1.5 Small Intestine

The small intestine is the primary site of chemical digestion and nutrient absorption in the digestive system. It is a long, coiled tube connecting the stomach to the large intestine.

Overall Structure and Function

• Location: Situated between the stomach and the large intestine.
• Primary Functions:
  1. Chemical Digestion: To break down chyme (partially digested food from the stomach) into its constituent macromolecules.
  2. Nutrient Absorption: To absorb digested nutrients into the bloodstream.
  3. Waste Solidification: To pass undigested waste to the large intestine.
• Size: The longest part of the gastrointestinal (GI) tract, approximately 6 meters long.
• Divisions: The small intestine is divided into three distinct sections:
  1. Duodenum
  2. Jejunum
  3. Ileum

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Duodenum

The first and shortest section of the small intestine.

• Length: 20-25 cm.
• Shape: A "C"-shaped structure that curves around the head of the pancreas.
• Function: Receives chyme from the stomach, along with digestive secretions from the pancreas and gallbladder, initiating the main phase of chemical digestion.

Secretions into the Duodenum

1. Pancreatic Juice

Secreted by the pancreas.

• pH: Alkaline (approximately 8.0) to neutralize the acidic chyme from the stomach.
• Purpose: To create an optimal pH environment for digestive enzymes in the small intestine.

2. Enzyme Activation

Pancreatic protein-digesting enzymes are secreted as inactive precursors (zymogens) to prevent self-digestion of the pancreas.

• Trypsinogen Activation: Intestinal glands secrete the enzyme enterokinase, which converts inactive trypsinogen into active trypsin. Trypsin can then activate more trypsinogen molecules.

• Chymotrypsinogen Activation: The active enzyme trypsin converts inactive chymotrypsinogen into active chymotrypsin.

3. Bile

• Production and Storage: Produced by the liver and stored in the gallbladder.
• Function: Bile performs emulsification, breaking down large fat globules into numerous small droplets. This dramatically increases the surface area of the lipids, allowing pancreatic lipase to digest them more efficiently.

4. Mucus

• Source: Secreted by duodenal glands, intestinal glands, and goblet cells.
• Function: Protects the intestinal wall from the corrosive effects of acidic chyme and the digestive action of enzymes.

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Jejunum and Ileum

The middle and final sections of the small intestine where digestion is completed and most absorption occurs.

• Length: Jejunum is about 2.5 m; Ileum is about 3.5 m.
• Function: Final digestion of carbohydrates, proteins, and fats, followed by absorption of nutrients. The digested mixture is converted into a watery emulsion called chyle.

Intestinal Enzymes

The lining of the jejunum and ileum secretes its own set of enzymes to complete digestion. These are often referred to as "Brush Border Enzymes".

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Summary

• The small intestine is divided into the duodenum, jejunum, and ileum and is the primary site for chemical digestion and nutrient absorption.
• The duodenum receives acidic chyme and neutralizes it with alkaline pancreatic juice. It also receives bile for fat emulsification and pancreatic enzymes for digestion.
• Pancreatic enzymes like amylase (digests carbohydrates), lipase (digests fats), and the precursors trypsinogen and chymotrypsinogen (digest proteins) are critical.
• The jejunum and ileum complete the digestive process using intestinal enzymes that break down dipeptides and disaccharides.
• The final products of digestion are small, absorbable molecules:

Biological Significance: The highly efficient, multi-stage digestive process in the small intestine ensures that complex food matter is broken down into simple molecules that can be absorbed into the bloodstream and used by the body's cells for energy, growth, and repair.