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Unlock This CourseThe small intestine is like a super-efficient nutrient extraction factory. It's where most of the food you eat gets broken down and absorbed into your bloodstream. But here's the amazing part - it's not just a simple tube. The small intestine has incredible adaptations that make it one of the most efficient absorption systems in nature.
Think about it: you need to get nutrients from a sandwich into every cell in your body. That's a massive job! The small intestine has evolved brilliant solutions to make this happen quickly and efficiently.
Key Definitions:
The small intestine is about 6 metres long and 2.5cm wide. But if you could unfold all its internal surfaces, it would cover about 200 square metres - roughly the size of a tennis court! This massive surface area is the secret to efficient absorption.
The small intestine uses a clever three-level system to maximise surface area. It's like Russian dolls - each level creates more space for absorption.
The inner wall of the small intestine isn't smooth - it's covered in circular folds called plicae circulares. These are permanent folds that increase the surface area by about 3 times. Think of them like the ridges inside a crisp packet that help you grip more crisps!
Permanent circular folds in the intestine wall that don't flatten out when the intestine fills with food.
Slows down food movement and increases surface area for absorption by 3x.
More time and space for nutrients to be absorbed efficiently.
On top of the circular folds are millions of tiny finger-like projections called villi. Each villus is about 0.5-1.5mm long. These increase the surface area by another 10 times! Imagine a shag carpet - that's what the inside of your small intestine looks like.
Each square centimetre of small intestine contains about 3,000-4,000 villi. That's like having thousands of tiny absorption fingers working non-stop to grab nutrients from your food!
What makes villi so special?
But wait, there's more! Each cell on the surface of a villus has its own tiny projections called microvilli. These are microscopic - about 600 times smaller than villi. They increase surface area by another 20 times and form what's called the "brush border".
Microvilli aren't just for surface area - they're covered in digestive enzymes that complete the breakdown of nutrients right at the absorption site. It's like having tiny molecular scissors cutting food into absorbable pieces.
Not all nutrients are absorbed the same way. The small intestine has different strategies for different types of food molecules.
Carbohydrates are broken down into simple sugars like glucose. These are absorbed by special transport proteins in the microvilli membrane. It's like having specific doorways for different types of sugar molecules.
Uses sodium-glucose transporters to actively pump glucose into cells, even against concentration gradients.
Uses different transporters and moves by facilitated diffusion down concentration gradients.
Most carbohydrates are absorbed within 1-3 hours of eating.
Proteins are broken down into amino acids and small peptides. These are absorbed by specific amino acid transporters. Your body has different transporters for different types of amino acids - it's like having separate conveyor belts for different shaped packages.
Fats are the trickiest to absorb because they don't dissolve in water. The small intestine uses a clever system involving bile salts to create tiny fat droplets called micelles. These are absorbed into the villus cells and then packaged into chylomicrons before entering the lymphatic system.
Some people can't properly digest lactose (milk sugar) because they don't produce enough lactase enzyme in their microvilli. This shows how important the brush border enzymes are - without them, nutrients can't be absorbed properly and cause digestive problems.
Once nutrients are absorbed into villus cells, they need to get into the bloodstream. The small intestine uses two main transport routes:
Water-soluble nutrients like amino acids, simple sugars and water-soluble vitamins go directly into the blood capillaries and travel to the liver via the hepatic portal vein.
Fat-soluble nutrients and fat-soluble vitamins (A, D, E, K) are packaged into chylomicrons and enter the lacteal (lymph vessel) in the centre of each villus.
Let's see how all these adaptations work together when you eat a meal:
Imagine you've just eaten a cheese sandwich. Here's what happens:
In coeliac disease, eating gluten damages the villi, making them shorter and reducing surface area. This shows how crucial healthy villi are for absorption. People with untreated coeliac disease can become malnourished even if they eat plenty of food, because their damaged intestine can't absorb nutrients properly.
The small intestine's adaptations are essential for survival. Without them, we couldn't extract enough nutrients from food to stay healthy. The combination of:
Makes the small intestine one of the most efficient absorption systems in the natural world. It's a perfect example of how structure relates to function in biology.