Explain how the undamag ed villi are adapted for their function. Explain why children with untreated coeliac disease may grow more slowly and become tired more easily than children without coeliac disease.

4. (c) (i)
The undamaged villi are highly adapted for efficient absorption of digested nutrients due to several features:

• They are finger-like projections, which greatly increase the surface area available for absorption in the small intestine.
• The epithelial cells lining the villi have a brush border (microvilli), further increasing the surface area for absorption.
• The wall of each villus is only one cell thick, providing a short diffusion distance for nutrients to pass into the bloodstream.
• Each villus has a rich blood supply (capillary network) to maintain a steep concentration gradient, ensuring absorbed nutrients are quickly transported away.

4. (c) (ii)
Children with untreated coeliac disease have damaged villi, as shown in Fig. 4.2.

• The damaged villi are flattened, which significantly reduces the surface area available for nutrient absorption in the small intestine.
• This leads to malabsorption of essential nutrients like carbohydrates, proteins, fats, vitamins, and minerals.
• Slow growth occurs because the body does not absorb enough nutrients required for cell division, tissue repair, and overall development.
• Tiredness results from insufficient absorption of energy-providing nutrients (like glucose from carbohydrates) and vital minerals such as iron. Iron is crucial for producing haemoglobin, which transports oxygen in the blood. A lack of iron can lead to anaemia, causing fatigue and reduced energy levels.