Overview of Digestion

We digest our food using our gastrointestinal tract, also called the GI tract. Food moves from the mouth to the esophagus, through the lower esophageal sphincter, to the stomach, through the pyloric sphincter, to the small intestine, and then to the large intestine. Accessory organs include the liver, gall bladder, and pancreas.

What happens in your mouth? A few things. These include mechanical digestion as you physically chew your foods, chemical digestion using lingual amylase (also called salivary amylase) lingual lipase, and lubrication of our food to enable easy swallowing. Although our nose does the main job of detecting flavors, we can also detect flavors from the food aromas that travel from the back of our mouth up into the nasal cavity. Remember that we said we taste with our tongue and detect flavors with our nose. The instrument for tasting is our taste buds. These “buddies” of ours detect certain chemicals on our tongue and send a signal to our brain to tell us what we are tasting. The five tastes include: salty, sour, umami, bitter, and sweet.

Once we are done chewing, we swallow. We did not talk about this but it is important to note that as we swallow, a valve called the epiglottis covers the entrance to our windpipe so that food or drinks do not get into our respiratory system. That’s why we can’t breathe and swallow at the same time.
After swallowing, the food moves quickly towards the stomach by powerful muscles that relax and contract in the esophagus, allowing food to get to the stomach in seconds. Just before going into the stomach there is a valve or you may say gate, called the lower esophageal sphincter. The health of this gate is important. People suffering from disfunction of the LES will experience acid reflux, that is, food moving back up into the esophagus. If you have ever experienced this, you know the burning sensation that it causes – not surprising given the acidic nature of the stomach.

The stomach is a powerful muscle designed to churn the food we eat into a soupy mass called chyme. Digestive juices including hydrochloric acid (HCl), gastric lipase, pepsin (a protease), and intrinsic factor (for B12 absorption) ensures that our food is sufficiently broken down so that the rest of the production line can handle the job.

After digestion in the stomach, chyme is handed off to the small intestine. As you can imagine, food from the stomach is very acidic. The small intestine cannot handle this level of acidity, so as food enters, sensors in the duodenum send a message to the pancreas to release bicarbonate ions which it uses to neutralize the acid. Also secreted from the pancreas are various enzymes that are needed to finish up digestion e.g., amylase, lipases, and proteases. Some of these enzymes are also produced in the lining of the small intestine. Since fats tend to stick together, bile salts stored in the gall bladder are released to break the fats apart and prevent them from associating. This is much like the function of soap which disperses oil and grease from your plate as you do the dishes.

The small intestine is where we complete the digestion process, and where we also begin to absorb nutrients which gets transported to its various destinations via the liver. Two of the important features we talked about in class concerning the small intestine is that it is long (about 20 feet) and has a rough surface. This facilitates maximum absorption of nutrients. What are the tiny projections in the small intestine called that gives it its rough texture? If you said villi, you are perfectly correct. Once digested, nutrients are absorbed through the cells lining the villi called enterocytes. Each villus has tiny capillaries that allows the absorption of amino acids, sugars, and small fatty acids. The villi also have another transportation line called lacteals which are hooked up to our lymphatic system. That’s where large fatty acids and fat-soluble vitamins are absorbed as these molecules are too large and hydrophobic (water hating) to be absorbed into the blood via the tiny capillaries in the villi.

Next stop, we go to the large intestine, otherwise known as the colon. Here is where water and electrolytes (dissolved minerals) remaining in the food is absorbed. This increases efficiency and no doubt prevents your stool from being too watery – otherwise you would have a permanent diarrhea. Who wants that? Apart from absorption, the large intestine is where you have another cool thing happening, i.e., fermentation. Here, good bacteria (also called probiotics) break down fiber (also called prebiotics) to produce important byproducts for our health. For example, they produce vitamin K and B vitamins, and short chain fatty acids (SCFA) which have been found to have several beneficial effects on the brain, immune and gut health.

Undigested food is temporarily stored in the rectum and eventually eliminated via the anus.