Villi: Function, Definition & Structure
Do you think your small intestine is just a simple tube, transporting your dinner and providing you with nutrients? Read on and learn just how interesting a 'simple tube' can be.
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 5 feet squared. That is, you have 5 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 3200 feet squared? Impossible? Read on.
How does our small intestine maximize absorption?
Before we continue, try this simple experiment. Take two identical pieces of notebook paper. Lay one down flat. Now, 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 3200 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.
It is the combination of these structural features that takes our small intestine surface area from 5 feet squared, about the size of half of a ping pong table, to an amazing 3200 feet squared, nearly the size of a basketball court!
Functions of the Villi
We've already stated that the small intestine's main job is to absorb nutrients from the food you eat and that your villi help by increasing the surface area the intestine has for absorption. Once the villi have moved the nutrients into the villi cells, where do the nutrients go? Each villus has its own set of circulatory vessels. Capillaries move blood the entire length of the villus. When the villi absorb nutrients like water soluble vitamins, amino acids, and sugars, they are transported into the capillaries for distribution and use throughout the body.
The villi also contain vessels called lacteals. Lacteals are part of the lymphatic system. Any fats, fat soluble vitamins, and excess liquids absorbed by the villi are first moved into the lymph fluid circulation before being emptied into the blood stream.
The villus structures are also involved in digestion. Brush border enzymes are chemicals secreted into the microvilli-covered cells that aid in digestion. These enzymes are capable of breaking down sugar, protein, and nucleic acids (DNA and RNA) directly on the surface of the microvilli. When the cells on the upper surface of the villus are sloughed off, these enzymes are released directly into the intestinal lumen, aiding in digestion there as well.
Putting It All Back Together
The small intestine has evolved perfectly for efficient absorption of nutrients. It is responsible for about 90% off all nutrient absorption that takes place in your digestive system. Length, aided by circular folds, villi, and microvilli, transform what seems like a simple tube into a complex machine essential for life. Think about that the next time you sit down to dinner.
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