The majority of digestion and absorption of nutrients takes place in the small intestine. In this lesson, you will learn about unique modifications within the walls of the small intestine, such as microvilli, villi, and circular folds. These structures increase absorption of nutrients.
In order to get the nutritional benefit from the foods you eat, the food must first be broken down into its most basic nutrient elements. These tiny nutrients are then absorbed out of the digestive tract and into your bloodstream. Your small intestine is the section of your digestive tract where the majority of nutrient absorption takes place. In this lesson, you will learn about the unique modifications within the wall of the small intestine that allow for maximum absorption.
We previously learned that the small intestine is about 20 feet long. So, right off the bat, we see that the mere fact that this organ is so long provides more area for nutrients to be absorbed. Of course, if the food, which by this time is in the semi-liquid form that we call chyme, simply flowed through your small intestine like water flowing through a hose, there would not be much time for nutrient absorption.
Instead of chyme flowing calmly through the small intestine, we remember that food is constantly tossed and sloshed around while it travels the 20 or so feet of intestine. This sloshing around is due to two movements that we previously learned about. The first movement was segmentation, and it was the rhythmic contraction of the circular muscles of the wall of the small intestine - almost like the wall is being pinched. The second movement was pendular movements, which were caused by the rhythmic contraction of the longitudinal muscles - almost like a Slinky being stretched and contracted.
As these different muscles contract and relax, your small intestine goes through a slew of gyrations which mix the chyme with the digestive juices and bring particles of food into contact with the small intestine wall. Within the wall, we see a number of special modifications that allow for maximum nutrient absorption.
Microvilli are hairs that aid in absorption of nutrients.
Nearly all of the nutrients you take in through your mouth are absorbed through the wall of the small intestine. This is no surprise when you take a look at the surface of the small intestine wall and notice all of the special modifications that are designed to increase nutrient absorption. We had previously learned that the mucosa is the lining of the digestive tract, and it consists of simple columnar epithelial cells.
In the small intestine, these cells contain microvilli, which are tiny hair-like projections that increase nutrient absorption. These projections increase the surface area of the small intestine allowing more area for nutrients to be absorbed.
Microvilli are microscopic and can only be seen with a very powerful microscope, but they give the cell surface a fuzzy appearance because there are so many of them. This fuzzy appearance leads some anatomists to refer to microvilli as a brush border because the structure looks like the bristles of a paintbrush. So, we see that the brush border can be defined as the microvilli-covered surface of the simple columnar epithelium cells of the small intestine.
Villi and Lacteals
Microvilli are not the only adaptation we see within the wall of the small intestine that helps boost absorption. Another structural adaptation we see in the small intestine is villi. Villi are small, finger-like projections that protrude from the mucosal lining and increase nutrient absorption. Each villus has many microvilli. So, it's almost like the villi are fingers protruding from the small intestine wall, and microvilli are hairs on those fingers. Both work to increase surface area so more nutrients can be absorbed.
Lacteals absorb dietary fats.
Within each villus, you will find plentiful capillary beds, along with lacteals. Lacteals are lymphatic capillaries that absorb dietary fats. We previously learned how bile, along with digestive enzymes, break down fats. Lacteals represent the unique way fats are absorbed. For example, other nutrients take a fairly direct route by passing through the small intestine wall and going into the blood capillary beds. In the case of digested fats, we see them being recombined in the intestinal lining and then absorbed into the lacteals. The fluid content in the lacteals then passes into larger lymphatic vessels before making its way back to your bloodstream.
In addition to microvilli and villi, there is a third adaptation to the small intestine wall that helps increase absorption, called circular folds. Circular folds are also referred to as plicae circulars, and we see that the word plicae is Latin for 'fold.' So, either term gives us an idea of what these adaptations might look like.
Circular folds in the wall of the intestine increase absorption.
The circular folds are permanent, deep folds in the wall of the small intestine that increase absorption. When chyme passes by the circular folds, it causes the chyme to spiral and makes the flow turbulent, bringing it in contact with the walls, where it can be absorbed more efficiently.
Let's review. The surface of the small intestine wall has a number of special modifications that are designed to increase nutrient absorption. The simple columnar epithelial cells that make up the mucosa contain one of these modifications, called microvilli. Microvilli are tiny, hair-like projections that increase nutrient absorption. This microvilli-covered surface makes the cell look fuzzy, and this is referred to as the brush border.
These microvilli can be found on top of villi. Villi are small, finger-like projections that protrude from the mucosal lining and increase nutrient absorption. Within each villus, you will find plentiful capillary beds along with lacteals. Lacteals are lymphatic capillaries that absorb dietary fats.
In addition to the microvilli and villi, there's a third adaptation in the small intestine wall that helps increase absorption, called circular folds. Circular folds are permanent, deep folds in the wall of the small intestine that increase absorption.
Following this lesson, you'll be able to describe the structure and function of three adaptations in the wall of the small intestine that allow for maximum nutrient absorption: villi, microvilli, and circular folds.