The small intestine is a vital part of your digestive system, located between the stomach and the large intestine. It's where the final stages of digestion happen and where most of the nutrients from your food are absorbed. Despite its name, it's the longest part of the digestive tract, measuring about 6 meters (20 feet) in an adult. Its 'small' name comes from its narrow width, which is about the size of your finger.

The small intestine is a long, coiled tube that continues the work of the stomach. Its main jobs are:

• Completing Digestion: It mixes the partially digested food from the stomach (called chyme) with digestive juices from the pancreas and bile from the liver. These juices break down carbohydrates, proteins, and fats into smaller, simpler molecules.

• Absorbing Nutrients: Once food is fully broken down, the small intestine absorbs the tiny nutrient molecules (like glucose, amino acids, and fatty acids) into the bloodstream. This is how the body gets the energy and building blocks it needs.

• Moving Food Along: The walls of the small intestine have muscles that contract in a wave-like motion, called peristalsis. This movement pushes the food mixture through the tube towards the large intestine.

The three parts of the small intestine

The small intestine is divided into three distinct parts:

• Duodenum: This is the first and shortest part, shaped like a 'C'. It receives the chyme from the stomach. This is where most of the chemical digestion happens. The pancreas secretes enzymes here to break down food, and the gallbladder releases bile to help with fat digestion.

• Jejunum: This is the middle section. Its main role is to absorb nutrients, including carbohydrates, amino acids, and fatty acids. It has many folds and blood vessels to help with this process.

• Ileum: This is the final and longest part. It absorbs any remaining nutrients that weren't absorbed in the jejunum. This includes important substances like vitamin B12 and bile salts. It connects the small intestine to the large intestine.

  
  

The small intestine is specially designed to be excellent at its job of absorbing nutrients. It has a huge internal surface area, which allows for maximum contact between the food and the intestinal wall.

• Villi: The inner wall of the small intestine is covered in millions of tiny, finger-like projections called villi (singular: villus). These greatly increase the surface area.

• Microvilli: On the surface of each villus are even smaller projections called microvilli. This further increases the surface area, making it even more efficient for absorption.

• Rich Blood Supply: Each villus contains a network of tiny blood capillaries and a lymph vessel called a lacteal.

  • Blood capillaries absorb simple sugars (like glucose) and amino acids.

  • Lacteals absorb fatty acids and glycerol.

  • This allows the nutrients to be quickly transported away from the intestine to the rest of the body.

• Thin Walls: The walls of the small intestine are very thin, just one cell thick in some places. This allows the digested food molecules to pass through easily into the blood or lymph.

Notes: Small Intestine

• The small intestine is the main site for the digestion and absorption of nutrients.

• It is approximately 6 meters long in adults but is called 'small' due to its narrow diameter.

• It has three parts:

  • Duodenum: Receives food from the stomach; where most chemical digestion begins with enzymes from the pancreas and bile from the liver.

  • Jejunum: Primary site for absorption of carbohydrates, proteins, and fats.

  • Ileum: Absorbs remaining nutrients, including vitamin B12 and bile salts.

• It is adapted for absorption by:

  • Having a very large surface area due to numerous folds and millions of villi (finger-like projections) and microvilli on the villi.

  • Having a rich blood supply in each villus to quickly transport absorbed nutrients.

  • Having thin walls to allow easy passage of food molecules.

•