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# 13.39: Small Intestine

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These projections absorb. Absorb what?

Imagine the inside walls of the 23 feet of your small intestine covered with these finger-like projections. Why? What's their purpose, and why is the small intestine so long? These projections absorb. Absorb what? Minerals and nutrients from food. And the length of the small intestine allows as much of these important substances to be absorbed as possible.

### Digestion and Absorption: The Small Intestine

The small intestine is a narrow tube about 7 meters (23 feet) long in adults. It is the site of most chemical digestion and virtually all absorption. The small intestine consists of three parts: the duodenum, jejunum, and ileum (see Figure above ).

#### Digestion in the Small Intestine

The duodenum is the first and shortest part of the small intestine. Most chemical digestion takes place here, and many digestive enzymes are active in the duodenum (see Table below ). Some are produced by the duodenum itself. Others are produced by the pancreas and secreted into the duodenum.

Enzyme What It Digests Where It Is Made
Amylase carbohydrates pancreas
Trypsin proteins pancreas
Lipase lipids pancreas, duodenum
Maltase carbohydrates duodenum
Peptidase proteins duodenum

The liver is an organ of both digestion and excretion. It produces a fluid called bile , which is secreted into the duodenum. Some bile also goes to the gall bladder , a sac-like organ that stores and concentrates bile and then secretes it into the small intestine. In the duodenum, bile breaks up large globules of lipids into smaller globules that are easier for enzymes to break down. Bile also reduces the acidity of food entering from the highly acidic stomach. This is important because digestive enzymes that work in the duodenum need a neutral environment. The pancreas contributes to the neutral environment by secreting bicarbonate, a basic substance that neutralizes acid.

#### Absorption in the Small Intestine

The jejunum is the second part of the small intestine, where most nutrients are absorbed into the blood. As shown in Figure below , the mucous membrane lining the jejunum is covered with millions of microscopic, fingerlike projections called villi (singular, villus). Villi contain many capillaries, and nutrients pass from the villi into the bloodstream through the capillaries. Because there are so many villi, they greatly increase the surface area for absorption. In fact, they make the inner surface of the small intestine as large as a tennis court!

This image shows intestinal villi greatly magnified. They are actually microscopic.

You can watch an animation of absorption across intestinal villi at the link below. http://www.youtube.com/watch?v=P1sDOJM65Bc&feature=related

The ileum is the third part of the small intestine. A few remaining nutrients are absorbed here. Like the jejunum, the inner surface of the ileum is covered with villi that increase the surface area for absorption.

### Summary

• Virtually all absorption of nutrients takes place in the small intestine, which has a very large inner surface area because it is covered with millions of microscopic villi.

### Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology $\rightarrow$ Biology for AP* $\rightarrow$ Search: The Human Digestive System
1. What is the role of the small intestine?
2. Describe the structure of the small intestine wall.
3. Describe how fats, proteins, and carbohydrates are digested.
4. List the enzymes involved in breaking down organic compounds.

### Review

1. Where are most nutrients absorbed?

2. Describe the functions of the three parts of the small intestine.

3. What role do villi play in absorption?

### Vocabulary Language: English Spanish

bile

bile

Fluid produced by the liver and stored in the gall bladder that is secreted into the small intestine to help digest lipids and neutralize acid from the stomach.
duodenum

duodenum

The first part of the small intestine; site of most chemical digestion.

Sac-like organ that stores bile from the liver and secretes it into the duodenum of the small intestine.
ileum

ileum

The third part of the small intestine; site of absorption of remaining nutrient.
jejunum

jejunum

The second part of the small intestine; site where most nutrients are absorbed into the blood.
liver

liver

Organ of digestion and excretion that secretes bile for lipid digestion and breaks down excess amino acids and toxins in the blood.
small intestine

small intestine

Long, narrow, tube-like organ of the digestive system where most chemical digestion of food and virtually all absorption of nutrients take place.
villi

villi

(singular, villus): Microscopic, finger-like projections in the mucous membrane lining the small intestine that form a large surface area for the absorption of nutrients.

Feb 24, 2012

Oct 03, 2014