# Villi: Function, Definition & Structure

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• 0:03 Surface Area of the…
• 1:11 Small Intestine…
• 3:37 Functions of the Villi
• 4:53 Lesson Summary

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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Do you think your small intestine is just a simple tube, transporting your dinner and providing you with nutrients? Read on to learn just how interesting this 'simple tube' can be.

## Surface Area of the Small Intestine

Right now, you have inside of you a long, coiled mass of small intestine. The average person has an astounding 23 feet of small intestine! The fact that the human body has invested that much length and interior space for absorption of nutrients shines a light on how crucial it is for our survival. The simple length, however, does not even come close to describing the amount of surface area the small intestine contains. Let's take a minute, do a little math, and look a little closer at your small intestine.

If we could take out your small intestine, lay it out in a 23-foot straight line, and open up the tube so it lies flat (about 0.22 feet wide), we could determine the average surface area of your small intestine by multiplying the length of 23 feet by the width of 0.22 feet to get the interior surface area of five feet squared. That is, you have five feet squared of intestinal surface area to use for absorbing nutrients from your meals. Now, what would you say if I told you that the actual surface area of the small intestine was closer to 3,200 feet squared? Impossible? Read on.

## How Our Small Intestine Maximizes Absorption

Before we continue, let's try this simple experiment. Take two identical pieces of notebook paper. Lay one down flat, then take the second one and fold it back and forth on itself until you have a fan shape. Set that page next to the flat page. Each piece of paper has the same surface area but the folded page takes up much less space. This is exactly what your small intestine does to fit a huge amount of nutrient-absorbing surface area into such a small starting area.

The surface area of the small intestine is increased in three ways. First of all, the small intestine is not a smooth tube. The inner lining is filled with circular folds that protrude out into the intestinal space (called the lumen), creating a series of peaks and valleys nearly the entire length of the intestine. These circular folds increase the surface area to about 16 feet squared. We still have a long way to go to reach 3,200 feet squared!

The next feature used to increase surface area is called a villus (plural: villi). The villi are small, finger-like projections about a millimeter in length that protrude from the circular folds. They cover the entire surface of the folds. The villi are separated by small crypts, which are small pockets where the cells grow and divide rapidly. This rapid growth and division pushes new cells up to the top of the villi to constantly replace cells that are sloughed off during the movement of food through the intestine. The villi increase the surface area to about 162 feet squared. We still have a long way to go.

The last, and largest, increase in surface area is related to the villi. Each cell on the surface of the villus that is exposed to the lumen is lined with additional, even smaller villi, called microvilli. These structures are tiny. When I say tiny, I mean that in every square inch of intestinal lining, there are about 129 billion microvilli. The cells along the surface of the villi, covered with their microvilli, form the brush border of the small intestine.

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