1.6 Absorption of Digestion Products

Absorption is the process by which digested nutrients pass from the lumen of the small intestine into the blood or lymph. The ileum is the major site of absorption, and its structure is specially adapted to maximize the surface area for efficient nutrient uptake.

Definition

Absorption: Absorption is the movement of digested food molecules (monosaccharides, amino acids, fatty acids, glycerol, vitamins, minerals, and water) from the lumen of the small intestine across the epithelial lining into the blood capillaries or lacteals (lymph vessels).

Main Idea

The ileum is the major site of nutrient absorption. Its inner surface is highly folded and covered with millions of tiny finger-like projections called villi, which are further covered with microvilli. These structures enormously increase the surface area for absorption. Nutrients cross the epithelial cells and enter either the blood (for carbohydrates and proteins) or the lymph (for fats).

Core Concept

• Ileum = major site of absorption
• Villi = finger-like projections (0.5–1.5 mm) that increase surface area
• Microvilli = tiny projections on epithelial cells that further increase surface area
• Lacteal = lymph capillary inside each villus; absorbs fats
• Capillary network = blood capillaries inside each villus; absorb carbohydrates and proteins
• Carbohydrates → absorbed into blood → hepatic portal vein → liver
• Proteins → absorbed into blood → hepatic portal vein → liver
• Fats → absorbed into lacteals → lymph → blood

Detailed Explanation

Structure of a Villus

Each villus is a finger-like projection of the intestinal mucosa. It is 0.5–1.5 mm in length and is covered by simple columnar epithelium.

Components of a Villus:

• Folds in the internal walls of the small intestine
• Villi — finger-like projections
• Microvilli — tiny projections on the surface of epithelial cells

These features make the small intestine the largest part of the alimentary canal in terms of surface area.

Pathway of Absorption

• An epithelial cell of the intestinal lining
• A cell lining the blood capillaries or lymph vessel

Absorption of Carbohydrates

Digestion Products: - Monosaccharides (glucose, galactose, fructose)

• Absorption occurs by a combination of simple diffusion and active transport.
• Monosaccharides are transferred by facilitated diffusion to the capillaries of intestinal villi.
• They are carried by the hepatic portal system to the liver.
• In the liver, non-glucose sugars are converted to glucose.
• Glucose enters the cell through facilitated diffusion.

Pathway: Lumen → Epithelial cell → Capillary → Hepatic portal vein → Liver → General circulation

Absorption of Lipids

• Fatty acids
• Glycerol
• Monoglycerides

• Lipids are digested into fatty acids and glycerol.
• After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells.
• The fats are then mixed with cholesterol and proteins, forming small globules called chylomicrons.
• Most chylomicrons are transported by exocytosis out of epithelial cells into lacteals.
• Lymph containing chylomicrons eventually drains from the lymphatic system into large veins that return blood to the heart.

Pathway: Lumen → Epithelial cell → Recombined into fats → Chylomicrons → Lacteal → Lymph → Thoracic duct → Subclavian vein → Blood

Absorption of Proteins

Digestion Products: - Amino acids

• Individual amino acids are absorbed in epithelial cells of villi.
• They enter the hepatic portal system, which transports them to the liver.
• The amino acids may be modified in the liver or released into the bloodstream.
• Most amino acids are used as building blocks to form new proteins.
• Some amino acids may be used for energy.

Pathway: Lumen → Epithelial cell → Capillary → Hepatic portal vein → Liver → General circulation

Intermediate Level

Comparison of Absorption Pathways

Structural Adaptations for Absorption

Role of the Liver in Absorption

After absorption, nutrients enter the hepatic portal vein and go to the liver first.

• Converts non-glucose sugars to glucose — galactose and fructose → glucose
• Stores glucose as glycogen — glycogenesis
• Releases glucose from glycogen — glycogenolysis
• Modifies amino acids — deamination, transamination
• Synthesizes proteins — plasma proteins
• Stores vitamins — A, D, B12
• Stores iron — from hemoglobin breakdown
• Detoxifies harmful substances — drugs, alcohol

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Ileum is the major site of nutrient absorption.
• 2. Villi are finger-like projections (0.5–1.5 mm) that increase surface area.
• 3. Microvilli are tiny projections on epithelial cells that further increase surface area.
• 4. Each villus contains a blood capillary network and a lacteal.
• 5. Carbohydrates are absorbed as monosaccharides into blood.
• 6. Proteins are absorbed as amino acids into blood.
• 7. Fats are absorbed as fatty acids and glycerol into lacteals.
• 8. Fats are recombined into triglycerides and packaged into chylomicrons.
• 9. Chylomicrons are transported by exocytosis into lacteals.
• 10. Lymph containing chylomicrons drains into large veins.
• 11. Glucose and amino acids enter the hepatic portal system.
• 12. The liver processes nutrients before they enter general circulation.
• 13. Non-glucose sugars are converted to glucose in the liver.
• 14. Amino acids are used as building blocks or for energy.
• 15. The structural features of the small intestine make it the largest part of the alimentary canal in terms of surface area.

Remember

• - Ileum = major site of absorption
• - Villi = finger-like projections
• - Microvilli = tiny projections on epithelial cells
• - Lacteal = lymph capillary in villus
• - Capillary network = blood capillaries in villus
• - Carbohydrates → blood → hepatic portal vein → liver
• - Proteins → blood → hepatic portal vein → liver
• - Fats → lacteals → lymph → thoracic duct → subclavian vein → blood
• - Chylomicrons = transport fats from epithelial cells to lacteals
• - Liver = processes nutrients before general circulation
• - Non-glucose sugars → converted to glucose in liver
• - Amino acids → building blocks or energy

Common Mistakes

• 1. Thinking fats are absorbed into blood — Fats are absorbed into lymph (lacteals).
• 2. Confusing villi and microvilli — Villi are finger-like projections; microvilli are tiny projections on epithelial cells.
• 3. Believing all nutrients are absorbed in the stomach — Most absorption occurs in the small intestine.
• 4. Mixing up the pathways — Carbohydrates and proteins go to blood; fats go to lymph.
• 5. Forgetting the role of the liver — The liver processes nutrients before they enter general circulation.
• 6. Thinking glucose is absorbed by simple diffusion only — Glucose is absorbed by facilitated diffusion and active transport.
• 7. Confusing chyle with chyme — Chyme is in the stomach; chyle is in the small intestine.
• 8. Forgetting the role of bile salts — Bile salts emulsify fats; bile pigments are waste products.
• 9. Mixing up the three parts of the small intestine — Duodenum (first), jejunum (middle), ileum (last).
• 10. Believing the small intestine is short — It is about 6 meters long.

Quick Revision

The ileum is the major site of nutrient absorption. Its inner surface has villi (finger-like projections) and microvilli (tiny projections on epithelial cells) that greatly increase surface area. Each villus contains a blood capillary network and a lacteal. Carbohydrates are absorbed as monosaccharides into blood capillaries; they travel via the hepatic portal vein to the liver. Proteins are absorbed as amino acids into blood capillaries; they also travel via the hepatic portal vein to the liver. Fats are absorbed as fatty acids and glycerol into epithelial cells, recombined into triglycerides, packaged into chylomicrons, and transported by exocytosis into lacteals. Lymph containing chylomicrons drains into large veins. The liver processes nutrients before they enter general circulation.

60-Second Revision

• Ileum = major site of absorption
• Villi = finger-like; microvilli = tiny projections
• Lacteal = lymph capillary; absorbs fats
• Capillary network = absorbs carbohydrates and proteins
• Carbohydrates → blood → hepatic portal vein → liver
• Proteins → blood → hepatic portal vein → liver
• Fats → lacteals → lymph → thoracic duct → subclavian vein → blood
• Chylomicrons = transport fats to lacteals
• Liver = processes nutrients
• Warning: Fats go to lymph, not blood; carbohydrates and proteins go to blood

NEXT TOPIC

• Chapter:
• 1. Digestive System of Man

• Topic:
• 1.7 LARGE INTESTINE

Source: