileum: ~3.6 m (12 ft), attached posteriorly by mesentery; joins large intestine at ileocecal valve.

Relationship to Other Organs

The duodenum is closely associated with the liver, gallbladder, and pancreas, which deliver bile and pancreatic juices essential for digestion.

Blood and Nerve Supply

• Blood supply:

  • Superior mesenteric artery supplies blood.

  • Veins drain into superior mesenteric veins, then into hepatic portal vein, and finally into the liver.

• Nerve supply:

  • Parasympathetic innervation via vagus nerve.

  • Sympathetic innervation from thoracic splanchnic nerves.

Microscopic Anatomy of the Small Intestine

The small intestine is highly specialized for absorption, with structural modifications that increase its surface area up to 600 times, reaching about 200 m2 (size of a tennis court).

• Circular folds: Permanent folds (~1 cm deep) that force chyme to spiral, increasing contact time for absorption.

• Villi: Fingerlike projections (~1 mm high) of mucosa, each containing a dense capillary bed and a lymphatic capillary called a lacteal for absorption.

• Microvilli: Cytoplasmic extensions of mucosal cells forming the brush border, which contains membrane-bound brush border enzymes for final carbohydrate and protein digestion.

Histology of the Small Intestine Wall

• Intestinal crypts: Tubular glands scattered between villi.

• Cell types:

  • Enterocytes: Simple columnar absorptive cells with microvilli; absorb nutrients and electrolytes.

  • Goblet cells: Mucus-secreting cells found in epithelia of villi and crypts.

  • Enteroendocrine cells: Source of hormones (e.g., CCK, secretin) regulating digestion.

  • Paneth cells: Deep in crypts; secrete antimicrobial agents (defensins, lysozyme).

  • Stem cells: Continuously divide to produce other cell types; villus epithelium renewed every 2–4 days.

• Mucosa-associated lymphoid tissue (MALT): Protects against microorganisms. Includes:

  • Individual lymphoid follicles

  • Peyer's patches: Aggregated lymphoid nodules, especially in distal small intestine

  • Lamina propria with plasma cells secreting IgA

• Submucosa: Areolar tissue; duodenal glands secrete alkaline mucus to neutralize acidic chyme.

Clinical – Homeostatic Imbalance

• Chemotherapy: Targets rapidly dividing cells, including GI tract epithelium, leading to symptoms such as nausea, vomiting, and diarrhea.

Intestinal Juice

• 1–2 L secreted daily in response to mucosal distension or irritation.

• Stimulated by hypertonic or acidic chyme.

• Slightly alkaline and isotonic with blood plasma.

• Consists mainly of water and mucus (from duodenal glands and goblet cells).

Digestive Processes in the Small Intestine

• Chyme from stomach contains partially digested carbohydrates, proteins, and undigested fats.

• Small intestine absorbs all nutrients and most water in 3–6 hours.

• Sources of enzymes:

  • Bile, bicarbonate, and digestive enzymes (from liver and pancreas).

  • Brush border enzymes (membrane-bound) for final digestion.

• Regulating chyme entry:

  • Chyme is usually hypertonic; slow delivery prevents osmotic water loss.

  • Low pH must be adjusted upward.

  • Chyme mixed with bile and pancreatic juice for digestion.

  • Enterogastric reflex and hormones control movement into duodenum.

• Motility:

  • Segmentation: Most common after a meal; mixes and moves contents toward ileocecal valve. Initiated by pacemaker cells; intensity altered by reflexes and hormones.

  • Peristalsis: Increases between meals, initiated by motilin; moves remnants, bacteria, and debris toward large intestine.

• Ileocecal valve control:

  • Sphincter relaxes to admit chyme into large intestine when gastroileal reflex and gastrin increase segmentation.

  • Valve closes when chyme exerts backward pressure, preventing regurgitation.

Table: Control of Small Intestinal Motility

Stimulus	Effect on Small Intestine Motility	Mechanism
Gastric motility and emptying	↑ Motility in ileum; relaxation of ileocecal valve	Gastroileal reflex (long neural reflex) and gastrin
Distension of small intestine, reduced intestinal volume	↑ Strength of segmentation; intestinal contents moved toward large intestine	Long and short neural reflexes; motilin (initiates long and short neural reflexes)

Table: Overview of the Functions of the Gastrointestinal Organs

Organ	Major Functions
Small Intestine	Digestion and absorption of nutrients; mixing and propulsion of contents
Large Intestine	Absorption of water and electrolytes; formation and elimination of feces
Stomach	Mechanical and chemical breakdown of food; initial protein digestion
Liver	Production of bile; metabolism of nutrients
Pancreas	Secretion of digestive enzymes and bicarbonate

23.9 The Large Intestine

Gross Anatomy of the Large Intestine

The large intestine absorbs water and electrolytes, forming and eliminating feces. It has unique features:

• Teniae coli: Three bands of longitudinal smooth muscle in muscularis.

• Haustra: Pocketlike sacs caused by tone of teniae coli.

• Epiploic appendages: Fat-filled pouches of visceral peritoneum.

Subdivisions

• Cecum: First part; receives contents from ileum.

• Appendix: Masses of lymphoid tissue; part of MALT, stores bacteria.

• Colon:

  • Ascending colon: Up right side; ends at right colic (hepatic) flexure.

  • Transverse colon: Across abdomen; ends at left colic (splenic) flexure.

  • Descending colon: Down left side.

  • Sigmoid colon: S-shaped, through pelvis.

• Rectum: Three rectal valves prevent passage of gas with feces.

• Anal canal: Opens to exterior at anus; has internal (smooth muscle) and external (skeletal muscle) sphincters.

Relationship to Peritoneum

• Cecum, appendix, and rectum are retroperitoneal.

• Colon is retroperitoneal except for transverse and sigmoid parts.

• Intraperitoneal regions anchored by mesocolons.

Mesenteries of Abdominal Digestive Organs

• Mesenteries are double layers of peritoneum that support and anchor digestive organs.

• Include greater omentum, lesser omentum, and mesocolon.

Clinical – Homeostatic Imbalance

• Appendicitis: Acute inflammation, often due to blockage by feces; can cause peritonitis if ruptured. Symptoms include pain, nausea, and vomiting. Treatment is surgical removal or antibiotics.

Microscopic Anatomy of the Large Intestine

• Thicker mucosa of simple columnar epithelium (except anal canal, which is stratified squamous for abrasion resistance).

• No circular folds, villi, or digestive secretions.

• Abundant deep crypts with mucus-producing goblet cells.

• Anal canal mucosa forms long ridges called anal columns.

• Anal recesses: Between columns, secrete mucus for emptying.

• Pectinate line: Divides regions of sensory innervation (visceral above, somatic below).

• Superficial venous plexuses can form hemorrhoids if inflamed.

Additional info:

• These notes are based on textbook-style lecture slides and cover the anatomy, histology, and physiology of the small and large intestines, including clinical correlations and tabular summaries of organ functions and motility control.