1.6 Absorption of Digestion Products

This section outlines the process by which the final products of digestion—monosaccharides, fatty acids, glycerol, and amino acids—are absorbed from the small intestine into the body's transport systems.

Site of Absorption: The Ileum

• The ileum, a part of the small intestine, is the primary site for the absorption of digested nutrients.
• Its structure is highly adapted to maximize the efficiency of this process.

Structural Adaptations for Maximum Absorption

• The internal walls of the small intestine are extensively folded to increase the available surface area.
• Villi: These are tiny, finger-like projections of the intestinal lining (mucosa), about 0.5-1.5 mm long. They vastly increase the surface area for absorption.
• Microvilli: The surface of the epithelial cells covering each villus is further folded into microscopic projections called microvilli (forming a "brush border"), which magnifies the absorptive surface even more.

Structure of a Villus

• Epithelium: Each villus is covered by a single layer of simple columnar epithelial cells, through which nutrients must pass.
• Blood Capillaries: A dense network of blood capillaries is present within each villus to absorb most nutrients.
• Lacteal: A central lymph capillary, called a lacteal, is located within each villus and is specialized for absorbing fats.

Absorption of Macronutrients

Absorption of Carbohydrates

• Absorbed Form: Monosaccharides (e.g., glucose, fructose, galactose).
• Mechanism: A combination of simple diffusion, facilitated diffusion, and active transport.
• Pathway: Monosaccharides are absorbed into the blood capillaries of the villi. This nutrient-rich blood then travels via the hepatic portal system to the liver, where non-glucose sugars are converted to glucose for energy or storage.

Absorption of Lipids

• Absorbed Form: Fatty acids and glycerol.
• Mechanism: These molecules diffuse into the epithelial cells.
• Pathway:
  1. Inside the epithelial cells, fatty acids and glycerol are re-combined to form fats (triglycerides).
  2. These fats are then packaged with cholesterol and proteins into small globules called chylomicrons.
  3. Chylomicrons are released from the epithelial cells via exocytosis and enter the lacteals.
  4. The lymph transports the chylomicrons, eventually releasing them into the bloodstream.
• Note: Lipoproteins like chylomicrons are classified by density. Chylomicrons have a very low density due to their high lipid content (99%) and low protein content (1%).

Absorption of Proteins

• Absorbed Form: Individual amino acids.
• Mechanism: Active transport into the epithelial cells.
• Pathway: Amino acids enter the blood capillaries in the villi and are transported via the hepatic portal system to the liver. The liver may modify them or release them into the general circulation for body cells to use in building new proteins.

Possible Questions and Answers

• Q: How is the small intestine adapted for nutrient absorption? A: It has a very large surface area due to its long length, internal folds, and the presence of millions of villi and microvilli.
• Q: What is the function of a lacteal? A: A lacteal is a specialized lymph capillary within a villus that absorbs digested fats in the form of chylomicrons.
• Q: What are chylomicrons and why are they formed? A: Chylomicrons are globules of reformed fats, cholesterol, and proteins. They are formed within the intestinal epithelial cells to transport water-insoluble lipids through the lymphatic system and into the bloodstream.
• Q: What is the hepatic portal system? A: It is a system of veins that carries nutrient-rich blood (containing absorbed carbohydrates and amino acids) from the capillaries of the small intestine directly to the liver for processing before it enters general circulation.

Summary

• The ileum is specialized for nutrient absorption with a massive surface area created by folds, villi, and microvilli.
• Each villus contains blood capillaries for absorbing monosaccharides and amino acids, and a lacteal for absorbing fats.
• The destination of absorbed nutrients varies:

• Biological Significance: This highly efficient absorption process ensures that the body receives the necessary building blocks and energy molecules from ingested food to maintain homeostasis, grow, and repair tissues.