1.5 Small Intestine

The small intestine is the longest part of the gastrointestinal tract and the primary site of digestion and absorption of nutrients. It receives chyme from the stomach, pancreatic juice and bile from accessory organs, and completes the digestion of carbohydrates, proteins, and fats.

Definition

The small intestine is a long, coiled tubular organ of the digestive system that connects the stomach to the large intestine. It is about 6 meters long and consists of three parts: the duodenum, the jejunum, and the ileum. It is the primary site of digestion and absorption of nutrients.

Main Idea

• Chyme from the stomach
• Pancreatic juice from the pancreas
• Bile from the liver/gallbladder

• Duodenum — receives pancreatic juice and bile; major site of chemical digestion
• Jejunum — completes digestion of proteins, carbohydrates, and fats
• Ileum — major site of nutrient absorption

Core Concept

• Duodenum = first part; receives pancreatic juice and bile; chemical digestion
• Jejunum = middle part; completes digestion
• Ileum = last part; major site of absorption
• Pancreatic juice = contains enzymes for all major nutrients
• Bile = emulsifies fats
• Brush border enzymes = complete digestion of carbohydrates and proteins
• Chyle = watery emulsion formed after digestion

Detailed Explanation

Structure of the Small Intestine

The small intestine is about 6 meters long and consists of three parts:

Duodenum

• The first part of the small intestine.
• Ranging from 20–25 cm (8–10 inches) in length.
• Extends from the pyloric sphincter of the stomach.
• Short structure shaped like a "C".
• Surrounds the head of the pancreas.

• The secretion of the pancreas is called pancreatic juice.
• It is poured through the pancreatic duct.
• Pancreatic juice is slightly alkaline (pH about 8).
• It neutralizes the acidic chyme from the stomach.

Enzymes in Pancreatic Juice:

• Starch-digesting enzyme.
• Hydrolyses polysaccharides to maltose and even to glucose.

• Principal enzyme for the hydrolysis of fats.
• Hydrolyses fats to:
• Mono- and diglycerides
• Glycerol
• Fatty acids

• Two important enzyme precursors are found in pancreatic juice: trypsinogen and chymotrypsinogen.
• Both are inactive forms.

• The intestinal glands secrete an activator enzyme called enterokinase.
• Enterokinase converts trypsinogen into trypsin.
• Trypsin then activates more trypsinogen.
• Trypsin is the active form, which acts on proteins and converts them into polypeptides.

Chymotrypsinogen: - The inactive chymotrypsinogen is converted to active form chymotrypsin by trypsin.

• Bile is manufactured in the liver but stored in the gall bladder.
• Bile emulsifies fat, causing them to break down into numerous small droplets called emulsion.
• Emulsification provides a relatively large surface area of lipid for the action of lipase enzyme.
• This speeds up the digestion of fats and oils.

Jejunum and Ileum

• About 2.5 m long.
• Completes the digestion of proteins, carbohydrates, and fats.

• About 3.5 m long.
• Major site of nutrient absorption.

Enzymes Secreted by the Lining of Jejunum and Ileum:

Pancreatic Lipase: - Completes the digestion of fats into fatty acids and glycerol.

Chyle: - By the action of enzymes, chyme is turned into a watery emulsion called chyle.

Absorption of Digestion Products

The ileum is the major site of nutrient absorption.

• Tiny finger-like projections of the mucosa.
• 0.5–1.5 mm in length.
• Each villus is covered by simple columnar epithelium.
• Contains a blood capillary network and a lymph capillary called a lacteal.

• Folds in the internal walls
• Villi — finger-like projections
• Microvilli — tiny projections on the surface of epithelial cells

• Absorption occurs by a combination of simple diffusion and active transport.
• Monosaccharides are transferred by facilitated diffusion to the capillaries of intestinal villi.
• They are carried by the hepatic portal system to the liver.
• In the liver, non-glucose sugars are converted to glucose.
• Glucose enters the cell through facilitated diffusion.

• Lipids are digested into fatty acids and glycerol.
• After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells.
• The fats are then mixed with cholesterol and proteins, forming small globules called chylomicrons.
• Most chylomicrons are transported by exocytosis out of epithelial cells into lacteals.
• Lymph containing chylomicrons eventually drains from the lymphatic system into large veins that return blood to the heart.

• Individual amino acids are absorbed in epithelial cells of villi.
• They enter the hepatic portal system, which transports them to the liver.
• The amino acids may be modified in the liver or released into the bloodstream.
• Most amino acids are used as building blocks to form new proteins.
• Some amino acids may be used for energy.

Intermediate Level

Three Regions of the Small Intestine

Pancreatic Juice

Bile

Note: Bile contains no digestive enzymes. It emulsifies fats, which increases surface area for lipase action.

Brush Border Enzymes

Absorption

Structural Adaptations for Absorption

• Length — 6 meters provides long time for digestion and absorption.
• Folds — plicae circulares increase surface area.
• Villi — finger-like projections increase surface area.
• Microvilli — tiny projections on epithelial cells increase surface area.
• Rich blood supply — capillaries in villi absorb nutrients.
• Lacteals — lymph capillaries absorb fats.
• Thin epithelium — single layer of cells for rapid diffusion.
• Mitochondria — provide ATP for active transport.

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Small intestine is about 6 meters long.
• 2. Three parts: duodenum, jejunum, ileum.
• 3. Duodenum receives pancreatic juice and bile.
• 4. Pancreatic juice is alkaline (pH 8) and neutralizes acidic chyme.
• 5. Pancreatic juice contains enzymes for all major nutrients.
• 6. Bile emulsifies fats but contains no digestive enzymes.
• 7. Trypsinogen is activated to trypsin by enterokinase.
• 8. Chymotrypsinogen is activated to chymotrypsin by trypsin.
• 9. Jejunum and ileum secrete brush border enzymes.
• 10. Ileum is the major site of nutrient absorption.
• 11. Villi and microvilli increase surface area for absorption.
• 12. Carbohydrates and proteins are absorbed into blood.
• 13. Fats are absorbed into lymph (lacteals).
• 14. Chyle is the watery emulsion formed after digestion.
• 15. Chylomicrons transport fats from epithelial cells to lacteals.

Remember

• - Small intestine = 6 m long; 3 parts
• - Duodenum = 20–25 cm; receives pancreatic juice and bile
• - Jejunum = 2.5 m; completes digestion
• - Ileum = 3.5 m; major site of absorption
• - Pancreatic juice = alkaline (pH 8); contains amylase, lipase, trypsinogen, chymotrypsinogen
• - Bile = emulsifies fats; no enzymes
• - Enterokinase = activates trypsinogen to trypsin
• - Trypsin = activates more trypsinogen and chymotrypsinogen
• - Brush border enzymes = maltase, sucrase, lactase, aminopeptidase, erepsin
• - Villi = finger-like projections; contain blood capillaries and lacteals
• - Microvilli = tiny projections on epithelial cells
• - Chyle = watery emulsion
• - Chylomicrons = transport fats to lacteals

Common Mistakes

• 1. Thinking bile contains digestive enzymes — Bile contains no enzymes; it emulsifies fats.
• 2. Confusing trypsinogen and trypsin — Trypsinogen is inactive; trypsin is active.
• 3. Mixing up enterokinase and trypsin — Enterokinase activates trypsinogen; trypsin activates more trypsinogen and chymotrypsinogen.
• 4. Believing most absorption occurs in the stomach — Most absorption occurs in the small intestine.
• 5. Thinking fats are absorbed into blood — Fats are absorbed into lymph (lacteals).
• 6. Confusing chyle with chyme — Chyme is in the stomach; chyle is in the small intestine.
• 7. Forgetting the role of bile salts — Bile salts emulsify fats; bile pigments are waste products.
• 8. Mixing up the three parts of the small intestine — Duodenum (first), jejunum (middle), ileum (last).
• 9. Thinking pancreatic juice is acidic — Pancreatic juice is alkaline (pH 8).
• 10. Confusing villi and microvilli — Villi are finger-like projections; microvilli are tiny projections on epithelial cells.

Quick Revision

The small intestine is about 6 meters long and consists of three parts: duodenum, jejunum, and ileum. The duodenum receives pancreatic juice (alkaline, pH 8) and bile. Pancreatic juice contains amylase, lipase, trypsinogen, and chymotrypsinogen. Bile emulsifies fats but contains no enzymes. Trypsinogen is activated to trypsin by enterokinase; trypsin activates more trypsinogen and chymotrypsinogen. The jejunum and ileum secrete brush border enzymes (maltase, sucrase, lactase, aminopeptidase, erepsin). The ileum is the major site of absorption. Villi and microvilli increase surface area. Carbohydrates and proteins are absorbed into blood; fats are absorbed into lymph (lacteals) as chylomicrons. Chyle is the watery emulsion formed after digestion.

60-Second Revision

• Small intestine = 6 m; duodenum, jejunum, ileum
• Duodenum = receives pancreatic juice and bile
• Pancreatic juice = pH 8; amylase, lipase, trypsinogen, chymotrypsinogen
• Bile = emulsifies fats; no enzymes
• Enterokinase = activates trypsinogen to trypsin
• Trypsin = activates more trypsinogen and chymotrypsinogen
• Brush border enzymes = maltase, sucrase, lactase, aminopeptidase, erepsin
• Ileum = major site of absorption
• Villi = finger-like; contain capillaries and lacteals
• Fats → chylomicrons → lacteals → lymph
• Warning: Bile has no enzymes; it only emulsifies fats

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• Chapter:
• 1. Digestive System of Man

• Topic:
• 1.6 ABSORPTION OF DIGESTION PRODUCTS

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