In this article we will discuss the organs of the digestive system, the location of these organs, and the general functions each organ provides to aid the overall digestive process. A separate article will detail more specific information about the digestive processes. In this article we are primarily concerned with introducing the digestive organs and their functions.
The digestive system is a complex part of the human anatomy and can be challenging to understand. This is particularly true for younger individuals. Children under the age of 12 are only required to know the fundamental and largest components of the digestive system and have some general idea of the function of each such component. The knowledge requirements should generally fit the abilities and learning capabilities of the child and need not cover portions of the anatomy that may be inappropriate for young children. Students between the ages of 12 and 15, inclusive, should develop a sound appreciation for the function of each major component of the digestive system, know their general location in the body, and know the major components by their common names. Individuals age 16 and older should know the names of all components of the system as well their location and primary functions.
We do not require anyone to recall minor details such as what percentage of the total saliva output is produced by specific salivary glands under different conditions. These details are provided as points of interest and clarification. We are primarily interested in ensuring students know, to their level of maturity, what organs comprise the digestive system, what each organ contributes to the digestive process, and where these organs are located within the human body.
The following chart provides a pictorial representation of the human digestive system and each of its components. You will notice that the organs of the digestive system comprise fully half of the vital organs in the human body. We will discuss each component in more detail following the diagram.
The mouth forms the initial portion of the digestive system. It is, for lack of a better term, the intake portal. The food and liquids that will ultimately be processed by the digestive system enter the system here. The mouth contains the teeth, tongue, and salivary glands that perform the initial processing of the material to be digested.
The teeth have two primary purposes. The first is to cut and tear food into manageable pieces, and the second is to crush and grind the food into a form that is better suited for subsequent digestive processes.
There are four types of teeth in the human mouth. These are the incisors, canines, premolars, and molars. We will discuss each type of tooth working from the front of the mouth toward the teeth closest to the throat.
The incisors are the eight teeth in the front of the mouth. They are used for cutting off pieces of food to be subsequently chewed by teeth toward the back of the mouth. If you bite an apple these are the teeth that cut out a piece of the apple to be consumed. These teeth are flat with a sharp upper edge. They also have a single root.
There are four canine teeth that are used for tearing off pieces of food. These teeth are located just to the back of each set of incisors. There are two canine teeth on the bottom set of teeth and two canine teeth on the upper set of teeth. They have a single sharp point designed to pierce and shred tough food such as meats. These teeth have a single root.
The premolars are a bit of a cross between a molar and a canine. These eight teeth have a fairly broad base, like a molar, but have several sharp pointed protrusions similar to the points on a canine tooth. These teeth, also call bicuspids, have a single root. These teeth help perforate and begin the grinding process for foods being consumed.
There are twelve molars in the human mouth. These teeth are large with relatively flat surfaces and are the teeth set furthest back in the mouth. They are used to grind food into a paste that can be easily swallowed.
A tooth is commonly thought of as having two sections. The crown is that portion of the tooth that protrudes largely above the gum line. The root is primarily that portion of the tooth that resides below the gum line. As we will see, these two portions of a tooth are composed, in part, of different materials.
Each tooth is a living organ composed of four separate parts. The exposed part of the tooth is called the enamel and is comprised of the hardest material in the human body. The crown is the portion of the tooth which is covered in enamel and protrudes above the gum line. As the tooth submerges below the gum the enamel ends and an outer layer called the cementum begins. Cementum is softer than enamel and primarily covers the root of the tooth (depicted as the white layer between the gums and the Dentin in the diagram at right). Inside the outer coverings comprised of the enamel and the cementum is the dentin. This is a softer part of the tooth and is present in both the crown and the root of the tooth. The innermost part of the tooth is called the pulp and this is where the nerves and blood vessels of the tooth are found.
The tongue is a large muscle in the floor of the mouth that has four primary purposes. First of all it helps manipulate food so it can be fully chewed by the teeth. The tongue is extremely mobile and flexible allowing it to reach nearly every part of the mouth. This allows the tongue to move food from other areas in the mouth to the teeth where the food can be fully ground. This flexibility in movement also allows the tongue to reach and clean the teeth to dislodge residue that has stuck to the teeth.
The tongue also contains numerous taste buds along its upper surface and is the primary way in which you taste food. Taste buds allow us to taste things that are salty, sweet, spicy, sour, bitter, and what is referred to as Umami, or pleasant tasting. In some cases the taste buds can identify toxins or foods that are not fit for consumption.
The tongue is also critical to the process of swallowing. The front of the tongue rises so the material to be swallowed moves to the back of the mouth, and then the back of the tongue descends to pass food from the mouth to the pharynx.
A final function of the tongue is to aid in the production of various sounds that comprise human speech.
There are innumerable salivary glands in and around the mouth. These glands produce two different types of fluids referred to as serous and mucous fluids, which collectively from our saliva. Saliva is approximately 98% water. The remaining components in saliva are various minerals and enzymes that help provide protection and hasten the digestive processes.
Serous fluids are thin watery fluids that contain an enzyme called amylase which begins the digestion of carbohydrates by converting starches into simple sugars.
Mucous fluids are thicker and more viscous fluids (often described as being somewhat slimy) that help chewed food move more readily through the remainder of the digestive system. These fluids also contain Lysozyme and Immunoglobulins that provide the first level of immunity-defense against food-borne pathogens.
An enzyme called lipase is also found in saliva in limited quantities. Lipase begins to break down fat by converting it to fatty acids.
Saliva has the following essential functions
- Moisten food so it can be formed into a small, soft, and easy to swallow mass called a bolus.
- Moisten dry food so it can be more readily tasted by the tongue.
- Begin the digestive process for carbohydrates and fats
- Provide immunity protection against food-borne pathogens
- Provide continual cleaning of the teeth, including breaking down food particles caught between teeth.
- Provide a mineral rich coating for the teeth that helps forestall decay and inhibit the loss of phosphate and calcium in teeth.
- Maintain moisture in the mouth that helps protect sensitive tissues
There are several salivary glands in the human body. The three largest glands are referred to as the Major Salivary Glands. The largest of these salivary glands are the parotid glands. These glands, located on either side of the head in the region between the jaw and the ear, produce 30% of the saliva in humans. The parotid glands produce only serous fluids, which are delivered via the Stensen duct to the upper part of the mouth in the vicinity of the second rear molar on each side of the mouth.
Nestled inside the back portions of the jaw bone and below the floor of the mouth on either side of the jaw, are two salivary glands called the submandibular glands. These walnut-sized glands produce both serous and mucous fluids which enter the floor of the mouth, forward of the tongue, via Wharton’s duct.
The Sublingual glands are situated under the floor of the mouth and generally below the canine teeth. These are small glands, roughly the size of an almond, and produce only about 5% of the saliva. These glands predominantly produce mucous fluid which is delivered via numerous ducts into the floor of the mouth forward of the tongue.
The amount of saliva produced by these glands varies. When you are sleeping very little saliva is produced. During mealtimes the maximum amount of saliva is produced. Throughout the active part of the day these glands produce the following percentages of your saliva.
These percentages represent the total amount of saliva produced by the Major Salivary Glands. The above values total 100%, but this does not mean these glands produce 100% of an individual’s saliva. In reality the Major Salivary Glands produce roughly 90% of a person’s saliva.
There are approximately 900 additional salivary glands in the mouth and throat. These are collectively called the Minor Salivary Glands and primarily produce mucous fluid. These glands are located in the inner lips, inner cheeks, hard palette, and floor of the mouth. They can also be found in the larynx, trachea, and base of the tongue. If you rub your tongue on the inside of your lips and cheeks and notice a great many small bumps, these are primarily your minor salivary glands. These glands have individual small ducts that excrete saliva directly into the oral cavity. Together the minor salivary glands produce just under 10% of the total volume of saliva. The salivary glands at the base of the tongue primarily produce serous fluid.
The pharynx is a relatively long muscular tube that extends from the base of the skull downward behind the nasal and oral cavities to the middle part of the neck (near the C6 vertebrae). It is used by both the respiratory and digestive systems and is divided into three primary sections, the Nasopharynx, Oropharynx, and Laryngopharynx. In general terms the pharynx is what you generally would refer to as your throat.
The Nasopharynx is the portion of the Pharynx that extends down from the back of the nasal cavity to the soft pallet at the back of the oral cavity. The Eustachian tubes connect from the ears into the Nasopharynx to help drain the inner ear and to maintain pressure in the ear as atmospheric pressures change.
At the back of the Nasopharynx is a cluster of lymphatic tissue called the pharyngeal tonsil, or more commonly referred to as the adenoids.
The Nasopharynx is part of the respiratory system and is not part of the digestive system. But if you have ever coughed a fluid up through your nose you now know how that can occur.
The portion of the Pharynx that extends from the soft palette down to the base of the tongue is called the Oropharynx. Air, food, and liquids all pass through this common part of the throat so the Oropharynx is part of both the respiratory and digestive systems.
On the outside walls (left and right side) of the Oropharynx are the palatine tonsils. These are lymphatic tissues that are often seen on either side of the throat when looking into the mouth. It is not uncommon for these tissues to become chronically inflamed leading to their surgical removal.
The Oropharynx is the only portion of the pharynx that can be viewed through the open mouth without the use of specialized instruments.
The Laryngopharynx, also called the Hypopharynx, is also part of both the respiratory and digestive systems. The primary function of this portion of the Pharynx is to route air to and from the windpipe and to route swallowed food and liquids to the Esophagus.
When you swallow the larynx rises and the Epiglottis tilts down and toward the back. This combined action helps to close off the windpipe (trachea) so that food is directed into the esophagus. When you breathe, the Epiglottis returns to a more vertical orientation and the larynx descends, opening the airway to allow air to pass into and out of the lungs.
The esophagus extends from the laryngopharynx down to the stomach. It is a muscular tube that is approximately eight inches in length in a typical adult with a group of muscles at either end. A group of muscles near the stomach, called the lower esophageal sphincter (LSC), are controlled by the autonomous nervous system and normally prevent stomach acids from entering the esophagus. The muscles near the top of the esophagus, called the upper esophageal sphincter (USC), close during breathing to keep air from entering the esophagus. They open during swallowing to allow food and liquid to enter the esophagus. They also open during belching and vomiting to permit gasses and stomach contents to pass upward into the pharynx.
The esophagus passes through the lower part of the neck in front of the vertebrae. As it descends it passes behind the heart and then penetrates through the diaphragm. It then enters the upper part of the stomach.
After you swallow a bolus a series of muscle contractions called peristalsis cause a wave-like action within the esophagus that pushes the bolus in one direction toward the stomach. Once begun, this wave travels the entire length of the esophagus. If a bolus does not move into the stomach quickly enough then a series of sensors in the esophagus detect its presence and the peristalsis process is repeated until the bolus has exited the esophagus.
The stomach is a large muscular organ that produces acids and enzymes that aid in the digestion of food. It is located on the left side of the upper abdomen. A typical adult stomach will hold approximately one liter of material, but may in some cases hold up to 1.5 gallons of foods and liquids.
The stomach continues the digestive process by releasing hydrochloric acid and protease enzymes. Protease enzymes begin to break down protein so it can be absorbed into the body. Hydrochloric acid helps to kill bacteria and provides the chemical environment necessary for some of the protease enzymes to properly function. Through a series of involuntary peristalsis actions the stomach contents are periodically churned and moved toward the small intestine. The stomach contents are converted into a digestive mixture referred to as chyme, which passes from the stomach into the small intestine.
At the bottom of the stomach is the pyloric sphincter that controls the flow of chyme into the small intestine.
Some materials may be absorbed directly by the stomach and provided to the bloodstream. This includes certain medications (aspirin, for one), water (in limited quantities), amino acids, and some substances such as caffeine and ethanol.
To protect itself from the effects of hydrochloric acid and protease enzymes the stomach is coated with a mucous secretion.
A full stomach is normally emptied within about four hours. Some materials, particularly carbohydrates, may move through the stomach much more quickly because they have already been partially digested by the enzymes in saliva.
The small intestine is a long muscular tube located in the abdomen below the stomach. In an adult the small intestine is about 22 feet in length, though there can be considerable variation in this length within the adult population. This organ is about one inch in diameter. The primary purpose of the small intestine is to absorb the nutrients and minerals found in the chyme produced by the stomach.
There are three primary sections of the small intestine. These are the duodenum, jejunum, and ileum. Each has a different primary function that aids in the digestion, absorption and recycling processes in the digestive system.
Most of the small intestine contains numerous folds and wrinkles called plicae circulara that provide increased surface area to better accommodate the absorption of nutrients. These folds also contain small hair-like projections called villi that provided additional surface area. Projecting from the villi are microvilli that offer substantial additional surface area to facilitate the secretion of enzymes and absorption of nutrients. The villi and microvilli are so numerous that they increase the total surface area of the small intestine by more than 600 times. When viewed through a microscope these villi look like the surface of a thick brush. As a result the coating of villi on the lining of the small intestine are often referred to as the brush border.
As we studied earlier in the circulatory system, the nutrients and minerals absorbed by the small intestines are transported as non-oxygenated blood via the hepatic portal vein to the liver.
The duodenum is a ten inch long C-shaped part of the small intestine that directly receives chyme from the stomach. It also receives a solution called bile from the gallbladder and/or liver and additional digestive enzymes and bicarbonates from the pancreas.
Another important function of the duodenum is the secretion of an alkaline mucous containing bicarbonate ions that work, together with the bicarbonates from the pancreas, to neutralize the powerful hydrochloric acid contained in the chyme. The duodenum absorbs iron, calcium, water, and phosphates from the digesting content, though calcium, water, and phosphates are also absorbed elsewhere in the small intestine as well.
The duodenum regulates the rate of digestion in the small intestine. It does this by signaling the stomach when to open and close the pyloric sphincter. It also signals the pancreas and gallbladder at the same time so that appropriate digestive juices can be provided into the duodenum to encourage associated digestive processes. As the duodenum empties it signals the stomach to provide additional chyme. When the duodenum has been filled it signals the stomach to close the pyloric sphincter.
Peristalsis continues to form a wave action that presses digestive content forward in the small intestine. This also helps mix the chyme, bicarbonate, bile, and digestive enzymes to ensure that the digestive processes will be quite thorough and that destructive acids are neutralized. The bile and digestive enzymes break down complex protein, carbohydrate, and fat molecules in the chyme into simpler molecular forms that can be readily absorbed and transported throughout the body.
The jejunum portion of the small intestine is located in the central part of the abdomen and is commonly about eight and a half feet in length. There is an extensive network of blood vessels and capillaries servicing the jejunum that facilitate the transfer of nutrients and minerals into the blood steam. As a result the jejunum is the portion of the small intestine where the majority of the glucose, amino acids, fatty acids, vitamins, minerals, and water is absorbed into the bloodstream. Some significant portion of the bile is also absorbed back into the bloodstream in the jejunum.
The ileum receives content from the jejunum via peristalsis action. It absorbs amino acids, vitamin B-12 and some additional water and nutrients. Importantly it also re-absorbs the majority of the remaining bile salts that were previously introduced into the duodenum by the gallbladder so that the bile salts can be recovered and recycled back to the liver.
The ileum connects to the large intestine via the ileocecal sphincter. This sphincter prevents material in the large intestine from flowing back into the small intestine.
The human liver is a large multi-function gland situated on the right side of the body. It sits to the right of the stomach, just below the diaphragm, and just above the pancreas, gallbladder, and intestines. The liver is largely, though not fully protected by the lower right rib cage. A portion of the liver is exposed under the rib cage and sternum in the vicinity of the solar plexus.
The liver is referred to as a gland because it generates chemical secretions used by other parts of the body. In an adult the liver generally weighs a little over three pounds, making it the second largest organ of the human body.
The liver receives and processes a large quantity of blood. Much of this is nutrient-rich blood received from the intestines. But a good deal of arterial blood is also supplied to enable proper functioning of the liver. The liver receives approximately 1.5 quarts of blood every minute.
There are four main lobes or physical portions of the liver. The two largest are the left lobe and the right lobe. The right lobe is the largest lobe and is largely concealed behind the right ribs. The left lobe is somewhat smaller and is largely exposed below the ribs and sternum. The caudate lobe is on the back side (posterior) of the liver near the top and positioned generally between the left and right lobes. The quadrate lobe is also on the posterior side of the liver generally between the left and right lobes, but it is situated at the bottom (inferior side) of the liver adjacent to the gallbladder. The quadrate lobe has a vertical orientation or appearance since it is longer (taller) than it is wide. All of the lobes of the liver perform the same functions. There is no functional specialization in any of the lobes.
The liver is responsible for the following functions:
- Detoxification of the blood
- Cleanse the blood by destroying aging blood cells, parasites, bacteria, fungi, and cellular waste.
- Produces several protein compounds that enable blood clotting and help regulate cellular fluid levels
- Conversion of simple sugars into glucose which are stored in the liver for future use
- Converts stored glucose into usable sugars when needed by the body
- Metabolize fats and proteins into useful nutrients and waste materials.
- Create numerous hormones used in the body.
- Stores vitamins, copper, and iron for future use and supplies them to the body when needed
- Converts ammonia to urea
- Breaks down various hormones, many of which are produced by the body’s other glands.
- Provide bile salts for the small intestines to aid in the digestion of fats.
So the liver functions much like a warehouse. It receives products (nutrients and waste in the blood), repackages them as necessary, and stores them for future use. When the need arises, it repackages these stored substances as necessary and sends them via the blood to other parts of the body where they are consumed or assimilated as waste.
Hepatocyte cells in the liver produce bile salts that are secreted into small tubes called bile canaliculi. These small tubes then connect to larger ducts, which in turn join other ducts to form ever larger ducts. The myriad ducts eventually empty into the left and right hepatic ducts. These two ducts collect bile salts from the left and right lobes of the liver and exit the liver where they subsequently join together to form the common hepatic duct.
Because of its critical function the liver is a vital organ. It functions as part of the circulatory, digestive, and immune systems.
The gallbladder is a large sack-like organ that stores bile salts until they are needed by the duodenum of the small intestine. The cystic duct connects the gallbladder to the common hepatic duct. Excess bile salts produced by the liver but not requested by the duodenum are accumulated and stored in the gallbladder via the cystic duct. The bile salts move into the gallbladder as a result of peristalsis action in the bile ducts. As bile salts accumulate in the gallbladder it swells. The gallbladder extracts liquids from the stored bile salts so that the bile salts become more concentrated. A normal gallbladder is roughly pear shaped and about three inches long and one and a half inches in diameter.
After a meal has been digested the gallbladder may be completely deflated and may contain very few bile salts. Prior to a meal the gallbladder may be quite large having stored bile salts for many hours.
When the duodenum indicates a need for bile salts the gallbladder undergoes muscle contractions that force bile salts out of the gallbladder and into the cystic duct. The bile salts then continue on, to be joined by secretions from the pancreas just prior to the duodenum. The common bile duct and the pancreatic duct join together in what is known as the ampulla of Vater. The sphincter of Oddi surrounds the ampulla of Vater and is used to regulate the flow of digestive juices into the duodenum.
The gallbladder is not a vital organ. Medical complications may require that the gallbladder be removed. When the gallbladder is removed bile salts continue to flow from the liver to the duodenum, albeit in lesser volumes and in lesser concentrations that would have occurred via the gallbladder. But in general this does not impact an individual significantly, though it may result in less efficient absorption of fats.
The pancreas is a vital organ. It produces numerous enzymes including trypsin which digests protein, lipase which digests fat, and amylase which digests carbohydrates. The pancreas also produces alkaline which is ultimately used to neutralize hydrochloric stomach acids in the duodenum.
In addition to digestive juices the pancreas generates several critical hormones. These include insulin, glucagon, and somatostatin.
Insulin is used to regulate blood glucose levels. It is used to signal the liver, muscles, and fat cells to take in glucose from blood. Insulin is produced when there is excess glucose in the blood.
If there is insufficient glucose in the blood then the pancreas secrets the glucagon hormone. This signals the liver to convert stored glycogen into glucose so that blood sugar levels are increased.
So the pancreas, via its secretion of insulin and glucagon helps keep blood sugar levels at a desirable level. The hormone somatostatin plays a role here as well. This hormone inhibits production of both insulin and glucagon. It also inhibits production of stomach acid, and various other hormones including the growth hormone and gastrointestinal hormones.
The pancreas sits laterally behind and below the stomach and is about six inches long. The head of the pancreas sits directly under the C-shaped section of the duodenum. The tail (smaller end) of the pancreas extends toward the left side of the body generally parallel to the stomach.
The pancreatic duct runs the entire length of the pancreas and collects secretions that then flow toward the ampulla of Vater.
The large intestine connects via the ileocecal sphincter to the ileum of the small intestine. The large intestine is a large muscular tube that is generally about five feet in length and about two and a half inches in diameter. It is positioned around the lower portion of the abdomen and begins on the lower right side of the abdomen. It then rises upward before moving laterally across the entire abdomen. When it reaches the left side of the body it descends downward to the rectum.
Like the small intestine, the large intestine contain numerous folds that provide increased surface area. The large intestine, however, does not contain villi.
The large intestine is divided into several parts.
The ileum deposits its contents via the ileocecal sphincter into the portion of the large intestine called the cecum. The cecum is located on the lower right side of the body just inside of the right hip bone. Here a large colony of bacteria begins to further digest the chyme. These extremely beneficial bacteria subject the chyme to fermentation processes that yield vitamins and produce gasses as a byproduct.
Descending from the cecum is the appendix. This is a small, thin, hollow organ about four inches in length and a quarter of an inch in diameter. The purpose of the appendix, if any, is not entirely known. The appendix is often removed due to inflammation or other medical conditions. Individuals usually show no adverse results following removal of the appendix. There has been some recent (since 2007) thought that the appendix may serve as a safe repository for beneficial bacteria. If disease or digestive distress eliminates beneficial bacteria in the cecum then the appendix may serve to introduce these bacteria back into the large intestine. This remains an area of study and has not yet been widely accepted as the primary purpose of the appendix.
From the cecum the large intestine moves upward along the right side of the body. This portion of the colon is called the ascending colon. The ascending colon is typically about eight inches in length. As fecal matter travels through the ascending colon bacteria continue to digest the intestine contents, emitting gasses and various vitamins in the process. The vitamins are absorbed by water in the large intestine, which in turn is absorbed by the intestinal wall and delivered into the blood stream.
At the top of the ascending colon the large intestine turns 90° toward the left side of the body. The large intestine then stretches horizontally across the entire abdomen. This section of the large intestine is referred to as the transverse colon. Bacterial fermentation and water removal continues in the transverse colon. After sufficient water has been removed the intestinal contents are converted into feces.
At the left side of the body the transverse colon turns downward 90°. This is the start of the descending colon. This ten inch long segment of the large intestine moves downward from just below the stomach to the inside of the left hip bone. The descending colon removes additional water and vitamins from the fecal matter and serves as a repository for this material until it can be eliminated from the body.
At the bottom of the descending colon is the next section of the large intestine. This section is referred to as the Sigmoid colon and transports fecal matter from near the left hip back toward the center of the body. This is an S-shaped segment measuring some 16 inches in length. Like other sections of the large intestine it continues to absorb vitamins and water from the intestinal content.
The rectum is the final part of the large intestine. It descends from the end of the Sigmoid colon downward to the anus. It stores fecal matter until it can be eliminated from the body. When full it sends nerve impulses to the brain to indicate there is fecal matter to be eliminated.
The anus is the very end of the large intestine and is short tube terminated by two sphincter muscles; the inner anal sphincter and the outer anal sphincter. These muscles can be relaxed at will to allow stored fecal matter and/or gasses to be released.