Sign up to access the complete lesson and track your progress!
Unlock This CourseThe small intestine is where most of our digestion and absorption happens. It's about 6-7 metres long in adults and has several clever adaptations that help it do its job well. These adaptations make sure we can get as many nutrients as possible from our food.
Key Definitions:
The small intestine has three parts:
The small intestine has a soft, pink lining with a velvety texture due to millions of tiny finger-like projections called villi.
The small intestine needs to absorb huge amounts of nutrients every day. To do this efficiently, it has evolved several special features that:
The small intestine has several amazing adaptations that make it perfectly suited for absorbing nutrients. Let's look at each one:
The most important adaptation of the small intestine is its massive surface area. If you were to flatten out an adult's small intestine, it would cover about 250 square metres - that's about the size of a tennis court! This huge surface area is created by three levels of folding:
The inner lining of the small intestine is folded into circular ridges called plicae circulares. These folds increase the surface area by about 3 times and help slow down food movement.
Covering the circular folds are millions of tiny finger-like projections called villi. Each villus is about 0.5-1.0mm long. Villi increase the surface area by about 10 times more.
The cells on the surface of each villus (epithelial cells) have thousands of microscopic projections called microvilli. These form a "brush border" that increases the surface area by another 20 times.
The three levels of folding in the small intestine increase its surface area by about 600 times compared to a simple tube of the same length. Without these adaptations, we would need a small intestine about 60 metres long to absorb enough nutrients!
Each villus is a tiny but complex structure perfectly designed for absorption:
The epithelial cells are joined by tight junctions, which control what can pass between them. This helps prevent harmful substances from entering the bloodstream.
The small intestine contains several types of specialised cells:
These cells (enterocytes) have microvilli on their surface and contain enzymes for the final stages of digestion. They absorb most nutrients including:
These cells produce mucus that:
The small intestine has an extensive network of blood vessels that:
This rich blood supply ensures that as nutrients are absorbed, they're quickly removed from the area, maintaining a concentration gradient that encourages more absorption.
Different nutrients are absorbed by different mechanisms:
Some nutrients like glucose and amino acids are absorbed by active transport. This process:
Other substances move by diffusion or facilitated diffusion:
Coeliac disease shows how important healthy villi are. In this condition, eating gluten triggers an immune response that damages the villi in the small intestine. The villi become flattened (villous atrophy), drastically reducing the surface area for absorption. This leads to symptoms like:
Treatment involves a strict gluten-free diet, which allows the villi to heal and restore normal absorption.
Different nutrients are absorbed in different ways and in different parts of the small intestine:
Broken down to monosaccharides (glucose, fructose, galactose). Absorbed mainly in the duodenum and jejunum by active transport or facilitated diffusion.
Broken down to amino acids. Absorbed throughout the small intestine, mainly by active transport using carrier proteins.
Broken down to fatty acids and glycerol. Form tiny droplets called micelles. Absorbed mainly in the jejunum and ileum, entering the lacteal in the villus.
All these adaptations work together to make the small intestine incredibly efficient at absorbing nutrients. Without them:
Understanding these adaptations helps us appreciate how our bodies are perfectly designed for efficient nutrition and why certain digestive disorders can have such serious effects on our health.