The Small Intestine & Its Parts

Overview

The small intestine is a long narrow tube located between the stomach and the large intestine. It plays a crucial role in digestion and absorption of nutrients.

Digestion in the Duodenum

• Pancreatic juice enters the duodenum and starts the digestion of proteins.

• Digestion of carbohydrates occurs as pancreatic amylase converts polysaccharides (starches) not digested by salivary amylase into disaccharides.

• Digestion of fats is aided by lipase, which converts fats into fatty acids and glycerol.

• Bile salts emulsify fats, reducing the size of fat globules and increasing their surface area for lipase action.

Role of the Small Intestine in Digestion

• The small intestine completes the digestion of food.

• Digested food, water, and nutrients are absorbed into the bloodstream through the ileum.

Digestive Liquids in the Small Intestine

1. Bile Juice

   • Produced by the liver and stored in the gall bladder.

   • Released into the duodenum via the bile duct.

   • Emulsifies fats, breaking them into tiny droplets to aid lipase digestion (emulsification).

2. Pancreatic Juice

   • Produced by the pancreas and secreted via the pancreatic duct.

   • Contains enzymes and sodium hydrogen carbonate which neutralizes stomach acid.

   • Enzymes include:

     • Amylase: digests starch into maltose.

     • Trypsin: digests proteins into polypeptides.

     • Lipase: digests fats into fatty acids and glycerol.

Absorption of Nutrients in the Small Intestine

• Absorption mainly occurs in the ileum.

• The ileum walls are covered with villi, which are further covered with microvilli to increase surface area.

• Each villus contains blood capillaries and a lacteal (lymph vessel).

• The lacteal absorbs fats and lipids with dissolved vitamins into the lymphatic system.

Mechanisms of Absorption

1. Diffusion

   • Monosaccharides, amino acids, fatty acids, and glycerol diffuse down their concentration gradients into enterocytes from the intestinal lumen.

2. Active Transport

   • Monosaccharides, amino acids, fatty acids, and glycerol are actively transported into the villi.

   • Disaccharides, dipeptides, and tripeptides are actively transported into enterocytes where digestion completes before transfer to capillaries.

   • Monosaccharides and amino acids enter blood capillaries; fatty acids and glycerol enter lacteals.

Key Takeaways

• The small intestine is divided into three parts: duodenum, jejunum, and ileum.

• Digestion in the duodenum is aided by bile and pancreatic juices.

• The ileum is the main site of nutrient absorption, facilitated by villi and microvilli.

• Both diffusion and active transport are mechanisms for nutrient absorption.

• The small intestine is the final and most important site for digestion and absorption in the human body.

Activity

"Digestive Journey Mapping"

• Step 1: Draw a simple diagram of the small intestine showing the duodenum, jejunum, and ileum.

• Step 2: Label where bile juice and pancreatic juice enter.

• Step 3: Use arrows to show the path of carbohydrates, proteins, and fats as they are digested and absorbed.

• Step 4: In pairs, explain to each other how nutrients move from the intestine into the blood or lymph.