Small Intestine
The small intestine is the primary site for nutrient absorption and is divided into the duodenum, jejunum, and ileum. The intestinal wall is highly specialized, featuring villi that dramatically increase surface area for absorption. These villi contain specialized cells, including enterocytes (absorptive cells), goblet cells (mucus secretion), endocrine cells, and Paneth cells.
Large Intestine
The large intestine (approximately 1.5 m long) consists of the cecum, appendix, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal. Its main roles are the absorption of water and electrolytes and the formation/elimination of waste. Defecation is controlled by the internal and external anal sphincters.
Accessory Organs and Chemical Digestion
While food does not pass through them, the accessory organs—the liver, gallbladder, and exocrine pancreas—are essential for delivering the chemical tools necessary to break down chyme.
The Liver and Bile Secretion
The liver is a metabolic powerhouse. It secretes 700 to 1200 mL of bile daily. Bile is an alkaline fluid containing bile salts, cholesterol, bilirubin, and electrolytes. Bile salts are critical for the emulsification of fats, which increases the surface area for enzymes to work.
Beyond digestion, the liver performs metabolic detoxification (biotransformation), altering drugs, hormones, and foreign molecules to make them less toxic for excretion. It also regulates blood glucose through processes like glycogenesis (storing glucose as glycogen) and gluconeogenesis (creating glucose from amino acids or glycerol).
To maintain efficiency, the body uses enterohepatic circulation: most bile salts are actively reabsorbed in the terminal ileum and returned to the liver via the portal circulation.
The Gallbladder: Storage and Release
The gallbladder acts as a reservoir, storing and concentrating bile between meals by absorbing water and electrolytes. When fat enters the duodenum, the mucosa secretes the hormone cholecystokinin (CCK). CCK triggers the gallbladder to contract, forcing concentrated bile through the cystic duct and common bile duct into the duodenum via the sphincter of Oddi.
The Exocrine Pancreas and Enzyme Activation
The exocrine pancreas provides two vital components: digestive enzymes and an alkaline fluid to neutralize acidic chyme.
- Proteases: To prevent self-digestion, the pancreas secretes proteases (like trypsin) as inactive proenzymes (e.g., trypsinogen). Once in the duodenum, the enzyme enterokinase activates trypsinogen into trypsin, which then activates the other proteases.
- Amylase: Secreted in active form, it breaks down carbohydrates into glucose and maltose.
- Lipase: Works alongside bile to hydrolyze lipids into fatty acids and monoglycerides.
Digestion and Absorption Mechanisms
Fat Emulsification and Micelle Formation
Fat digestion is a unique two-step process. First, bile salts emulsify large fat droplets into smaller particles. Second, pancreatic lipase breaks these down into fatty acids and monoglycerides. These products then aggregate with bile salts to form micelles—tiny, water-soluble spheres with a hydrophobic core and a hydrophilic shell. These micelles transport the fats through the intestinal lumen to the brush border, where they can diffuse into enterocytes.
Nutrient Absorption Distribution
Absorption is highly specialized by location:
- Duodenum: Primarily absorbs calcium, magnesium, and iron.
- Jejunum: The primary site for carbohydrates, amino acids, water-soluble and fat-soluble vitamins, and most water (90%).
- Ileum: Absorbs Vitamin B12 and bile salts.
- Colon: Absorbs remaining water and electrolytes (sodium and potassium).
Exam Tip: Remember that while the small intestine absorbs the vast majority of nutrients, the terminal ileum has a unique and critical role in reabsorbing bile salts for enterohepatic circulation and Vitamin B12.
Clinical Manifestations and Disorders
Pain and Bleeding
Abdominal pain is a key diagnostic tool. Its location (e.g., epigastric vs. lower abdomen) and character (e.g., gnawing vs. colicky) help clinicians differentiate between mechanical, inflammatory, or ischemic causes.
Gastrointestinal bleeding presents in two distinct ways:
- Melena: Dark, "coffee-ground" stools indicating upper GI bleeding (esophagus, stomach, or duodenum) where blood has been digested.
- Hematochezia: Bright red blood indicating lower GI bleeding (colon or rectum) where blood has not been digested.
Jaundice and Metabolic Imbalance
Jaundice (yellowing of skin/sclera) results from elevated bilirubin. It can be hemolytic (excessive red blood cell destruction creating too much unconjugated bilirubin) or obstructive (a blockage in the bile ducts preventing conjugated bilirubin from reaching the intestine).
Digestive disorders can also cause severe electrolyte imbalances. For example, prolonged vomiting can lead to metabolic alkalosis due to the loss of hydrogen ions, while intestinal obstructions can lead to metabolic acidosis as bicarbonate from pancreatic secretions is lost.
Pediatric Considerations
In infants, specific conditions require rapid identification:
- Infantile Hypertrophic Pyloric Stenosis (IHPS): Characterized by forceful, nonbilious projectile vomiting and an "olive-sized" mass in the abdomen.
- Intussusception: A medical emergency where one part of the bowel telescopes into another, causing acute pain and potential perforation.
