August 1, 2007
Look up in the sky! Look down at the ground! In fact, look all around you! What you see are objects that are each made up of chemical elements configured in a specific pattern to form things such as clouds, the soil of the earth, and all of nature.
So too, the human body we each possess is composed of various chemical elements molded together in a certain way that allows for life on earth. The cells in the body need oxygen, water, and nutrients such as carbohydrates, proteins, fats, minerals, and vitamins for survival. We know that the body obtains oxygen and releases the toxic by-product of respiration, carbon dioxide, by way of the lungs. But the rest of what our cells need for energy, growth, tissue repair, and all other bodily function comes through the gastrointestinal system.
In other words; you literally are what you eat! Look at yourself in the mirror and reflect on the fact that everything you see that is part of your natural body has had to come through your gastrointestinal system first on its way to making your physical self; you. The combination of carbohydrates, proteins, fats, minerals, vitamins, and water have had to be able to enter into your body in order to be used for what they’re needed for. But most of the food that enters is in a complex form that cannot be readily accessed and used so the body has to be able to break it down by digestion in order to absorb the nutrients.
Digestion and Absorption: An Overview
Here’s a good way to think about the process of digestion and absorption of what we eat and drink. The digestive tract is basically a long muscular tube, with its own blood supply, that pushes its contents along from the mouth to the anus. Food and drink are taken into the mouth and are swallowed in a manner as described in the last column. The contents are then transported along this hollow muscular tube along its inner tract (lumen) at a speed that allows for optimal digestion. Fluid is secreted into the lumen which contains various chemicals and proteins to help breakdown the complicated chemical structures within our food and drink. The inner lining of the gastrointestinal tract then absorbs these more simplified chemicals, such as glucose, amino acids, free fatty acids, and cholesterol, which are transported to where they are needed in the body.
The Players and What They Do
Your ability to digest and absorb the necessary nutrients for life and finally excrete what is not needed is dependent on many components doing different tasks. These include the mouth, the teeth, the tongue, the salivary glands, the esophagus, the stomach, the intestine the pancreas, the liver, the gall bladder, the colorectum, and the anus.
( taken online from NIDDK-Image Library)
The combination of the mouth, teeth, tongue and salivary glands, is the starting point for food and fluid digestion and absorption by way of ingestion. It is here where we decide that what we are taking into ourselves is agreeable and necessary. The process of converting the food to a consistency that allows for absorption now begins. The salivary glands secrete fluid that contains enzymes (amylase) which help to start to breakdown starch. The contents are then sent to the esophagus by swallowing using a complex neuromuscular system which at the same time prevents aspiration into the lungs and asphyxia…..and death.
The esophagus is simply a conduit for the food and fluid to travel through the chest cavity into the stomach in the abdominal cavity. Here it is mixed with chemicals such as hydro-chloric acid and protein digestive enzyme called pepsin and is further churned and mixed in order to further prepare its contents, now called chyme, to be absorbed.
The stomach absorbs very little and regulates the slow release of its chyme through the pyloric sphincter into the intestine. Both the stomach and the upper part of the intestine (duodenum) by detection of the chemicals within the lumen are able to send out hormonal messages to the gall bladder and the pancreas telling them to start secreting their fluids. The chyme that leaves the stomach and enters the intestine then mixes with bile from the liver and gall bladder, pancreatic juice containing digestive enzymes, and intestinal secretions containing mucus, alkaline fluid of bicarbonate offsetting the hydrochloric acid from the stomach, and intestinal lining digestive enzymes as well.
This mixture now moves along the intestine where digestion takes place and because of this digestion, later on, absorption. In fact, the intestine is where most of digestion and absorption in the body occurs. In addition to simple sugars like glucose, amino acids and small protein peptide chains, and simple fats like monoglycerides, free fatty acids, and cholesterol, the intestine also absorbs minerals like calcium, phosphorus, and iron, electrolytes like sodium, potassium and chloride ions, and all the necessary vitamins including Vitamin B12 which is dependent on the stomach’s secretion of intrinsic factor (without which someone would have pernicious anemia) and finally, that most necessary of factors for survival…good old H2O……water.
Finally, about 1.5 liters of chyme makes its way into the colon daily where mostly water and sodium and chloride ions are absorbed. The remaining 100-150 gm of feces that daily exits the gastrointestinal tract through the rectum and anus usually consists of about 70% water, and 30% solids from undigested plant fibers like cellulose, cells shed from the lining of the GI tract, and bacteria. The brown discoloration of feces derives from the bacterial breakdown of bilirubin which comes from the metabolism of heme from hemoglobin. This is the main pigment in bile and its characteristic odor relates to bacterial action on amines in the remaining chyme contained in the colorectum.
Intelligent Design? NOT !!
Anyone who is knowledgeable about what is considered intelligent design when it comes to life can clearly see that the human gastrointestinal system was not intelligently designed. After all, we have to suffer with stomach and duodenal ulcers because of disorders of hyperacidity, gallbladder attacks due to the formation of gallstones, and anal canal hemorrhoids which although not life-threatening, can really be a pain in the - - - !!
I mean: how can you consider this system to have been intelligently designed if humanity has to suffer from these conditions? One could raise the bar further and also consider that if humans are not immortal then they couldn’t have been intelligently designed.
Control: That’s the Key
But seriously folks, the skeptical critics of intelligent design, tend to focus their arguments on the intelligent which at its most basic refers only to the presence or absence of “a mind at work” and not its perceived sophistication, and they ignore the design which speaks to specified complexity: that is the presence of a system of high information content requiring complex instructions for control. And speaking of control let’s take a look at some of the control mechanisms that are present in the GI system.
I’m sure we’re all familiar with what happens when we approach a set of automatic revolving doors at a luxury hotel. A detector becomes “aware” of our presence and automatically starts the doors spinning at a speed that will allow us to try to coordinate our entry into the hotel. Whenever I encounter one of these systems my attempts to time my entry without touching anything always reminds me of playing skip rope with my classmates back in elementary school. The nostalgia always makes me want to go through these doors again and again, much to the chagrin of my wife.
It goes without saying that if the doors were to be continuously revolving irrespective of anyone needing to enter or leave the hotel, that this would be a colossal waste of energy. Well the same can be said of the gastrointestinal tract. The secretion of gastric, intestinal, and pancreatic fluids, each containing their own specific enzymes and chemicals, and the muscular contractions which help digestion, are not processes that are happening all of the time. Just like the scenario of the revolving doors, if they were to be ongoing without any food around, it would be a colossal waste of energy. After all, it takes energy for the body to digest and absorb food. If the energy required to do this were to be more than the potential energy present in what was absorbed, then I would not be able to sit here and write this article and you wouldn’t be able to read it…we’d be dead. So control of gastric, intestinal, and pancreatic secretion and gastrointestinal motility must be regulated to occur when there’s really something around to be digested and absorbed, in order for survival to be possible. So how does the body do it? Basically in two ways: nervous and endocrine control. (messages from the nervous system and hormonal messages)
Regulation of Gastric Secretion
The stomach secretes its own unique juice containing cells that form mucus, hydrochloric acid, and pepsinogen. Pepsinogen is the inactivated enzyme that when exposed to acid becomes activated to pepsin which starts the process of protein breakdown. The stomach also contains specialized cells which, when properly stimulated, secrete a hormone called gastrin which affects stomach gland cell function as well.
Just anticipating, seeing, smelling, or tasting food will trigger messages from various regions in the brain to travel along one of the main nerves arising from the brainstem, specifically the vagus nerve, which has connections with the stomach. Stimulation of the vagus nerve causes it to neurochemically tell the stomach glands to start secreting mucus, hydrochloric acid, and pepsinogen in preparation for what is about to come into the stomach by way of the mouth, pharynx, and esophagus. The vagus nerve also tells the specialized cells in the stomach that make the hormone gastrin to gear up for action as well. The gastrin enters the bloodstream and has its own effect on the stomach glands, also telling them to start putting out mucus, hydrochloric acid, and pepsinogen as well. This is the first, or cephalic phase,of gastric secretion regulation by the body.
But that’s only the beginning. Because once the food actually enters the stomach it starts up the gastric phase by stimulating nerve cells that are sensitive to gastric distension and other cells that respond to the chemicals contained in the food we‘re eating. We are all familiar with what our stomach feels like after we’ve eaten. It feels like its enlarged and we can just imagine the walls of our stomach further separating from each other and becoming distended as we keep piling the food in.
When this happens, the nerve cells in the walls of the stomach, called mechanoreceptors, become more and more stimulated because they’re sensitive to this tugging and pulling on the tissue in which they are located. This causes them to send out stronger and stronger messages to other nerves located nearby which by reflex action send back neurochemical messages to the stomach cells telling them to secrete more mucus, hydrochloric acid, pepsinogen, and gastrin. In addition, some of the early breakdown products of protein are able to stimulate gastrin secretion as well.
So one can immediately see that there are two levels of stomach stimulation whether in the cephalic phase or in the gastric phase. The first level occurs directly on the gastric cells which make mucus, hydrochloric acid, and the enzyme precursor called pepsinogen which needs acid to be activated. The second level is accomplished by stimulating the specialized cells in the stomach that secrete the hormone gastrin which enters the bloodstream and further stimulates the gastric cells to make the same products.
Regulation of Gastric Emptying
Now that the stomach has mixed the food with lots of acid and activated pepsin, and has churned it around and around to help digest it, it must now pass its contents through the pylorus into the first part of the intestine known as the duodenum.
(taken from The Free Dictionary by Farlex).
In order for the most efficient digestion and absorption to take place in the intestine it is important that the stomach not send the chyme hurtling down willy nilly to overwhelm the capacity of the intestine to accomplish its task of digestion and absorption. In this I am reminded of that classic episode of I Love Lucy in which Lucy is unable to keep up with her job of dipping chocolates along a conveyer belt, so she starts to try to eat them instead. It’s an hilarious scene but I think you get the picture.
Another problem is that the stomach contents are very acidic and although its cells are protected from the effects of acid, they can become injured from bile if it refluxes from the duodenum into the stomach: a condition known as bile gastritis. And the same can be said for the duodenum. Its cells are naturally protected from bile but when exposed to acid this can be toxic for them resulting in duodenitis or even a duodenal ulcer, which can be life threatening if it bleeds or perforates.
So gastric emptying, like gastric secretion, has to be carefully controlled to allow for efficient gastrointestinal function and to prevent injury to both the stomach and the duodenal cells. But how does this take place and how is it controlled?
As luck would have it, or depending on your perspective, maybe from intelligent design, the opening from the stomach to the duodenum is controlled by the pyloric sphincter, a specialized ring-like band of muscle that is able to constrict and relax in order to send out as much chyme as is necessary into the duodenum. If you want to get an idea of how this muscle works, consider the muscular action involved in emptying your rectum through what is called the anal sphincter. (sorry, but it seemed the easiest example to use)
There are several factors that affect how fast the stomach empties. If your stomach is very full then, just based on the pressure behind that fullness, the stomach tends to empty quicker. But we’ve all experienced eating a heavy meal and several hours later, on burping, noticing the taste still emanating from our stomach. Another critical factor that affects gastric emptying is what’s in the stomach. Receptors in the stomach and the duodenum that are able to detect nutrients such as fat and protein send neurochemical and hormonal messages that ultimately result in the tightening of the pyloric sphincter which slows down gastric emptying and allows for more efficient digestion of these heavier nutrients. That’s why when you have a fatty meal, your stomach feels fuller for a longer time and you may feel more bloated. It’s all in the name of digestion to help with absorption down the road.
Regulation of Pancreatic Secretion
Now that the food has started undergoing the process of digestion by being exposed to starch- breaking amylase from the salivary glands, and protein-breaking pepsin, activated by hydrochloric acid in the stomach, as the chyme enters the intestine it is further prepared for the heavy-duty breakdown enzymes that it will encounter from the pancreas.
Pancreatic juice is very alkaline, containing bicarbonate ions, which neutralizes the acidic chyme that has come into the intestine from the stomach. This juice also contains numerous different inactivated enzymes, some of which are turned on by the alkaline environment, and others by these now-activated enzymes.
Some examples of these enzymes are: trypsin (inactive form: trypsinogen) which is activated by the alkaline milieu and which itself activates numerous other enzymes and breaks down the bonds in protein: amylase: which is the dominant enzyme in the body that digests starch and other sugar combinations: and lipase: which breaks down fats into glycerol and fatty acids.
But how does the pancreas know when to send out these enzymes to do their work? Just as gastric secretion is regulated by neural and hormonal control, so is the release of pancreatic juice with its diversity of enzymes contained in alkaline fluid. The cephalic phase, mediated by the vagus nerve, and the gastric phase, brought on by gastric distension and some of the breakdown products of protein digestion, by way of nerve messages and the release of gastrin cause some of this to take place.
But the stimulation that causes the pancreas to release the majority of its juices comes from the intestinal phase. When the upper intestine is exposed to the breakdown products of fats and proteins, this causes some of its specialized cells to secrete two hormones which have the main effect on pancreatic secretion: cholecystokinin (CCK) and secretin Both of these important hormones work together to tell the pancreas to rain down its digestive juices into the intestine so that our food may be broken down to allow for absorption downstream.
Charles Darwin: You Have a Lot of Explaining to Do
If you look back and reflect on what we’ve talked about so far on the gastrointestinal system and how it allows for digestion and absorption of food for our survival, you can’t help but to notice how every part of the system has its own job to do. People who believe that all life came about solely by the forces of nature, (natural selection acting on random variation), without a mind being at work, must be able to explain how each component of the system came into being while staying functional every step along the way.
Besides the need to explain the development of the gross anatomy and the microscopic structures contained within each specialized part of the gastrointestinal system, what ultimately should be needed to satisfy all human inquiry in this matter is functional capacity. As I’ve said many times before: the mere existence of parts should not assume a system of function, and the mere existence of a functioning system should not assume adequate capacity for survival.
You only need to consider some of the main factors that turns the system on and keeps it going as long as there is food to be digested. The existence of the mechanoreceptors in the stomach, the way in which they sense the increased distension of the stomach, and the mechanism for how they tell the glands to start producing mucus, hydrochloric acid, pepsin, and gastrin, needs to be explained.
And likewise the ability of the cells in the stomach and intestine to detect the breakdown products of digestion and by way of neurochemical and hormonal messages are able to direct the stomach and pancreas to secrete the juices necessary for continued digestion and also affect gastric emptying appropriately, must be thoroughly explained and not merely assumed.
Finally, I wonder if you noticed something very interesting about the relationships between the inactivated enzyme released in the stomach (pepsinogen) and the main proteolytic enzyme released from the pancreas into the intestine (trypsinogen) and the environments in which they do their jobs. Pepsinogen becomes activated into pepsin when it encounters an acidic environment and trypsinogen becomes activated into trypsin when it is in an alkaline milieu.
And what do you know? It just so happens that the stomach makes hydrochloric acid which is needed to breakdown the tissue fibers contained within meat, kills off many would-be bacterial invaders, combines with calcium and iron to facilitate absorption, and provides the perfect setting for pepsin to do its job.
And the same goes for trypsin and pancreatic secretion. The pancreas pushes lots of bicarbonate ions into the intestine to offset the acid coming from the stomach after its done its job up there, and in so doing, provides the perfect environment to activate trypsinogen into trypsin so that it can start breaking down protein and activate most of the other digestive enzymes coming from the pancreas as well. Talk about luck !!
Next time we’ll complete the study of the gastrointestinal system by looking at the liver and the gall bladder: and you thought this stuff was complicated: wait and see!
Howard Glicksman M.D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being. Comments and questions about this column or any of the previous ones are welcome at firstname.lastname@example.org
Copyright 2007 Dr. Howard Glicksman. All rights reserved. International
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