Lesson 3 - Digestion in the Small Intestine

Question 1

What Happens in the Small Intestine?

Answer 1

Food that exits the stomach must pass through the pyloric sphincter. This sphincter periodically relaxes to release small portions of
chyme into the small intestine

This steady release prevents overloading and allows time for thorough digestion

The small intestine is a long tube about 2.5
cm in diameter but up to 7 m long.

Most digestion and absorption of nutrients
takes place in the small intestine (chemical
digestion of lipids, carbohydrates and any
remaining proteins, mechanical digestion via
peristalsis)

The inner layer of the small intestine is folded
into ridges which are covered in villi.

Villi are small finger-like projections that
maximum surface area for efficient nutrient
absorption

Each epithelial cells in the villi has even
smaller, microscopic projections of the cell
membrane called microvilli to further
increase the surface area available for
absorption

Within each villus is a network of tiny
blood vessels called capillaries.

All nutrients, except digested fats, enter the bloodstream through the capillaries.

Fats are too big for capillaries! Instead, they move through the lymphatic system,
and from there into the bloodstream

The small intestine has three (3) sections:

The Duodenum
- The first 25-30 cm
- Most enzymes are added
- Digestion in the small intestine begins

The Jejunum
- Digestion continues
- Some nutrients are absorbed

The Ileum
- The majority of nutrients are absorbed

Question 2

How does the Small Intestine Digest

Answer 2

When chyme (pH 2.5) enters the small intestine, it mixes with prosecretin (inactive, made in small intestine)

This activates it and produces Secretin
(hormone)

Secretin:
- Prevents more food from entering the duodenum until the current batch is digested
- Stimulates the liver to make bile
- Stimulates the pancreas to secrete lipid and protein enzymes
- Stimulate the pancreas to release bicarbonate ions (HCO3-), raising the pH of chyme from 2.5 to 9, which deactivates pepsin and protects the small intestine from stomach acids

In the Duodenum:
- When chyme enters the small intestine, specialized cells in the duodenum release a hormone called cholecystokinin (CCK) into the bloodstream.
- This hormone signals the pancreas to secrete a variety of substances through the pancreatic duct back into the duodenum
- CCK also signals the stomach to slow down digestion

Question 3

Lipid Digestion in the Small Intestine

Answer 3

The liver produces bile which is stored in the gall bladder. The presence of fats in the duodenum triggers the release of bile through the bile duct.

Bile emulsifies fats (aka: breaking them into tiny droplets). This allows lipase (enzyme, secreted by the pancreas) to break lipid chains into individual fatty acid molecules

Question 4

Protein and Carb Digestion in the Small Intestine

Answer 4

Now that the pH in the intestine is no longer
acidic, the pancreas can secrete pancreatic
amylase into the duodenum, resuming
carbohydrate digestion

The pancreas releases trypsinogen (inactive).
In the duodenum the enzyme enterokinase
converts it into active trypsin (active enzyme)
which breaks down proteins into smaller
pieces.