Small intestine
Digestion in the small and
large intestines
9.5
The structure of the small intestine
• Most digestion and
absorption of nutrients
takes place in the small
intestine.
• Only about 2.5 cm in
diameter.7m in length.
• Large intestine: 7.6cm
in diameter but 1.5 m
in length.
• Small intestine is made
up three sections:
• duodenum., the
jejunum, and the
ileum.
• carbohydrates, proteins and lipids are
digested in the small intestine with the
help of hormones and enzymes.
• Takes about 3h to 5h to digest a meal in
the small intestine.
The structure of the small intestine
• Inner surface of the small intestine gives the maximum surface area for
efficient nutrient absorption.
• Villi - finger like projections of the inner layer folded into ridges
• Microvillus - microscopic projection of the cell membrane; increases the surface
area of the cell
• Lacteal - a lymphatic vessel within a villus, through which digested fats enter
the circulatory system
• Duodenum - where most
enzymes are added; start
of digestion
• Jejunum - digestion
continues and some
nutrients are absorbed
• Ileum - most nutrients are
absorbed; the last section
of small intestine.
Chemical digestion in the small
intestine
The role of Pancreas in Digestion
• The pyloric sphincter controls the passage of food
from the stomach into the small intestine.
• When the food in the stomach is mixed properly with
gastric juice and the proteins are partially digested,
sphincter will relax and chyme will enter duodenum.
• This process is slow and steady -> prevents overloading
and allows time for thorough digestion.
• Pancreas is in between the stomach and the duodenum.
• Pancreas has two roles: 1. secrete enzymes for digestion,
2. secretes hormones that regulate the absorption and
storage of glucose form the blood.
• The enzyme amylase (in saliva) is also secreted by the
pancreas.
• Digestion of carbohydrates form the mouth continues to
duodenum.
Digestion in Duodenum
Proteins to amino acids
• Acidic chyme enters the small intestine, a
chemical called prosecretin is converted to
its active form, secretin - secreted by the
duodenum that stimulates pancreatic and
bile secretions (enzymes for digestion)
• Primary function: release bile to neutralize
the acidic chyme and raise the pH. Secretin
protects the small intestine from stomach
acids.