The wonderful digestive system

To understand how what we eat affects our bodies, it is important to understand how the digestive system works. The digestive system is in charge of converting what we eat into the tissues that make us. I don’t know about you, but I think this system is fascinating.

The digestive system consists of a gastrointestinal tract that's basically a tube that goes from the mouth to the anus. This tube has three layers:

• The first layer is just mucus; it protects the internal wall of the tube and works as a lubricant for the food that goes by.
• The second layer is made of epithelial cells; these are in charge or producing the mucus. In the intestine it allows some molecules to go through the wall and reach the bloodstream, everything else ends up in your poop.
• The third layer is called smooth muscle and its function is to move the food through the intestinal tract, it does this by contracting itself. These contractions are unwittingly (we do it without thinking about it) and are the ones that make our belly growl.

Besides the gastrointestinal tract, we also need other organs to do the digestion: tongue, salivary glands, pancreas, liver, and gallbladder.

The food goes through the intestinal tract, where is totally disintegrated, the body absorbs the nutrients and whatever is not used is excreted.

It all starts with smell and sight

Although it's hard to believe, the digestion process starts when we see or smell food, it's a brain process, that's why when we see food ads, we get hungry. This behavior has a lot of logic because in the wild if we saw or smelled something, we would eat it. Our primal behavior doesn't know what an ad about pizza means or that you can't have food that you smell because you don’t have money to pay for it. Food in nature is scarce and if you smell or see it, you will do everything to get it.

Mouth

When food enters the mouth, it's already being digested. To digest our food, we need some molecules called enzymes. If you want to know more about how enzymes work you can check this post What is an enzyme and what it does. There is an enzyme in the saliva called amylase which helps to digest starches. A starch is a complex carbohydrate made of a ton of glucose molecules bonded together. Starches are present in things like rice, potatoes, or bread.

The teeth and the tongue are in charge of grinding and mixing the food inside the mouth, when everything is grinded the tongue pressures the food against the throat to send the food to the esophagus.

The stomach

After passing through the esophagus the food reaches the stomach, where it’s mixed with hydrochloric acid which disintegrates the foods especially proteins. The stomach secretes an enzyme called pepsin to break down proteins into smaller proteins called peptides.

Small intestine

Once the food has been enough time in contact with the enzymes and the acid, then it goes to the small intestine. Here is where most of the nutrient’s absorption happens. The nutrients in the food we eat come as large molecules and we need to chop them up in order to be absorbed by the wall of the intestine. Here is where the other organs I mentioned before come into play.

The liver secrets something called bile, which is then stored in the gallbladder. We all know fats can’t be mixed with water, so to absorb fats we first need to dilute them, bile it’s like soap it breaks down fats and allows them to be mixed with water and be carried through the intestine.

The pancreas produces some of enzyme to break down some nutrients, for example lipase breaks down fats, amylase breaks down starches and protease breaks down proteins (specifically trypsin).

The small intestine also produces some enzymes like sucrase or protease. Sucrase breaks down white sugar into two different molecules: glucose and fructose. Thanks to the enzymes the food we eat is now transformed into smaller molecules like fatty acids, amino acids and simple sugars, these molecules need to reach the bloodstream, but they have to go across the intestine wall. Let’s learn more about the intestine wall to understand how nutrients go across it.

The intestine wall is made up of cells called enterocytes which separate the intestine from the rest of the body. Each enterocyte is close to another one and are glued together thanks to something called tight junctions which are proteins that act kind of like a seam between enterocytes.

Each enterocyte has something called microvilli at the top, these increase the contact surface between the dissolved nutrients and the enterocytes. Imagine you have a spring, when it’s resting its width is 10 centimeters, but if you stretch it, it can be as wide as 50 centimeters or more. It happens the same with the intestine, the microvilli are like the turns of the spring and if we stretch it out, the small intestine would be about 5 meters long (although it can go from 3 to 10 meters). Evolution made it this way to create a huge contact surface in a small space, this way the absorption process is more efficient and effective.

Everything that’s not a nutrient or hasn’t been digested goes to the large intestine. Here is where the undigested food sits for a long time since here is where the water and some minerals are absorbed. It’s also the place where the bacteria feast from everything that we didn’t absorb, and the place where farts are created. As you know beer is created by fermentation and that’s how it gets the gas in it, bacteria in the large intestine do the same, they ferment the food we don’t digest and they excrete gases that we know as farts. Bacteria also generate other molecules that are beneficial to us, some day I’ll talk about that in depth.

After this you know how the story ends, in the bathroom haha.

I hope you find this interesting and that you have learned something new, until next time, bye!