Small Intenstine

The small intestine is a long, coiled, tubular organ that forms a crucial part of the human digestive system. It is the primary site for digestion and absorption of nutrients from food. Extending from the stomach to the large intestine, the small intestine ensures that the digested food (chyme) is broken down into simple molecules that can be absorbed into the bloodstream to nourish the body. Despite its name, the small intestine is much longer than the large intestine, but it is called “small” because of its smaller diameter.

Structure and Location

The small intestine is located in the abdominal cavity, surrounded by the large intestine. It measures approximately 6 to 7 metres (20–23 feet) in length and about 2.5–3 cm in diameter. It is suspended from the posterior abdominal wall by a membranous fold known as the mesentery, which contains blood vessels, lymphatics, and nerves supplying the intestine.
The small intestine is divided into three main parts:

1. Duodenum
   • The first and shortest part, about 25 cm (10 inches) long.
   • It connects the stomach to the jejunum.
   • Receives bile from the liver and pancreatic juice from the pancreas through the common bile duct and pancreatic duct.
   • Major site for the chemical breakdown of food (especially fats, proteins, and carbohydrates).
2. Jejunum
   • The middle portion, approximately 2.5 metres long.
   • Has numerous folds and villi that increase the surface area for nutrient absorption.
   • Most of the digestion and absorption of nutrients occur here.
3. Ileum
   • The final and longest section, about 3.5 metres long.
   • It connects to the large intestine at the ileocaecal valve, which regulates the passage of digested material.
   • Responsible for the absorption of vitamin B₁₂, bile salts, and any remaining nutrients.

Histological Structure (Wall Layers)

The wall of the small intestine consists of four distinct layers, arranged from inner to outer:

1. Mucosa:
   • The innermost layer that secretes digestive enzymes and mucus.
   • Contains villi and microvilli, finger-like projections that greatly enhance the absorptive surface area (about 200 square metres).
   • Composed of epithelial cells, goblet cells (for mucus secretion), and intestinal glands (crypts of Lieberkühn).
2. Submucosa:
   • Made up of connective tissue containing blood vessels, lymphatic vessels, and nerves.
   • In the duodenum, it contains Brunner’s glands, which secrete alkaline mucus to neutralise stomach acid.
3. Muscularis externa:
   • Composed of two layers of smooth muscle:
     • Inner circular layer
     • Outer longitudinal layer
   • Responsible for peristaltic movements that propel the food forward and aid mixing with digestive juices.
4. Serosa:
   • The outermost thin membrane (visceral peritoneum) that reduces friction between the intestine and surrounding organs.

Digestive Secretions and Enzymes

The small intestine receives secretions from its own glands as well as from associated organs such as the liver and pancreas.

• Bile (from Liver and Gallbladder):
  • Contains bile salts that emulsify fats into tiny droplets, increasing the efficiency of enzymatic digestion.
• Pancreatic Juice (from Pancreas):
  • Contains enzymes such as trypsin, amylase, and lipase that break down proteins, carbohydrates, and fats.
• Intestinal Juice (Succus Entericus):
  • Secreted by intestinal glands; contains enzymes like maltase, lactase, sucrase, peptidase, and nucleotidase, which complete the final stages of digestion.

Functions of the Small Intestine

The small intestine performs several vital physiological functions essential for survival and health.
1. Digestion:

• Enzymatic breakdown of macromolecules into absorbable forms:
  • Carbohydrates → Simple sugars (glucose, fructose)
  • Proteins → Amino acids
  • Fats → Fatty acids and glycerol
  • Nucleic acids → Nucleotides

2. Absorption:

• The principal site for nutrient absorption into the bloodstream or lymphatic system.
• Mechanisms include:
  • Simple diffusion – for small, fat-soluble molecules.
  • Facilitated diffusion – for specific nutrients such as fructose.
  • Active transport – for glucose, amino acids, and ions.
  • Endocytosis – for larger molecules like immunoglobulins in infants.
• Absorbed nutrients enter blood capillaries (for sugars and amino acids) or lacteals (for fats).

3. Secretion:

• Produces intestinal juice containing enzymes and mucus for digestion and lubrication.

4. Transport:

• Moves chyme gradually from duodenum to ileum through peristalsis and segmentation.

5. Immunological Defence:

• The intestinal wall contains Peyer’s patches (aggregations of lymphoid tissue in the ileum) that protect against pathogens and support gut immunity.

Adaptations for Absorption

The small intestine is highly adapted for efficient absorption of nutrients through several structural modifications:

• Length: Provides a large surface area for digestion and absorption.
• Villi and Microvilli: Greatly increase surface area (by about 600 times).
• Rich Blood Supply: Facilitates rapid transport of absorbed nutrients.
• Thin Epithelium: Allows easy diffusion of digested molecules.
• Peristaltic Movements: Ensure constant mixing and movement of food.

Common Disorders of the Small Intestine

1. Celiac Disease:
   • An autoimmune disorder triggered by gluten intake, leading to damage of the intestinal villi and poor nutrient absorption.
2. Crohn’s Disease:
   • A type of inflammatory bowel disease affecting any part of the gastrointestinal tract, often involving the ileum.
3. Malabsorption Syndrome:
   • Inability to absorb nutrients properly, caused by enzyme deficiency or infection.
4. Intestinal Obstruction:
   • Blockage of the small intestine due to adhesions, hernia, or tumour.
5. Peptic Ulcer (Duodenal Ulcer):
   • Erosion of the duodenal lining due to excessive acid or bacterial infection (Helicobacter pylori).
6. Diarrhoea and Gastroenteritis:
   • Caused by bacterial or viral infections leading to inflammation and rapid transit of intestinal contents.

Importance in Human Health

The small intestine is vital for maintaining nutritional balance and metabolic health. Its efficient function ensures that the body receives essential nutrients required for energy, growth, repair, and immune function. Any impairment in its activity can result in nutrient deficiencies, digestive disorders, or metabolic imbalances.