You Are What You Eat: The Beginnings of the Digestive Process
Editor's note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that's because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, "The Designed Body." For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
Take a good look at yourself. Everything you see (and can't see) is made up of chemicals that had to be taken into your body before they could be formed into organs and tissues. These chemicals are things like water, glucose and other sugars, amino acids, cholesterol, sodium, potassium, calcium, iron, and vitamins, just to name a few. In fact, other than molecular oxygen (O2), which is used for energy and is taken in through the lungs, all the other atoms and molecules the body needs to live, grow, and work properly enter through the gastrointestinal system. In other words, you are, quite literally, what you eat.
Since the body is made of matter, it must follow the laws of nature. Many of the nutrients the body needs are chemically locked up inside more complex molecules that cannot enter the body through the gastrointestinal system because they are too big. These include carbohydrates, which consist of chains of sugar molecules; proteins, which are made up of many different types of amino acids joined together; and fats, which consist of fatty acids and glycerol. To procure what the body needs, the gastrointestinal system must use enzymes to first breakdown these complex molecules into smaller ones in a process called digestion. Then it absorbs these simpler molecules so the body can use them to construct the complex molecules it needs to survive.
The gastrointestinal system depends on many different parts to digest and absorb the necessary nutrients for life and excrete what is not needed. These include the mouth, teeth, tongue, salivary glands, pharynx, esophagus, stomach, intestine, pancreas, liver, gall bladder, colorectum, and anus. Here's how it all works. The gastrointestinal tract is basically a long, muscular, hollow tube that contracts to push its contents along. Food and drink enter the mouth and are swallowed into the tract. The contents are then slowly transported at a speed that allows for optimal digestion.
Fluids, containing various chemicals and different enzymes to help breakdown the complicated chemical structures within its contents, are secreted into the lumen. These fluids break down the molecules into simpler ones, like glucose, amino acids, free fatty acids, and cholesterol. Once the complex molecules have been broken down, the cells that line the lumen of the gastrointestinal tract absorbs them into the body and places them in the blood. From here they are transported to where they are needed. Let's consider how the body follows the rules and takes control to begin this process.
The process of digestion is similar to how a pulp and paper mill works. The mill uses mechanics and chemicals to take huge logs and chop and mash them into pulp for paper and other products. Similarly, the first job of the gastrointestinal system is to use mechanical and chemical means to take in complex molecules and unlock the nutrients within them so the nutrients can be absorbed into the body. However, the equipment and the chemicals at the mill don't do anything unless there are actually logs to be processed, otherwise it would be wasting energy and other resources. So too, the gastrointestinal system begins the process of digestion in earnest only when you eat or drink.
There are many different sensors in the body that stimulate the hunger center, which is the integrator that analyzes the data, and then notifies the conscious mind to take in calories. By responding to this urge the body activates the gastrointestinal system, the effector that actually does something about the situation. The process of digestion begins as soon as food enters the mouth. Its presence, along with its taste and smell, are detected by the nervous system, which stimulates the release of saliva from glands in the mouth. Saliva is a fluid that contains many different chemicals that not only help oral and dental health, but also the swallowing of food. Saliva also contains digestive enzymes, like amylase and lipase, which are the first enzymes to start working on the complex molecules that have been ingested. Amylase breaks the chemical bonds between sugar molecules, like glucose, that are joined together in large carbohydrate molecules, like starch. And lipase breaks the chemical bonds between molecules, like fatty acids and glycerol that are joined together in large fat molecules.
As the contents of the mouth mixes with saliva, it is mashed by the teeth and the tongue, formed into a small mushy lump called a bolus, and moved back toward the throat (pharynx). Up until this point everything the body has done, bringing in food and drink, chewing, and moving it into the pharynx, has been voluntary. But once the bolus reaches the pharynx, the body must automatically take control to be sure that it goes into the esophagus and not the lungs. Sensors in the pharynx detect the bolus and send information to the brain where it is integrated. The brain then initiates the swallow reflex by sending out nerve messages that cause coordinated muscle contraction to propel the bolus into the esophagus. The esophagus is a muscular passageway that propels the bolus in a wave-like movement, called peristalsis, down through the chest cavity toward the stomach that is located within the abdomen.
Anyone who has ever had something "go down the wrong tube" can attest to the extremely sensitive cough reflex that prevents accidental aspiration (inhalation). If enough food or water goes down the trachea toward the lungs, instead of the esophagus, this can quickly lead to respiratory failure. So, merely trying to explain how all of the above parts came together, as evolutionary biologists claim to do, without explaining how coordinated swallowing developed, is not sufficient to explain the systems development. After all, once the bolus goes over the precipice of the pharynx and by the power of gravity and muscular action moves downward, there is only so much time available for the body to react.
While looking in a mirror, if you gently place your fingers on the front of your neck just below the jaw and swallow a few times, you will see and feel the tissue in the neck move up and down. What you are feeling and seeing are the upper parts of your respiratory system being moved up and out of the way so the airway can be protected from what is being sent down into your esophagus. This requires about twenty-five different pairs of muscles under the direction of the swallow center in the brain, and is carried out in about a second, usually a thousand times a day.
Because some people with neuromuscular conditions (like brainstem stroke, Multiple Sclerosis, Lou Gehrig's disease) have problems with swallowing, they are at high risk for aspiration. Clearly, for our earliest ancestors to survive would have required them to have had all of the right parts working in a coordinated fashion. But we have only shown how the body is able to safely move what it eats and drinks from the mouth to the esophagus. What then? Next time we will look at what happens within the stomach and beyond to complete digestion and allow for the absorption of the nutrients the body needs to live, grow, and work properly.
Image credit: © muro / Dollar Photo Club.