Digestion 101

One of the most idiosyncratic things about American culture is its relationship to food.  The citizens of the United States are some of the most diet-obsessed people in the world, counting calories and and studying ingredients while at the same time battling out of control obesity rates.  One of the reasons for this is that while people are more educated than ever about aspects of how food can have specific effects on the human body, many people lack a basic understanding of how the various elements of the digestive system work together to turn food into energy.  The goal of this guide is to look at how those elements fit together and how each one contributes to the process of turning food into the raw materials that the body needs to fuel and maintain itself.  While the focus will mostly be at the organ level (as opposed to the cellular level), there will be times during the discussion where it’ll be necessary to examine what is happening on a smaller level.  For the most part, however, this will be a general guide to the digestion and its component parts.  So, with that in mind, let’s start this exploration at the beginning of the digestive tract – the mouth.

The Mouth

Most people think of digestion as something that occurs once food reaches the stomach, but in fact the process of breaking down food begins when before the first mouthful ever crosses the lips.  As Pavlov famously documented, salivation begins to occur in anticipation of food, not with its arrival. There’s an important reason for this. Saliva not only aids in consumption by moistening the food as it is consumed (which makes it easier to swallow) but it also contains enzymes which begin the break down of nutrients in the food that was just consumed.  Specifically, it contains two enzymes which aid in the breakdown of starches and fats.  These nutrients tend to be denser than simple foods like carbohydrates and therefore need more time to break down in order to get the most nutritional value from them.  This is also why they provide more energy to the body over a more extended period of time.  Saliva also assists the digestion by neutralizing certain harmful bacteria that may be on the food, easing the demands on the immune system while food is being broken down later in the digestive process.

The tongue and teeth also play an important function in digestion.  Things are needed or wanted by the body and brain tend to taste better than things that are less needed.  Many of the strange cravings that people experience at various points in their lives are related to this phenomena.   For example, it is not uncommon for women to crave dirt during pregnancy.  This is related to a need for certain minerals that may be insufficient in the diet that are needed for the development of the fetus.  Because these minerals may be more abundant in the soil, the brain will translate this need into a craving and a material that would not usually be considered a normal “food” suddenly seems not just edible, but needed.

Another example is sugar cravings.  As explained in previous articles on this blog, the brain loves sugar. Unlike other organs and tissues in the body, the brain’s preferred source of energy is carbohydrates, and simple sugars are the fastest way to deliver those carbs to the brain.  So during times of need or anticipated need – such as when extreme or prolonged stress is ongoing or anticipated – the brain will communicate to the body that it wants those carbohydrates.  This is will manifest as a craving for sweet foods, as that flavor is what is associated with simple carbohydrate content.  Many times, these cravings are actually reflecting either anxiety or true hunger and are better satisfied by more substantial fare, but, especially in an environment where simple sugars are plentiful and easily acquired, a more thoughtful approach to stemming the cravings must be learned as cravings are a powerful instinct.

The role of the teeth in digestion is much more straightforward and mechanical. Teeth are there to make big pieces of food into smaller pieces.  Much like cooking (or most chemical reactions), the more surface area of a substance that is exposed to the forces affecting it, either in energy or chemical form, the faster and more efficient the break down of the material will be.  The design of the teeth and mouth evolve in very different ways depending on what that creature’s diet consists of.  Animals who exist primarily on plant materials tend to have broad, flat teeth that grind food into an almost paste-like substance that is easy for them to break down.  Species whose diet consists primarily of other animals have sharp, pointed teeth that are mostly designed to make it easier to rend the flesh from the body of the animal they are consuming.  Most of the non-protein elements of their diets comes from eating the digestive organs of vegetarian animals, where the plant matter is already mostly broken down and therefore easier for their own digestive tracts to extract what the carnivore needs dietarily.

Humans, along with a few other species of animal, are omnivores, meaning they have the theoretical capacity to consume both plants and animals (due to dietary, cultural, or religious reasons, individuals may choose to limit their diet, but in terms of anatomy, most of humanity is born with the same basic mouth design).  This is reflected in the mouth by the variety of teeth present.  The incisors cut plant and animal material into smaller pieces while the canines allow for the grasping and tearing of larger pieces of flesh. Finally, the molars grind both plant and animal material down into smaller matter clumps that are held together with saliva called a “bolus”.  When discussing digestion, the bolus (in layman’s terms, one swallow’s worth of chewed food and saliva) is the general term used as the consumed food works it’s way through the upper digestive tract, so this will be the primary term used as the discussion continues.

As was mentioned before, the primary function of the teeth is to break down the food being consumed so that as much of its surface area is exposed and broken down as possible to the chemicals and enzymes that will break it down into components the body can use. This function is so important that many digestive issues can arise simply by not chewing one’s food enough.  For example, if a large piece of food matter is swallowed intact, the chances that the digestive processes will be able to fully break it down as it travels through the organs becomes reduced, increasing the chances of undigested food in the stool.  While it remains in the digestive tract, the chances that the undigested food will contribute to issues such as gas, bloating, and added fermentation increases, leading to abdominal discomfort and blockages.  There are many schools of thought on this, but it’s is commonly agreed that each mouthful of food should be chewed at least fifteen to twenty times before swallowing.  One of the reasons why processed food is often more attractive to the body is because many of these foods have already been broken down and reconstituted, so that they require much less energy to digest.  (Unfortunately, that also means many more preservatives are required to keep the food from spoiling, which makes these foods overall much less nutritious that foods which are less processed.)

The Esophagus

After the food has been turned into a bolus by the teeth and the saliva in the mouth, swallowing then transports the bolus from the mouth to the stomach by way of the esophagus.  The esophagus is a hollow, muscle-lined tube that moves the bolus by the way of smooth muscle contractions.  These contractions further break down the bolus through agitation of the substance as it is passed down towards the stomach.  Much like clothes in a washing machine, agitation helps aid digestion by further breaking down the bolus and exposing more of its surface area to the chemical processes that are occurring in the digestive system.  This is an important factor in almost every step of digestion.  If a person attempts to swallow something too big to be digested or if the body is attempting to reject something from the throat, the same muscles which move food downward to the stomach will instead try to force the offending substance back out through the mouth.  If this occurs at the top of the throat, it often presents as gagging.  When the offending material is in the stomach, vomiting will occur, and if the substance becomes stuck in the esophagus in such a way that it blocks the flow of air to the lungs, the person will begin choking and may need additional assistance (such as the Heimlich maneuver) to clear the offending material.  If the bolus is appropriately sized and there are no complications during its passage through the esophagus, after about a minute, the bolus will arrive in the stomach.

The Stomach

As the bolus reaches the end of the esophagus, it passes through a sphincter to arrive in the stomach.  (When part of the stomach sits above this sphincter, a condition known as a hiatal hernia is the result and can contribute to such conditions as heartburn and GERD, as the body is less able to keep its contents from passing back up into the esophagus.)  The stomach contains nerves unrelated to the taste buds which can relay information to the body about the protein, starch, carbohydrate, and fat content of the food.  This data can then be used by the brain to determine if the food being consumed will meet the body’s nutritional needs.  Unfortunately, this information often comes either too late or is ignored, which contributes to over-eating and poor food choices, but it is valuable information for people who are trying to control their appetite as part of a program to eat healthier foods and amounts of those foods.  Digestively, the stomach exists to further the breakdown of proteins, decontaminate the substance that has been consumed, and to transform the bolus into chyme.  This is accomplished partly by the secretion of gastric acid which unbinds proteins, kills offending bacteria, and further breaks down other non-protein materials.  Gastric acid, which contains a concentration of .05% hydrochloric acid, is secreted by the walls of the stomach itself, and is an extremely necessary part of the digestive process.

When stomach acid is insufficient, proteins become harder to digest and the chances of bacteria surviving into the small intestine increases dramatically.  In a healthy individual, the stomach is coated with a layer of mucous that protects the lining of the stomach from the acid it produces.  However, there are certain conditions and situations which will cause this mucous layer to become insufficient to protect the lining and the results often include ulceration of the stomach lining, a painful and potentially dangerous situation.  As a result of the discomfort, a common treatment involves reducing the amount of acid being produced, which can create even problems such as those mentioned above.  Certain medications for issues unrelated to ulcers and heartburn can also reduce the amount of stomach acid being produced, which can create further problems down the road such as malnutrition and digestive upset if the person taking those medications is not prepared for them.

Another element in the breakdown of foods in the stomach is agitation of the stomach by the smooth muscles lining it.  As the bolus is exposed to the acid inside the stomach, certain enzymes are activated, further breaking down the proteins inside.  The stomach will churn during this period, helping to expose as much of the bolus as possible in order to maximize the amount of the bolus that is broken down into a more liquid-like substance called chyme that is transported to the small intestine by a second sphincter in the lower part of the stomach.  This agitation also helps moves the bolus through the stomach towards that opening.  Breaking down the bolus into chyme takes approximately forty minutes to an hour from the time it arrives in the stomach.  While some substances are absorbed by the stomach (alcohol being a notable example), most of the actual breakdown and absorption of the chyme is done in the next organ, the small intestine.  But before that, there are two other organs that contribute to the digestive process that should be looked at first.

The Pancreas

While food doesn’t pass directly through the pancreas as part of the digestive process, this organ is nonetheless contributes very important enzymes and chemicals which aid in the break down of food into something that the body can make use of.  A full analysis of these substances could be an entry on its own, but for the purpose of this article, there are two that are worth noting specifically: bicarbonate and insulin.

As chyme enters the small intestine from the stomach, it still contains a large amount of stomach acid.  The tissues of the small intestine need to be more absorbent than those of the stomach and therefore don’t have the thick layer of mucous to protect them from the acid.  The bicarbonate (an alkaline) released by the pancreas is thus necessary to neutralize this acid so that those tissues aren’t damaged.  As chyme passes through the sphincter between the stomach and the small intestine, the pancreas releases bicarbonate to raise the ph of the chyme to a more neutral level.  If the amount of bicarbonate is insufficient to fully combat the acidity of the chyme, it can result in ulcers in the upper tract of the small intestine that can be quite painful.

The second product of the pancreas that should be noted is insulin.  This substance is much better known than the first thanks to the dramatic increase in the number of cases of diabetes across developed nations.  Insulin is released by the pancreas in response to the increased presence of glucose in the blood stream and plays a large part in carbohydrate and fat metabolism.  The presence of insulin in the bloodstream also activates other metabolic and cellular functions, making it an important part of overall health as well as digestive function.  High glucose levels can be toxic to certain bodily tissues and can contribute to increased infection and tissue damage, so healthy pancreatic function and insulin response is essential in maintaining good health.  There are two types of diabetes which, when contrasted, illustrate how common problems with insulin can come about and, when compared, show how insufficient insulin response can create very serious health issues.

The oldest form of diabetes, Type 1, used to be called “child-onset” diabetes, but with the increasing number of children developing Type 2 diabetes, this moniker is no longer appropriate.  Type 1 diabetes develops out of an autoimmune disorder where the immune system attacks the insulin producing cells in the pancreas.  This process occurs over several years with the patient seeing gradually increasing symptoms that become fully realized around the beginning of adolescence, include mood swings, stomach pain, and other signs of digestive disfunction.  Because the body no longer produces insulin, the diet must be carefully managed and blood sugar must be closely monitored.  This condition has existed in humans through much of recorded history and was referred to by the Greeks as “sweet pee” (due to the body exuding extra sugars through the urine, giving it a sugary smell) and by the Chinese as “xiao ke” or “wasting thirst” (due to the greater occurrence of dehydration as the body uses water to exude the extra sugar, meaning greater amounts must be consumed).  In previous generations, control of the diet and some herbal remedies were used to control the condition, but in the early 1900’s, insulin harvested from pigs became available to control sugar levels as needed.  Today, much of that insulin is produced by plants using genetic engineering.  Because the stomach acids render insulin taken orally into an unusable form, it must be injected into the body as opposed to take orally.

Type 2 diabetes, or what was formerly known as “adult-onset” and is now commonly referred to a insulin-resistant, is a form of maladaptation that occurs in the human body when blood sugar levels remain high over an extended period of time.  The body begins to lose the ability to use its own insulin properly, leading to higher glucose blood levels than are healthy.  Poor diet and a lack of physical exertion over an extended period of time are a major factor in developing type 2 diabetes and correcting these poor habits can be an effective way to regulate and even reverse the damage that is being done.  While some type 2 sufferers may also be insulin deficient, in most cases using exogenous insulin injections is an insufficient treatment plan.

In both types of diabetes, the higher glucose levels have a toxic effect of sensitive body tissues such as the capillaries.  Over time, these tissues will die off, which starves nearby nerve tissues of the blood needed for their own health.  This is the primary cause of certain common diabetic side effects such as the loss of sensation in the feet and hands and over time can include blindness.  The lack of proper blood flow and sensation as well as the higher blood sugar levels also make the individual more vulnerable to infections and tissue necrosis, which is why, in extreme cases, limb amputation becomes necessary.  Also, because the extra sugar is excreted in the urine, the kidneys become exposed to much higher levels of glucose, a situation that causes them to become to damaged.  This reduction of functionality can be a precursor to complete kidney failure which will then require dialysis and in extreme cases, a complete kidney transplant.

Hopefully this detour into diabetes was not too distracting, but it seems an excellent way to both explain the function of insulin as well as showing the dangers that can be posed to health by its absence or the body’s failure to regulate it.  Before moving on to the small intestine, there is one more secondary digestive organ to cover.

The Gall Bladder

As its name suggests (a bladder being a hollow, yet flexible container), the gall bladder is a hollow organ designed to hold a substance, in this case bile.  Bile is released into the small intestine when lipids (or fats) are detected in the food being consumed.  Produced in the liver out of substances such as water, bile salts, and cholesterol, bile is stored in the gall bladder until it is needed.  When that time comes, it is squirted out and mixed with the chyme in the small intestine to help break down lipids contained within.  In cases where the gall bladder has been removed, usually due to gall stones, the liver continues to produce a steady stream of bile, but without the ability to control its release, either a low fat diet or bile in pill form must be consumed to avoid abdominal pain and nausea.

Gall stones can occur for a number of reasons, but some common ones include a low fat diet (so less bile is released, increasing the chances that the bile in the gall bladder will have time to form stones), dehydration (less water means the bile will become thicker and more likely to harden into stone form), and blockage of the duct by bacteria or other substances.  A common remedy is to remove the gall bladder surgically, but as mentioned above, this can create other issues or require the addition of a supplement to make up for its absence.

The Small Intestine

This is the longest organ in the human body (it can vary from fifteen to thirty-five feet in length) and arguably the most important aspect of the digestive system when it comes to absorbing nutrients from the substances that are consumed.  Fats, proteins, carbohydrates, vitamins, and minerals are all absorbed through the tissue walls of the small intestine as the chyme passes through it on its way to the large intestine.  While some of these substances are broken down by organs or substances outside of the small intestine (proteins by the stomach, fats by the bile from the gall bladder), without the small intestine, the body would still have no way to make use of them.  One of the reasons for the length of this organ is to allow as much of these materials to be absorbed as possible.  The tissues of the small intestine themselves are designed to maximize surface area so that the nutrients can be diffused through them to the capillaries where they enter the bloodstream to be distributed as needed.

Another reason that the small intestine is so long that the chemical breakdown of chyme into nutrients is an uneven process.  More complicated nutrients take longer to break down into usable components than less complicated ones.  As these materials pass through the intestines, easily obtained nutrients are absorbed earlier on while more complicated ones continue to work their way through the organ as they move towards the large intestine.  By the end of it’s trip through the small intestine, most of the nutrients and some of the substances released by the body to help with digestion (bile salts, for one example) have been absorbed through the cells lining the small intestine.

Unlike most of the other organs that nutrients pass through in the digestive tract, the small intestine does not use peristalsis to move the chyme through itself.  Peristalsis is the rhythmic contraction of smooth muscles lining the organ which fire in a wave-like sequence to push food through the organ.  Instead, the small intestine uses villi, small hair-like projections that line the inside of the organ, to gently push chyme along, similar to how a gondolier pushes a boat through water.  These villi are aided in their efforts by the contraction and expansion of muscles in the abdominal core such as the obliques and the transverse abdominis.  When a person moves in such a way that engages these muscles, predominately movements where the torso is engaged in horizontal rotation (walking is one such movement), the contraction and expansion of the muscles helps move food through the small intestine, increasing the amount of agitation that occurs and further breaking up the chyme, which in turn exposes more of it to the inner surface of the intestine.  This allows for more of the nutrients to be absorbed and for the chyme to move more quickly through the small intestine.

Because so much of the absorption and movement of the small intestine occurs along this inner surface, anything that blocks or coats that surface will have an overall negative effect on both the speed and efficacy of which the intestine absorbs nutrients.  Substances like mucous which are produced by the body in response to dietary elements which the body finds disagreeable either because of sensitivity to them (such as in a gluten sensitivity) or the presence of an offending or non-food element (which includes some preservatives in addition to other chemicals added to food) as well as the overgrowth of native bacteria and yeasts can occlude the inner surface of the intestine, slowing the transport of chyme through its length and reducing the amount of nutrients absorbed.  Also, the longer the chyme remains in the intestine, the more chances there are that gases will be produced from the chyme in a process similar to fermentation.  These added gases increase the pressure inside the small intestine, which can alter the shape of the organ, further reducing its efficacy as well as create sensations of pressure which lead to abdominal discomfort.

Speaking of bacteria and yeasts, there is one more organ that is involved in digestion that should be touched on before the large intestine is covered.  It’s not an active part of the system as it doesn’t actively transport food or secrete a substance which aids in digestion, but instead is thought to act as a storehouse for the healthy bacteria that live in the intestines and which aid in digestion.  For a long time, western medical science believed this organ to be vestigial, but recent research has revealed a potential purpose for it.

The Appendix

This small organ was once thought to be a vestigial element of the digestive system that was nothing more than a site for potential health problems.  A burst appendix is a life threatening emergency that can require extensive surgery and medical intervention if not caught in time.  Recent additions to medical knowledge have caused that opinion to change in recent years, however.  The current medical opinion is that the appendix exists as a storehouse for helpful bacteria that live symbiotically in the gut and which exists to ‘reboot’ the digestive system in cases where that bacterial population has been killed off or is otherwise compromised.  Before moving on, a closer look should be taken at the role microorganisms in the gut play a role in the digestive process and how there presence is so necessary that a system that can replace them would be helpful in maintaining good health.

Because of the variety of foods that are consumed, at some point in the evolution of living beings, a symbiotic relationship was began with certain microorganisms such as bacterias and yeast.  In return for a safe environment to grow and exist, these organisms provide living beings with aid in converting food into usable nutrients as well as fighting off other, more harmful organisms that might cause disease.  Predominately found in the large intestine (with a smaller population also occupying the small intestine), these organisms aid their host in a variety of mutually beneficial ways.  While the host can survive without them, functions such as immunity, maintenance, and overall metabolism are negatively impacted.

In mammals, these bacteria are passed along from the mother during birth (during passage through the birth canal) and breast feeding.  As they are some of the first microorganisms the newborn is exposed to, the immune system regards them as a natural presence and, with the exception of certain immunological conditions, doesn’t react to their presence.  As the young mammal develops and is weaned, they aid the body in breaking down foods that were a common part of the mother’s diet.  Some of the problems that exist with a radical diet change is the immune system reacting to either unknown bacteria entering the digestive tract or a lack of the proper bacteria to break down foods.  Common negative reactions include gas, bloating, and diarrhea, which can all be symptoms of the body improperly breaking down unfamiliar foods.

If the populations of these native bacterias are decimated, such as through antibiotic use, current theory holds that the appendix will then release some of it’s storehouse of “native” bacteria to help restore the body back to equilibrium.  Since most of these bacteria live in the large intestine, the location of the appendix is extremely fortuitous, as it is close enough to the beginning of the large intestine that it can deposit its load of bacteria in a way that the organisms can distribute themselves equally throughout the organ.  Its proximity to the small intestine also allows for some of the bacteria to migrate back up to the small intestine where they can aid that organ as well, though to a lesser extent.

Candida, a yeast, is another example of a native microorganism.  Inside the body, candida helps with extracting needed energy from sugars.  In recent history, however, due to the increase in sugar in the average person’s diet, the chances that the population of candida will surpass healthy levels of equilibrium has risen greatly.  If it becomes too great, this yeast can contribute to abdominal upset through increased levels of fermentation, greatly increases the levels of gas produced in the large and small intestines as well as taking on a parasitic relationship with the body, damaging the host as it overgrows.  One advantage of maintaining healthy bacteria in the body is that the native gut flora will act to control the growth of candida in the body.  When those bacteria are killed off through medications or poor diet, the odds that the yeasts will engage in massive overgrowth increases dramatically.  The immune system will attempt to control this to a degree, but it’s also a good strategy to replace the native bacteria as quickly as possible, using probiotic foods and supplements in addition to the storehouse of bacteria in the appendix to return the body to a state of equilibrium as quickly as possible.

The Large Intestine

The final organ involved in the digestive process is the large intestine.  This is where the water the body uses in the breakdown of food is resorbed and the last of the nutrients are synthesized and absorbed before the elements that can’t be used are combined with waste products from the body and expelled as feces.  As mentioned in the last section, the largest concentration of bacteria in the body can be found in the large intestine.  These bacteria break down the remaining nutrients that were not absorbed by the small intestine and convert them into substances the body can use.  Vitamin K, a nutrient essential in the creation of clotting factor and the prevention of hemorrhages, is one major example.  Without the bacteria in the lower intestine, the amount of vitamin K provided by the daily diet would be insufficient for most individuals.

Water reclamation is also a very important function of the large intestine.  The body’s ability to hold on to as much water as possible can be very impressive, especially when the amount lost through urination, respiration, and perspiration is figured in.  Since the large intestine is a mostly closed system, it’s one of the few locations where the body has a significant control of the moisture used.  That is the reason why in cases where a person becomes dehydrated, constipation and dry, hard stool are one of the first signs.  Most of the substances that the body produces and absorbs which can be toxic in larger amounts (such as sugar, mentioned above) are disposed through urination.  In cases where a person cannot urinate, negative symptoms start appearing fairly quickly.  Other substances, which may be less toxic, but which can still create problems if too concentrated in the body, are expelled through the feces.  When there is insufficient water intake to properly hydrate both functions, water is pulled in greater amounts from the large intestine in order to ensure that urination can still occur, hopefully long enough for a safe source of hydration to be found.

While the large intestine does use peristalsis to move substances along its length, it, like the small intestine, benefits greatly from voluntary movements of the body such as walking.  Contraction and extension of the muscles of the abdomen and hips aids in the breaking down the contents of the large intestine making it easier to absorb nutrients from them and moving them through the length of the organ.  In fact, leg movements might even be more beneficial to the large intestine than the small, as constipation is a major complication of a sedentary lifestyle.  A factor in this might be that the psoas major, a muscle which initiate ship flexion (or bringing the knees towards the chest), crosses the large intestine as the muscle passes through the abdomen.  When this muscle is tight, it can obstruct the large intestine, contributing to constipation by applying pressure to the organ, making it more difficult to pass the digested material through to end of the digestive tract.  Speaking of muscles, there is one last thing that should be mentioned before this article on the digestion can be closed out.


It may seem like a simple thing, but poor posture can have an extremely negative effect on the ability of the digest food and expel waste.  Humans have evolved to predominately be either standing, squatting, or lying flat.  In modern society, however, more time is spent in the seated position, where slouching often leads to the digestive organs being compressed, especially the organs found below the belly button.  In proper seated posture, the hips should be below the shoulders and the hips should bent to ninety degrees, ensuring the torso is as aligned as possible.  When the torso is curled or bent, the organs of digestion become folded, which makes the smooth movement of digesting food through the intestines more difficult as there is less room for them to pass.  This means that food passes through the system more slowly, which increases the amount of gas released, as well as the amount of food that may still be in the system when the next meal is consumed.  Either alone or combined, these factors increase the pressure in the system, increasing discomfort and decreasing digestive efficiency. Therefore, a simple step to increase digestive health is to maintain good posture whenever possible.


So concludes another chapter in the user’s guide to the human body.  Hopefully the information found within will be helpful in understanding why the human form functions the way it does and may even provide a few answers to lingering questions.  Thanks for reading, and as always, be safe and be healthy.