Small intestine

Digestive system diagram

The small intestine is a crucial part of the human digestive system. It is responsible for the final stages of digestion and the absorption of most nutrients. Located between the stomach and the large intestine, it measures approximately 20-25 feet in length in adults.

Structure

The small intestine can be divided into three sections:

• Duodenum: This is the shortest part, measuring about 10-12 inches. It is where the partially digested food from the stomach, known as chyme, mixes with digestive enzymes from the pancreas and bile from the liver and gallbladder.
• Jejunum: Measuring around 8 feet, this middle section is where the majority of digestion and absorption takes place.
• Ileum: The longest part, approximately 11-12 feet in length. It absorbs mainly vitamin B12 and bile acids as well as any other remaining nutrients.

Large and small intestine

Each section has a specific lining with tiny hair-like structures called villi. These increase the surface area to maximize nutrient absorption. The cells on the surface of villi, known as enterocytes, have further microscopic projections called microvilli, forming what's known as the "brush border."

Function

The primary function of the small intestine is to digest and absorb food. Digestive enzymes, including lipases, proteases, and carbohydrases, break down fats, proteins, and carbohydrates respectively. These enzymes are either produced by the small intestine itself or by accessory digestive organs like the pancreas.

Absorption in the small intestine happens in two ways:

• Passive Diffusion: Where nutrients move from a higher to a lower concentration.
• Active Transport: Where nutrients are moved against a concentration gradient, requiring energy.

Water and electrolytes are also absorbed in the small intestine, balancing the fluids in the body.

Clinical significance

The small intestine can be affected by various conditions and diseases. Some common ones include:

• Celiac disease: An autoimmune disorder where ingestion of gluten leads to damage in the small intestine.
• Crohn's disease: A type of inflammatory bowel disease that can affect any part of the digestive tract but is often found in the ileum.
• Small Intestinal Bacterial Overgrowth (SIBO): A condition in which abnormally large numbers of bacteria grow in the small intestine.
• Intestinal obstruction: A blockage that prevents food and liquid from passing through.

Symptoms of small intestine disorders often include abdominal pain, diarrhea, weight loss, and nutrient deficiencies.

See Also

• Digestive system
• Stomach
• Large intestine

References

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