9.3 Absorption
Absorption is the process where simple digested nutrient molecules pass through the epithelial lining of the small intestine into the bloodstream and lymphatic system. In humans, this occurs primarily in the ileum. The ileum possesses specialized structural adaptations—including internal folds, villi, and microvilli—that maximize surface area and ensure efficient nutrient uptake.
Learning Objectives
- Describe the structural adaptations of the ileum, villi, and microvilli in the absorption of digested food.
- Identify the specific roles of the epithelial layer, goblet cells, blood capillaries, and lacteals in a villus.
- Explain the different methods of absorption (simple diffusion, facilitated diffusion, active transport, and osmosis) for various nutrients into blood capillaries and lacteals.
Structure of a Villus and Pathways of Nutrient Absorption
A single villus contains a central lacteal, a surrounding blood capillary network, a one-cell-thick epithelium, goblet cells and apical microvilli. Water-soluble nutrients enter the blood capillaries, while lipid droplets enter the lacteal.
Into Blood Capillaries
Water-soluble nutrients: glucose, galactose, amino acids, fructose, vitamins B and C, and water.
Into Lacteal
Lipid-soluble nutrients: fatty acids and glycerol as recombined lipid droplets, plus fat-soluble vitamins A, D, E and K.
Short Explanation
What is absorption?
Absorption is the process where simple digested nutrient molecules pass through the epithelial lining of the small intestine into the bloodstream and lymphatic system. In humans, this occurs primarily in the ileum.
Adaptations of the ileum
The ileum is highly adapted for nutrient absorption due to its long length. Its internal layer is folded and covered by millions of tiny finger-like projections called villi.
Adaptations of a villus
The epithelial layer is only one cell thick, which shortens diffusion distance and accelerates nutrient absorption. Goblet cells secrete mucus to aid digestion and protect the lining. A network of blood capillaries surrounds the lacteal to rapidly absorb and transport water-soluble digestive products throughout the body. The lacteal is a central lymphatic vessel that absorbs and carries droplets of fatty acids and glycerol.
Microvilli
Microvilli are tiny projections located on the outer surface of the villus epithelial cells. They provide a very large surface area to increase the rate of nutrient absorption.
Two circulatory routes
Once across the epithelial border, nutrients split into two routes. Blood capillaries receive water-soluble molecules, while the lacteal receives lipid-soluble nutrients.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Name the lymphatic vessel inside a villus that transports droplets of fatty acids and glycerol.
State the method of transport used to absorb fructose across epithelial cells into the blood capillaries.
Explain why the epithelial layer of a villus is only one cell thick.
Common Mistakes
- Stating that fatty acids and glycerol are absorbed directly into the blood capillaries; they actually enter the lacteal as recombined lipid droplets.
- Confusing the transport mechanisms of simple sugars: assuming fructose is absorbed by active transport like glucose and galactose, whereas fructose is absorbed by facilitated diffusion.
- Thinking that vitamins B and C enter the lacteal; they are water-soluble and enter the blood capillaries with water.
Concept Misunderstandings
Digested lipids travel directly as free fatty acids and glycerol through the lymph fluid.
Fatty acids and glycerol diffuse into the epithelial cells first, where they recombine through condensation to form tiny lipid droplets before passing into the lacteal.
The terms “villus” and “microvillus” refer to the same structure.
A villus is a multicellular projection of the intestinal wall, while a microvillus is a tiny microscopic projection on the plasma membrane of an individual epithelial cell.
Summary
- The ileum is long, folded, and lined with villi and microvilli to provide a huge surface area for absorption.
- The villus epithelium is one cell thick to accelerate absorption, and goblet cells secrete mucus to aid digestion.
- Blood capillaries absorb glucose, galactose and amino acids by active transport; fructose by facilitated diffusion; water by osmosis; and vitamins B and C with water.
- Lacteal absorbs fatty acids and glycerol after they recombine into lipid droplets by condensation, plus fat-soluble vitamins A, D, E and K by simple diffusion.
Short Activity
Objective: identify absorption routes and transport mechanisms in the ileum.
A. Objective Quiz
Which nutrient is absorbed across the intestinal epithelial cells into the blood capillaries via facilitated diffusion?
What happens to fatty acids and glycerol inside the epithelial cells of a villus before entering the lacteal?
B. Fill in the Blanks
The epithelial layer of a villus is cell thick to accelerate nutrient absorption.
The central vessel in a villus that carries droplets of fatty acids and glycerol is the .
C. Matching / Drag and Drop
Match each nutrient to its correct absorption route and method in the ileum. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Nutrient to Its Route and Transport Method
Write the correct nutrient next to each route and transport method.
| Route and Transport Method | Digested Nutrient |
|---|---|
| 1. Absorbed into blood capillaries via active transport | |
| 2. Absorbed into blood capillaries via facilitated diffusion | |
| 3. Absorbed into blood capillaries via osmosis | |
| 4. Dissolve in lipids and absorb into lacteal via simple diffusion |
Answer choices: Glucose and galactose • Fructose • Water • Vitamins A, D, E, K
