Nutrient Absorption in Villi and Assimilation
Absorption is the movement of digested nutrients from the gut into the blood or lymph, mainly through the villi of the small intestine, and assimilation is the use of those nutrients by body cells. In brief: villi are finger-like projections that greatly increase the surface area of the small intestine. Glucose and amino acids enter the blood capillaries in the villi, while fatty acids and glycerol are taken up mostly into the lymph vessel. Egestion is the removal of food that was never absorbed.
Digestion has no value for the body until the products reach the cells. This guide explains how villi are built for absorption, how different nutrients cross the intestinal lining, what happens during assimilation and egestion, how to estimate the effect of folds on surface area and which mistakes to avoid.
What is absorption?
Absorption is the passage of small molecules from the lumen (the inside space) of the gut across the intestinal wall into the blood or lymph. It occurs mostly in the small intestine, which receives food already broken down by enzymes. Some absorption also occurs elsewhere: the stomach absorbs a little water and alcohol, and the large intestine absorbs water and minerals.
The small molecules that can be absorbed are glucose and other simple sugars, amino acids, fatty acids and glycerol, vitamins, minerals and water.
How are villi adapted for absorption?
The inner wall of the small intestine is folded into millions of villi, each about 0.5 to 1.5 millimeters long. The cells on the surface of each villus have microvilli, tiny projections on the cell membrane called the brush border. Together, folds, villi and microvilli increase the absorbing surface many times, to an estimated area of tens or even hundreds of square meters, depending on the method of estimate.
Several features make villi efficient:
- Large surface area from villi and microvilli.
- Thin walls: the lining is a single layer of cells, so the diffusion distance is short.
- Dense network of capillaries, which carries away absorbed glucose and amino acids and keeps the concentration gradient steep.
- A lacteal, which is a blind-ended lymph vessel in the center of each villus, absorbs fats.
- Many mitochondria in the cells to supply the energy for active transport.
- Muscle fibers that move the villi and mix the contents.
How do nutrients cross the intestinal lining?
Different methods are used depending on the molecule.
Diffusion and facilitated diffusion
When the concentration of a nutrient is higher in the gut than in the blood, it moves in by diffusion. Fructose crosses mainly by facilitated diffusion through carrier proteins. Blood flow removes absorbed molecules and keeps the gradient steep.
Active transport
After a meal, glucose and amino acids are often more concentrated in the cells than in the gut when digestion is nearly complete, and the body absorbs all it can. Active transport uses carrier proteins and ATP to move the molecules against the concentration gradient. Glucose absorption is linked to the movement of sodium ions. This process makes it possible to absorb nearly all of the glucose from food.
Absorption of fats
Fatty acids and glycerol are absorbed into the cells of the villus. Inside the cells, they are reassembled into triglycerides, packaged with protein into small particles called chylomicrons and released into the lacteals, which are part of the lymphatic system. The lymph eventually joins the bloodstream near the heart. For this reason fats do not go to the liver first.
Water
Water is absorbed by osmosis. Around 8 to 9 liters of fluid enter the gut each day, from food, drink and digestive secretions, and most of it is absorbed in the small intestine, with the rest taken up in the large intestine.
Where do absorbed nutrients go?
| Nutrient | Form absorbed | Main method | Route |
|---|---|---|---|
| Carbohydrate | Glucose | Active transport | Capillary, then liver |
| Protein | Amino acids | Active transport | Capillary, then liver |
| Fat | Fatty acids, glycerol | Diffusion | Lacteal (lymph) |
| Water-soluble vitamins | Vitamins | Diffusion or carriers | Capillary |
| Water | Water | Osmosis | Capillary |
What is assimilation?
Assimilation is the uptake and use of absorbed nutrients by the cells of the body. Blood from the intestine goes through the hepatic portal vein to the liver, which regulates what enters the general circulation. The liver stores excess glucose as glycogen, adjusts the amino acids and processes toxins.
Cells then use the nutrients:
- Glucose is used in respiration to release energy.
- Amino acids are used to build new proteins, such as enzymes and muscle proteins.
- Fatty acids and glycerol build cell membranes, store energy and supply energy.
Excess nutrients are stored. Glucose is held as glycogen in the liver and muscles, and any surplus is turned into fat in adipose tissue. The body cannot store amino acids, so the extra ones are converted in the liver to urea, which is excreted in urine.
What is egestion?
Egestion, also called defecation, is the removal of undigested material from the body as feces. The material includes fiber, dead cells, bacteria and bile pigments, which give feces their brown color. It is not excretion, because the material was never part of the body’s cells and was not produced by their metabolism.
Worked example: surface area of the intestine
To see why folds matter, compare a smooth tube with a tube that has villi. Model a section of intestine as a cylinder 10 cm long and 2.5 cm in diameter, and each villus as a small cylinder 1 mm long and 0.2 mm wide.
- Smooth tube: Surface area = π × diameter × length = 3.14 × 2.5 × 10 ≈ 78.5 cm².
- One villus (side surface): π × 0.02 cm × 0.1 cm ≈ 0.00628 cm². The tip adds a little, about 0.0003 cm², which can be ignored.
- Number of villi: Suppose there are 2 000 villi per cm² of the wall. In 78.5 cm² there are about 157 000 villi.
- Area added by villi: 157 000 × 0.00628 ≈ 986 cm².
- Total: about 78.5 + 986 ≈ 1 065 cm², which is about 13.6 times more than the smooth tube.
This simple model, with only villi included, already gives an increase of more than 13 times. The microvilli add much more. The numbers are illustrative and are not measurements of a real intestine.
How was absorption studied?
Early physiologists in the nineteenth century studied absorption by tying off sections of intestine in animals and measuring what disappeared from the enclosed fluid. Electron microscopes in the 1950s revealed the microvilli. The discovery of sodium-linked glucose transport in the 1960s, associated with Robert Crane, explained how glucose can be taken up against a gradient. Oral rehydration therapy for diarrhea, which uses a solution of salt and glucose, is based on this mechanism and has saved many lives.
Which mistakes are common?
Mistake 1: Using absorption and digestion as synonyms
Digestion breaks food into small molecules. Absorption moves those molecules into the blood or lymph.
Mistake 2: Saying all nutrients go into the blood capillaries
Most fat enters the lacteal. Glucose and amino acids enter the capillaries.
Mistake 3: Confusing egestion with excretion
Egestion removes undigested food. Excretion removes waste made by the body’s cells, such as urea and carbon dioxide.
Mistake 4: Thinking that villi digest food
Villi are for absorption. Digestion is done by enzymes, some of which are attached to the surface of the intestinal cells.