Describe the process of digestion and absorption in small intestine.

I. Digestion in the Small Intestine

Digestion in the small intestine involves both chemical and mechanical processes, working synergistically to break down complex food molecules into simpler, absorbable units.

A. Mechanical Digestion

Mechanical digestion in the small intestine primarily involves segmentation, rhythmic contractions of the intestinal muscles that mix the chyme with digestive juices and increase contact with the intestinal wall. This process doesn't propel the chyme forward like peristalsis, but rather mixes it thoroughly.

B. Chemical Digestion

Chemical digestion is facilitated by enzymes from the pancreas, liver, and the small intestinal wall itself.

• Pancreatic Enzymes: The pancreas secretes a variety of enzymes into the duodenum via the pancreatic duct. These include:
  • Pancreatic Amylase: Digests carbohydrates into disaccharides.
  • Trypsin, Chymotrypsin, and Carboxypeptidase: Digest proteins into peptides and amino acids.
  • Pancreatic Lipase: Digests fats into fatty acids and glycerol.
  • Nuclease: Digests nucleic acids into nucleotides.
• Bile: Produced by the liver and stored in the gallbladder, bile emulsifies fats, increasing their surface area for lipase action.
• Intestinal Enzymes: The intestinal wall secretes enzymes like:
  • Disaccharidases (Maltase, Sucrase, Lactase): Break down disaccharides into monosaccharides.
  • Peptidases: Break down peptides into amino acids.

II. Absorption in the Small Intestine

The small intestine is uniquely adapted for efficient absorption, owing to its large surface area created by circular folds, villi, and microvilli. Absorption occurs primarily in the jejunum and ileum.

A. Structural Adaptations for Absorption

The intestinal wall is highly folded, forming circular folds (plicae circulares). These folds are covered with finger-like projections called villi, which in turn are covered with microscopic projections called microvilli. This creates a brush border, dramatically increasing the surface area for absorption. Each villus contains a capillary network and a lacteal (lymphatic vessel).

B. Mechanisms of Absorption

Different nutrients are absorbed via different mechanisms:

• Monosaccharides (Glucose, Fructose, Galactose): Absorbed via active transport and facilitated diffusion. Glucose and galactose require a sodium-dependent transporter (SGLT1), while fructose uses facilitated diffusion (GLUT5).
• Amino Acids: Absorbed via active transport, often coupled with sodium ions.
• Fatty Acids and Glycerol: After emulsification by bile, fatty acids and glycerol are absorbed into the intestinal cells. They are then re-esterified into triglycerides and packaged into chylomicrons, which enter the lacteals.
• Water: Absorbed via osmosis, following the absorption of solutes.
• Vitamins: Fat-soluble vitamins (A, D, E, K) are absorbed along with fats. Water-soluble vitamins (B vitamins, C) are absorbed via diffusion or active transport.
• Electrolytes: Absorbed via active transport and diffusion.

C. Regional Specialization

Different sections of the small intestine specialize in absorbing different nutrients:

Section	Primary Absorption
Duodenum	Iron, Calcium
Jejunum	Sugars, Amino Acids, Fatty Acids
Ileum	Vitamin B12, Bile Salts, Water