Mini Lesson: Small Intestine

Video Transcript

Completing Digestion

When it comes to digesting foods and absorbing nutrients, the small intestine is really where it all comes together. Before food arrives in the small intestine, it has gone through some chemical and mechanical digestion in the mouth and the stomach, but most of the molecules are still too large to be taken in and used by the body.

Help from Accessory Organs

The small intestine is divided into three parts that all work just a bit differently.  When food enters the first part of the small intestine, signals are sent to the gallbladder and pancreas to let them know that it’s time for them to help. These organs are not technically part of the digestive tract, but they make some very important chemicals that help to complete the digestive process. They send these chemicals to the first part of the small intestine where they are mixed with the pieces of food and they help to break apart the fats, proteins, and carbohydrates into even smaller pieces.

Fats are broken into smaller chunks by the bile from the liver and gallbladder, then the enzymes from the pancreas finish the job by making them small enough to be taken into the body. Different enzymes from the pancreas also help to break down proteins and carbohydrates, but the cells of the small intestine make the enzymes that finally get these molecules small enough to be absorbed.

Nutrient Absorption

Absorbing nutrients is one of the small intestine’s most important jobs, so it has this amazing design that helps it to take in as many nutrients as possible. The cells lining the inside of the tube that is the small intestine take nutrients from inside the tube and pass it to the vessels on the other side of the cell, so the nutrients can be delivered to all parts of the body and get used. This means that the more cells there are, with more surface area available to grab onto nutrients, the more nutrients you can absorb. So, to fit more absorbing cells in, these cells join together side-by-side creating these finger like shapes the stick out into the hollow part of the tube. On top of that, each cell has what looks like little hairs that poke out into that hollow space. With the extra cells making little fingers and added surface area of the hairs, the combined surface area of the small intestine is pretty close to the size of a tennis court with a lot of absorption potential.

Role of the Intestinal Barrier

But another important job for this layer of cells is to keep out all the molecules that do not belong in the body. This can be kind of tricky with only one layer of cells, so there are a lot of important tools the cells use to keep this barrier strong. Tight junctions are special molecules on the sides of the cells that hold the cells together and keep molecules from sneaking between the cells. There are also some of those cells that make mucus to create a layer that divides the cells from the food in the hollow space, acting like bouncers that only let the good nutrients through.

So, there is a lot happening in the small intestine. All of these steps have to happen with the right timing for the food to be broken down enough so that the cells can take in all the nutrients. Food that is not broken down all the way or other problems that stop the cells from taking in nutrients properly can leave the body without enough nutrients to keep the body working the way that it should. 

Consequences of Inadequate Digestion

If large chunks of food are left to move through the small intestine, this can feed bacteria, which means they start to grow and take over in the small intestine where they really shouldn't be. These bacteria can not only keep us from absorbing the nutrients that our bodies need, but when the bacteria eat these foods, they make gas that can be painful, make us feel bloated, and may even cause diarrhea or constipation.

Another common problem is when the cells are not able to keep that barrier strong and large molecules and toxins leak through the weak areas and into the body. The body is not designed to deal with these types of molecules, so they can trigger reactions and can even trick the immune system into attacking the body itself. If this keeps going, that is when individuals start to develop the symptoms of autoimmune disease. This can cause damage all throughout the body, but some damage is targeted to the digestive tract itself, such as in the case of Celiac or Crohn’s disease.