The small intestine is a long, coiled tube where most digestion and absorption of nutrients takes place. It is divided into three main sections: 1. Duodenum: The first and shortest section, C-shaped, where it receives partially digested food from the stomach and mixes with bile from the liver and digestive enzymes from the pancreas. 2. Jejunum: The middle section, where most of the absorption of carbohydrates, amino acids, and fats occurs. 3. Ileum: The final section, responsible for absorbing vitamin B₁₂, bile salts, and any remaining nutrients. The inner lining of the small intestine has numerous folds, villi, and microvilli that greatly increase its surface area for efficient nutrient absorption.