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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Villus Structure
The small intestine is where most of our food digestion and nutrient absorption happens. To do this job well, it needs a special design. This is where villi come in – they're tiny finger-like projections that line the small intestine and are crucial for absorbing nutrients from our food into our bloodstream.
Key Definitions:
Imagine trying to soak up spilled water with a flat sheet versus a fluffy towel. The towel works better because it has more surface area to absorb the water. Similarly, villi increase the surface area of the small intestine by about 30 times! This massive increase allows your body to absorb nutrients much more efficiently than if the intestinal wall was flat.
Villi are found throughout the small intestine, which includes the duodenum, jejunum and ileum. They're most numerous in the jejunum, where most nutrient absorption takes place. The small intestine in humans is about 6-7 metres long and every square centimetre contains approximately 2,000 villi!
Each villus is a tiny structure, about 0.5-1.5 mm in length, but their design is incredibly sophisticated. Let's look at what makes up a villus:
A villus contains several important components that work together to absorb nutrients efficiently:
The outer layer of a villus is made of a single layer of columnar epithelial cells. These cells have microvilli on their surface (the brush border) that further increase surface area. Each epithelial cell may have up to 3,000 microvilli!
Each villus contains a network of blood capillaries. These tiny blood vessels collect absorbed nutrients like glucose, amino acids, vitamins and minerals and carry them away to the liver via the hepatic portal vein.
In the centre of each villus is a lymph vessel called a lacteal. This collects absorbed fatty acids and glycerol, which are too large to enter the blood capillaries directly. The lacteal appears milky white after a fatty meal.
Villi have several special features that make them perfect for absorbing nutrients:
The small intestine has three features that dramatically increase its surface area:
Together, these increase the surface area by about 600 times compared to a simple tube of the same size!
The epithelial cells of villi contain special transport proteins in their cell membranes that help move nutrients from the intestine into the bloodstream. Different nutrients use different transport methods:
Different nutrients are absorbed in different ways across the villus epithelium:
Carbohydrates are broken down into simple sugars like glucose. Glucose is absorbed by active transport using carrier proteins called GLUT transporters. This requires energy in the form of ATP. Once inside the epithelial cells, glucose diffuses into the blood capillaries.
Proteins are broken down into amino acids. These are absorbed by active transport using specific carrier proteins. Like glucose, they require energy for absorption and are transported into the blood capillaries.
Lipids (fats) are broken down into fatty acids and glycerol. These can diffuse directly through the cell membrane of the epithelial cells. Inside the cells, they're reassembled into fat droplets called chylomicrons, which enter the lacteal rather than the blood capillaries.
Coeliac disease is an autoimmune disorder where eating gluten (found in wheat, barley and rye) triggers an immune response that damages the villi. This condition affects about 1 in 100 people in the UK.
When someone with coeliac disease consumes gluten, their immune system attacks the lining of the small intestine, causing inflammation and damaging the villi. This damage, called villous atrophy, flattens the villi and reduces the surface area available for nutrient absorption.
Symptoms include diarrhoea, abdominal pain, bloating, fatigue and weight loss. The only treatment is a strict gluten-free diet, which allows the villi to heal and function properly again. Without treatment, coeliac disease can lead to serious complications like malnutrition, anaemia, osteoporosis and even certain cancers.
Villi aren't static structures – they're constantly in motion! Each villus can contract and relax thanks to smooth muscle cells in its core. This movement, called villus motility, helps mix the intestinal contents and brings different nutrients into contact with the absorptive surface.
The microvilli that make up the brush border aren't just for increasing surface area. They also contain digestive enzymes that complete the final stages of digestion right at the absorption surface. These include:
The epithelial cells of villi have one of the fastest renewal rates in the body. They're completely replaced every 4-5 days! New cells are produced in the crypts (the spaces between villi) and migrate up the villus as they mature. When they reach the tip, they're shed into the intestinal lumen.
This rapid renewal helps maintain the integrity of the intestinal lining despite the harsh digestive environment and potential damage from food particles or bacteria.
Several conditions can affect the structure and function of villi:
Certain bacteria like E. coli and Salmonella can damage villi, leading to diarrhoea and malabsorption. The damage is usually temporary and villi regenerate once the infection is cleared.
This occurs when there's a deficiency of lactase enzyme on the brush border. Without lactase, lactose from milk can't be digested properly, causing symptoms like bloating, gas and diarrhoea.
Some medications, particularly certain antibiotics and chemotherapy drugs, can damage villi. This is why these treatments sometimes cause digestive side effects and nutritional problems.
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