The small intestine is the main site where digested food becomes usable by the body. Its inner wall is shaped into folds, villi, and microvilli that greatly increase surface area for absorption. This design lets nutrients move efficiently from the intestine into transport systems that deliver them to cells.
Understanding this process connects digestion to energy, growth, repair, and homeostasis.
Absorption happens when molecules cross the epithelial lining of each villus and enter either blood capillaries or lymph vessels. Sugars, amino acids, water-soluble vitamins, minerals, and much of the water enter the bloodstream. Fatty acids and monoglycerides are rebuilt into triglycerides, packaged into chylomicrons, and sent into lacteals in the lymphatic system.
Different nutrients use diffusion, facilitated diffusion, active transport, and osmosis depending on their size, charge, concentration gradient, and solubility.
Key Facts
- Folds, villi, and microvilli increase the absorptive surface area of the small intestine by hundreds of times.
- Most nutrient absorption occurs in the jejunum, while the ileum absorbs bile salts and vitamin B12.
- Glucose and amino acids enter villus capillaries and travel through the hepatic portal vein to the liver.
- Fatty acids and monoglycerides enter epithelial cells, form chylomicrons, and move into lacteals.
- Osmosis moves water across the intestinal wall from higher water concentration to lower water concentration.
- Rate of absorption depends on surface area, concentration gradient, membrane transport proteins, and blood or lymph flow.
Vocabulary
- Villus
- A fingerlike projection of the small intestine lining that contains capillaries and a lacteal for nutrient absorption.
- Microvilli
- Tiny projections on epithelial cells that form the brush border and greatly increase surface area.
- Capillary
- A very small blood vessel where absorbed sugars, amino acids, minerals, and water-soluble vitamins enter the blood.
- Lacteal
- A lymphatic vessel inside a villus that absorbs fats packaged as chylomicrons.
- Active transport
- The movement of substances across a membrane using cellular energy, often against a concentration gradient.
Common Mistakes to Avoid
- Assuming all nutrients enter the bloodstream directly, which is wrong because most absorbed fats first enter lacteals and travel through lymph.
- Forgetting the role of microvilli, which is wrong because the brush border provides much of the surface area and contains enzymes and transport proteins.
- Thinking absorption and digestion are the same process, which is wrong because digestion breaks food into small molecules while absorption moves those molecules into body fluids.
- Treating diffusion as the only absorption method, which is wrong because glucose, amino acids, ions, and other nutrients often require carrier proteins or active transport.
Practice Questions
- 1 A villus has an exposed surface area of 0.25 mm2. If microvilli increase that surface area by a factor of 20, what is the effective surface area for absorption?
- 2 A student measures glucose concentration as 90 mg/dL in the intestinal fluid and 70 mg/dL in nearby capillary blood. What is the concentration difference, and which direction would glucose tend to move by diffusion?
- 3 Explain why damage that flattens villi can cause weight loss and nutrient deficiencies even if a person eats enough food.