Makindo Medical Notes

Anatomy of the Small Intestine

The small intestine is a key part of the digestive system, responsible for the majority of digestion and nutrient absorption. It is a long, coiled tube measuring about 6-7 meters in length and extends from the stomach to the large intestine. It is divided into three main sections: the duodenum, jejunum, and ileum.

Sections of the Small Intestine

• Duodenum: The first 25-30 cm (10-12 inches) of the small intestine. It begins at the pylorus of the stomach and forms a C-shaped curve around the head of the pancreas. The duodenum receives bile from the liver and gallbladder via the common bile duct and pancreatic enzymes from the pancreas via the pancreatic duct, making it a key site for chemical digestion.
• Jejunum: The middle section, approximately 2.5 meters long. The jejunum is responsible for absorbing most of the nutrients. It has thicker walls, a larger diameter, and more villi compared to the ileum, which enhances nutrient absorption.
• Ileum: The final part of the small intestine, measuring around 3-4 meters. It connects to the large intestine at the ileocecal valve. The ileum absorbs vitamin B12, bile salts, and any remaining nutrients not absorbed by the jejunum.

Layers of the Small Intestine

• Mucosa: The innermost layer lined with villi and microvilli to increase surface area for nutrient absorption. The mucosa contains specialized cells like enterocytes (absorptive cells), goblet cells (mucus secretion), and enteroendocrine cells (hormone secretion).
• Submucosa: A layer of connective tissue containing blood vessels, nerves, and lymphatics, which support nutrient absorption and transport.
• Muscularis Externa: Composed of smooth muscle layers responsible for peristalsis and segmentation, which mix and move food along the small intestine.
• Serosa: The outermost layer, a thin membrane that covers and protects the small intestine.

Physiology of the Small Intestine

The small intestine is the primary site for digestion and absorption of nutrients. It plays a critical role in breaking down food, absorbing essential nutrients, and transporting digested material to the large intestine.

Functions of the Small Intestine

• Digestion: The small intestine is where most of the chemical digestion of food occurs. Enzymes from the pancreas (e.g., amylase, lipase, proteases) and bile from the liver break down carbohydrates, proteins, and fats. The duodenum is the primary site of this enzymatic activity.
• Absorption: The villi and microvilli in the small intestine dramatically increase the surface area for absorption.
  • Carbohydrates: Broken down into monosaccharides (glucose, fructose, galactose), absorbed primarily in the jejunum.
  • Proteins: Broken down into amino acids and peptides, absorbed along the small intestine.
  • Fats: Broken down into fatty acids and monoglycerides, which are absorbed into lymphatic vessels called lacteals in the villi.
  • Vitamins and Minerals: Water-soluble vitamins (B, C), fat-soluble vitamins (A, D, E, K), and minerals like iron and calcium are absorbed along different parts of the small intestine.
• Secretion: The small intestine secretes intestinal juices containing water, mucus, and digestive enzymes. Hormones like secretin and cholecystokinin (CCK) regulate digestive activity and bile secretion.
• Immune Function: The small intestine contains lymphoid tissues known as Peyer's patches, which play a role in protecting against pathogens in the gastrointestinal tract.

Peristalsis and Segmentation

The small intestine uses two types of muscular contractions to move and mix food:

• Peristalsis: Rhythmic waves of muscle contraction that move food (chyme) along the intestine.
• Segmentation: Localized contractions that mix chyme with digestive juices and increase contact with the intestinal walls, enhancing nutrient absorption.

Blood Supply

• Arterial Supply: The small intestine is primarily supplied by the superior mesenteric artery, which branches into smaller arteries that supply the duodenum, jejunum, and ileum.
• Venous Drainage: Blood from the small intestine drains into the superior mesenteric vein, which joins with the splenic vein to form the portal vein. The portal vein transports nutrient-rich blood to the liver for further processing.

Nerve Supply

• Sympathetic Innervation: Reduces motility and secretion, while contracting the sphincters in the gastrointestinal tract.
• Parasympathetic Innervation: Increases motility, secretion, and blood flow to enhance digestion and absorption. The vagus nerve is the main parasympathetic nerve supplying the small intestine.

Clinical Relevance

• Malabsorption Syndromes: Conditions like celiac disease, Crohn's disease, and tropical sprue can damage the mucosa, leading to poor absorption of nutrients.
• Small Intestinal Bacterial Overgrowth (SIBO): Overgrowth of bacteria in the small intestine can cause bloating, diarrhoea, and malabsorption.
• Peptic Ulcers: Ulcers can develop in the duodenum due to increased gastric acid exposure, often caused by H. pylori infection or excessive NSAID use.
• Ileus: A condition where normal peristalsis is reduced or absent, leading to obstruction and buildup of contents in the small intestine.
• Intestinal Obstruction: Can be caused by adhesions, hernias, tumours, or other blockages that prevent the passage of food through the small intestine.