Chapter 3: Small Intestine, Digestion, and Nutrient Absorption

The small intestine plays a significant role in the digestion of macronutrients, along with the aid of three additional digestive organs: the liver, gallbladder, and pancreas. Its anatomical features can be classified into microscopic and macroscopic anatomy, with the macroscopic anatomy focusing on gross anatomical features, namely the duodenum, jejunum, ileum, and mesentery. The microscopic anatomy of the small intestine includes different layers of the intestinal wall, villi, and major cell types of the intestinal epithelium. The wall of the small intestine has four layers: serosa, muscularis propria, submucosa, and mucosa. The mucosa provides an enormous surface area for the absorption of nutrients and water, while the submucosa supports the mucosa in its specific function of electrolyte, nutrient, and fluid absorption. Intestinal motility is a crucial process that ensures the thorough mixing of intestinal secretions, pancreatic and liver secretions, and the gastric chyme entering the small intestine. The small intestine is the key location for the absorption of digested macronutrients, micronutrients, water, and electrolytes. The anatomy and physiology of the small intestine are thoroughly covered in this chapter. The digestion and absorption of several nutrients, including carbohydrates, proteins, fats, minerals, and vitamins, are also given special attention.