Science

  • Issue 107 / September - October 2015



    The Impeccable Digestive Tract

    Brian Turk

    Almost every single system in the human body works without rest, even during sleep. The mechanism is set, the gears are in place, and all the components are in order. The only thing the body needs to get going is energy!

    This energy is distributed via our blood. Our arteries and veins slip through every organ and serve them with energy. But, where does the blood get the energy?

    The answer to that is the topic of our article: the digestive system. Our digestive system is how we process what we eat and drink. Though it has multiple different parts, in essence, it's all just one long tube, starting from the mouth and ending at the rectum. The main goal of this system is to break down molecules so that they are able to enter our cells. There are three main molecule types this system is concerned with: carbohydrates, proteins, and fats. These are all digested at different levels in different parts of the digestive tract. Carbohydrates are broken down in the mouth, while proteins are digested in the stomach. The area that breaks down all three types of molecules is the small intestine. Here, all molecules are broken down into "bite-sized" pieces for our cells. After all the organic molecules are absorbed by the blood, all that is left is waste and water. This is where the large intestine comes into play. The large intestine is responsible for absorbing any water left in the feces. Of course, it can't just absorb all the water. This process is performed according to what the body needs. If an excess amount of water is absorbed, then constipation occurs. However, if a low level of water is drawn, we suffer from diarrhea. The absorption has to be very precise.

    We now have basic knowledge about the digestive tract. It all seems quite simple, doesn't it? What could possibly go wrong?

    Well, even though it looks fairly simple in theory, this system is designed to function very precisely. Every type of different molecule can only be digested by a specific enzyme in a specific environment. For instance, even with the right enzyme, a protein cannot be digested in an alkaline environment. Every compartment of the digestive tract is unique, and they all have to be able to work together harmoniously. First of all, food needs to get from one end of the digestive tract to the other. The digestive tract is lined with involuntary muscles that contract and relax to form a movement called peristalsis. These movements form small pockets in the tract that carry food. The muscles behind the pocket contract, pushing the food further. The muscles ahead of the pocket relax, forming an opening where the food can empty into.

    An area of interest is the junction of the airway and the esophagus (the tube leading to the stomach). When food passes through this area, a door-like structure called the epiglottis closes the airway and pushes food into the esophagus. These series of contractions and relaxations must be done in a precise order and timing, or else choking or vomiting may occur. The amazing thing here is that all we have to do to start this process is swallow! The rest is involuntary; we do not need to stop and think about when to contract or relax which muscle. Imagine if you had to be very careful while eating not to vomit or accidentally choke yourself; eating wouldn't be much fun then, would it?

    The large intestine also uses peristalsis. The phenomenon of pocket formation here helps separate fully-drained contents from partially-drained contents. This is very useful during defecation, since we expel one pocket at a time. This way, only fully-drained contents are expelled. Otherwise, we might unknowingly expel feces for which drainage is incomplete, causing our body to waste water. This intermittent procession of pockets also allows us to feel finished after visiting the toilet. Of course, this all raises the question of how long food stays in each compartment and the transition of food between the compartments. As a general rule, the period of time food stays in each compartment gets longer and longer as food proceeds in the system. For example, food stays in the mouth for around 5-10 seconds, while the average transit time throughout the large intestine is 40 hours.1 Even though the main purpose of each organ in the digestive tract is digestion, they also serve as storage areas. Food stays in these organs even after digestion is complete, until the next compartment has emptied and is ready.

    Even the process of transition is quite complex. "Why?" you may ask. The reason is that each compartment has a totally different environment. The stomach has a pH (acidity level) of around 2, with acids that can easily melt down a nail. Just a few centimeters into the small intestine, the pH becomes an average of 7. If the acid were to travel down the small intestine without being neutralized, it would digest the small intestine! So, a hormone called "secretin" is responsible for the secretion of bile (which is a great buffer) in the small intestine's entrance. This bile neutralizes the acid into a more "user-friendly" form for the small intestine. It really is unbelievable how each part of the digestive tract is aware of the others, and adjusts their movements accordingly. The timing and process of transporting food through the tract is amazing.

    So far, we have discussed the general procedures of the digestive tract. But there is no doubt that such a unique system will contain unique parts. Now, we will examine two specific structures and try to understand what makes them so special.

    First, let's check out that little bulge where the large and the small intestines meet. This bulge has a name we are all actually quite familiar with: the appendix. This structure is kind of mysterious and there have been many theories on what it actually does. According to some, it is a leftover of evolution. But there is another viable theory: that the appendix serves as a haven for useful bacteria when illness flushes harmful bacteria from the rest of the intestines.2 It is a safe spot because it is not within the normal pathway of digestion. This way, in times of infection when harmful bacteria are flushed from the intestines during diarrhea, the beneficial bacteria can seek shelter in our appendix. Of course, these bacteria are only beneficial when in the intestines. They could cause serious damage if they were to spread to other parts of the body. That is why the entire surface of the intestines is covered with mucus, which keeps bacteria under control inside the digestive tract. Of course, as most of us know, a common illness is appendicitis. Appendicitis is the inflammation of the appendix, and may cause the appendix to burst. A bursting appendix will, in turn, spread our beneficial bacteria and cause infection. This is why people with appendicitis are quickly taken into surgery and have their appendixes removed before bursting occurs. Even though the appendix may look like an oddity in the digestive tract, it, too, has its own purpose. This is what we would expect, because no organ in the body is dead weight.

    The other unique structure is the external sphincter. This structure is the doorway between the end of the digestive tract and the outside. There is also an internal sphincter right behind it. There is a special way these two doorways are coordinated that allows us to easily pass feces. These two doorways are both very much the same except for one thing: the internal sphincter is involuntary while the external is under our control. So, how do these work together? Whenever our rectum fills, feces will apply pressure onto the internal sphincter, which causes it to open. However, unless we consciously open the external sphincter, we will not let the feces out. As can easily be understood, this is extremely convenient.

    There is another mechanism that makes this process even better, and that is the closure of the internal sphincter after a few seconds. How does this affect us? Well, if that sphincter were always open, we would constantly have the urge to use the bathroom. The occasional opening of our internal sphincter is a reminder of our need to defecate. But there is a balance to this process, so we are not constantly in the bathroom. It is as if the body somehow "knows" about social life and how convenient it is that this mechanism manages our daily activities. Otherwise, like animals, we would satisfy our need on the spot at the first urge.

    Now that we have a better idea of our digestive system, we are able to see how flawless it really is. It is amazing how the compartments are placed centimeters apart and are still so different. It is awe-inspiring how each section manages to work harmoniously with the others for one common cause: to digest food into small pieces that cells can use for energy. It is wondrous how every little part of this system plays a huge role and how even the smallest dysfunction in any of them can cause huge problems. Every centimeter, every bulge, and every minute is crucial for the continuation of this system; yet, it manages to work 24 hours a day for up to 100 years. Only one word can be used for such a system: Impeccable.

    We don't often think about how our food is digested ÔÇô and this is part of what makes our digestive system remarkable. It fulfills an incredibly intricate, complex role without any conscious oversight.

    Notes

    1 Metcalf AM, et al. Simplified assessment of segmental colonic transit. Gastroenterology. 1987;92:40

    2 Associated Press. "Scientists may have found appendix's purpose". MSNBC, 5 October 2007.

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