Video: Digestive system

Did you know that there's not one, but two external surfaces which our bodies must protect? The first one, of course, is our skin which we're all familiar with. The second one is not often considered an external surface by most people, but believe it or not, the inside of the digestive tract is actually outside of our bodies. Think of it like this. Our body is effectively built like one big giant meaty doughnut. Yes, I said doughnut. In its simplest form, think of our digestive tract as a long nine-meter external passage or tunnel which runs right through our body beginning at the mouth and terminating at the other end as the anus.

Interestingly, though, just because our food passes inside this tunnel does not mean it passes inside our body. In fact, it's actually purposefully kept on the outside. Of course, it's all a little more complicated than that, otherwise, when we eat food and drink liquids, they'd just fall out the other side almost as soon as we put them in our mouths, right? But this doesn't happen, of course, because the digestive tract is a specialized system composed of different organs in sequence which are adapted to carry out a particular function that is important to the process of digestion.

In this tutorial, we'll be discussing the digestive tract quite literally from the beginning to the end, specifically, the organs of the digestive system and the accessory organs. So let's waste no time and let's explore the digestive system.

First, let's ask ourselves the obvious question, why do we eat? The simple answer, of course, is we eat to live. But what does this mean and why do we digest the food we eat anyway? Well, our bodies need energy for fuel in order to carry out all normal body functions such as breathing, circulating our blood, cleaning our bodies of waste products, and the list goes on and on, and we get this energy from food.

The food we eat is composed of complex macromolecules. For example, meat, to put it simply, is composed mainly of protein and fats while bread contains starch and other carbohydrates. When we ingest food, our body means to break down the macromolecules which are too large or complex for our bodies to absorb or process properly into smaller molecules such as amino acids, fatty acids, or sugars. These smaller molecules can more easily be absorbed by the body for energy as well as components needed for cell growth and cell repair.

The whole process of digestion can be summed up as ingestion of food and drink, propulsion of the food along the digestive tract, digestion or breaking down of the food ingested be it mechanically or chemically, absorption of nutrients and water to fuel the body, and finally, elimination of any waste products. So let's take a look at the alimentary canal.

The alimentary canal, also known as the gastrointestinal tract or digestive tract, is the part of the digestive system that at all times is in direct contact with the food bolus – the food that is ingested. It is comprised of several organs, all of which we'll discuss in this tutorial. We'll also be having a look at what are known as accessory organs which are indirectly involved in the process of digestion. They do not come into direct contact with the food bolus, however, they are active in the digestive process and affect the food bolus indirectly. But more on that a little bit later.

So let's start from the beginning of the alimentary canal with the place food begins its journey – the oral cavity proper a.k.a. the mouth – and it's the inlet or the entrance to the alimentary canal. So when we talk about the oral cavity, this encompasses the lips, the teeth, and the anterior two-thirds of the tongue to a point known as the isthmus of fauces which makes up its posterior border.

Now it goes without saying that when we put food in our mouths, it is then physically broken down through chewing, a process known as mastication to us anatomy nerds, but physical breakdown is not all that's happening here. Chemical digestion is already also at work at this point through enzymes present in our saliva which take the first steps in breaking complex carbohydrate molecules into simpler ones. And this is the first step in the digestion process.

The saliva secreted into the mouth or oral cavity is produced by the salivary glands which are accessory digestive organs, but we'll talk about them a little bit later on. For now, let's follow the mashed up ball of food now known as a bolus as it makes its way through the digestive tract.

The next step along the alimentary canal is the pharynx seen here highlighted in green and known in layman's terms as simply the throat. The pharynx is an approximately twelve point five centimeter long tube that runs from the nasal cavity, the oral cavity, and into the upper part of the esophagus and the respiratory tract. When we open our mouths wide enough, we can see part of the posterior wall of the pharynx just behind the isthmus of the fauces that we described earlier.

The pharynx connects the oral cavity proper to the esophagus for the transmission of the bolus down the digestive tract after mastication – that is, when the food bolus is swallowed. Another important function of the pharynx is that it connects the nasal cavity to the larynx and subsequently the trachea for the passage of air in and out of the lungs. The pharynx can be anatomically divided into three regions – the nasopharynx, the oropharynx, and the laryngopharynx. We're not going to be discussing the nasopharynx today as it is not part of the digestive tract, but we'll look at the oropharynx and the laryngopharynx.

Keep in mind though that the nasopharynx comprises the superior part of the pharynx extending from the fornix of the nasal cavity to the soft palate of the oral cavity. The oropharynx is the second part of the pharynx and you can see it highlighted here in green. Situated between the nasopharynx superiorly and the laryngopharynx inferiorly, the oropharynx can quite easily be seen when the mouth is open wide. If you remember, we saw this image earlier and talked about the posterior wall of the pharynx. And what we can see to be more precise is the posterior wall of the oropharynx.

If we look at the pharynx from a posterior view, we can get a better idea of roughly where the oropharynx begins and ends and you can see in this image that the posterior wall of the pharynx has been removed allowing us to look at it from the back. You can see the area of the oropharynx highlighted in green and here you can also see the posterior third of the tongue which can be used as an anatomical landmark for this part of the pharynx.

Situated inferior to the oropharynx is the final part of the pharynx known as the laryngopharynx, and this portion of the pharynx is also known as the hypopharynx and it begins at approximately the level of the hyoid bone and it is continuous with the oropharynx superiorly which we saw previously and with the larynx and with the esophagus inferiorly ending approximately at the level of the cricoid cartilage.

Looking at this image on the right, we're looking at the pharynx again from the posterior aspect with all the structures of the posterior neck such as the posterior wall and the vertebrae removed, and we can see that it is covered by a mucous membrane and also identify the opening into the larynx as well as the epiglottis.

The laryngopharynx is also a passageway for both the food bolus and air and it is the epiglottis which plays an important role in guiding them down the correct tubes with air going into the larynx and food being directed into the esophagus.

So we're now continuing distally into the esophagus highlighted in green and the esophagus is basically a long fibromuscular tube measuring approximately twenty three to thirty seven centimeters in length and is around about two centimeters in diameter. It extends from the pharynx to the stomach coursing down through the thorax in the posterior mediastinum and passing through the diaphragm through the esophageal hiatus to reach the stomach in the abdominal cavity.

The esophagus is not involved in the digestion of the bolus passing through, rather in the propulsion of it further along the alimentary canal, and this means that the sole purpose of the esophagus is to transmit the food bolus and liquids from the pharynx to the stomach to continue the digestion process. The muscle fibers of the esophagus contract and relax in order to transmit the food bolus down this tube to the stomach in a process known as peristalsis. It should be noted that although the esophagus is a long muscular tube, it can be broken down anatomically into three parts – the cervical part, the thoracic part, and the abdominal part.

The next stop for the food bolus along the digestive tract is the stomach, and here in this illustration, you can just about make out the stomach highlighted in green, but our view of it is slightly obstructed by its adjacent organs. For a better view of the stomach, let's remove all of the surrounding organs and tissues and focus specifically on the organ itself.

So the stomach wall is made up of three layers of smooth muscle coursing in longitudinal, transverse, and oblique directions. In addition, the stomach can also be anatomically divided into four regions. The cardiac part of the stomach is situated just below the diaphragm after the termination of the esophagus and here we find the gastroesophageal junction and the cardiac orifice through which food from the esophagus enters the stomach.

The fundus of the stomach is the dome-shaped part of the stomach that is formed by the upper curvature of the stomach and is filled with gas when in an upright position. The body or the corpus is the largest part of the stomach and it is this part that forms the lesser and greater curvatures of the stomach. And, finally, the pyloric part which is the most inferior part of the stomach and empties into the small intestines.

The stomach also possesses two functional constrictions of circular muscle and these are called sphincters, and we have the cardiac sphincter also known as the lower esophageal sphincter and this is situated at the junction of the esophagus and the stomach. It opens to allow the food bolus to enter the stomach and also constricts shut once the food enters the stomach to prevent it from flowing back into the esophagus. And we also have the pyloric sphincter, which is situated at the end of the pyloric part of the stomach. When it relaxes, it allows the food processed via chemical and mechanical digestion in the stomach to move into the duodenum for the next stage of the digestive process. Note that the upper and lower esophageal sphincters are functional, rather than true anatomical structures. The actual anatomical support for the upper sphincter is provided by the cricopharyngeus and inferior pharyngeal constrictor muscles, while the lower sphincter is supported by the diaphragmatic crura and phrenicoesophageal ligaments.

These smooth muscle sphincters are like gatekeepers controlling when the food bolus or liquids exit the stomach cavity. Removing the anterior wall of the stomach now we can see that the mucosal lining of the stomach is thrown into folds which are known as rugae and these contain specialized glands and secretory cells that secrete gastric juices such as hydrochloric acid and enzymes that aid in the digestion of the food bolus.

While in the stomach, food is broken down chemically by these gastric juices and enzymes and mechanically by the contraction and relaxation of the stomach muscles which make a wave-like mixing of the contents of the stomach. It is here that in the stomach that we can say chemical digestion truly gets going, but mechanical digestion also occurs here. In addition to gastric juices and enzymes, a protective alkaline mucus is secreted by mucous cells found in the stomach's epithelial lining and the alkaline mucus serves as a protective layer protecting the stomach from effectively digesting itself which really wouldn't end very well.

Travelling further down the digestive tract, the next stop after the stomach is the small intestine. The small intestines mark the beginning of what we know as the bowels and it is here that the bulk of absorption of nutrients from food and liquids takes place. Despite its name, the small intestine is named such due to its diameter in comparison to the large intestine which we're going to be discussing a little bit later.

So the small intestine also known as the small bowel is approximately six meters or twenty feet long which is about the same height as your average giraffe. It is effectively, of course, a direct continuation from the stomach which presents several anatomical features specialized for absorption as well as digestion. One such specialization are the intestinal villi folded into the mucosa of the small intestine increasing the surface area of absorption. The villi in turn also possess microvilli which even further increase the surface area. In fact, if we were to unfold all of the small intestine, its combined surface area would be enough to cover a whole tennis court. Imagine that!

Structurally, the small intestine can be divided into three parts – the duodenum which directly follows the pyloric sphincter of the stomach, the jejunum, and the ileum which is the most distal part of the small intestine. The duodenum is the first part or the first section of the small intestine and, once again, because of the location of the adjacent organs, the duodenum is obstructed from clear view. So in order to get a clearer view, let's first look at this illustration where the duodenum and the pancreas have been isolated to allow us a better view. And you may have noticed that this portion of the small intestine has a well-defined C-shape and that's because the duodenum curves around the head of the pancreas.

So remember how we talked about the next stop for the food bolus after the stomach as being the small intestine. Well, once the food has been acted upon by chemical and mechanical digestion, it is emptied into the duodenum, and at this stage, the food bolus is more of a soupy mix called chyme and even though the duodenum is only the first part of the small intestine as a whole, in itself can be divided into four parts. But don't worry, it's not that complicated.

So the four parts are the superior part which is the first part of the duodenum that starts just after the pylorus, the descending part of the duodenum which is the second part of the duodenum that courses caudally from the superior duodenal flexure – this is the part of the duodenum that contains the major duodenal papilla or the sphincter of Oddi seen here which is an opening for the pancreatic duct and the common bile duct in the small intestine and also the minor duodenal papilla which is the opening for the accessory pancreatic duct.

The horizontal part is the third part of the duodenum beginning at the inferior duodenal flexure and coursing horizontally to the left side up ‘til the start of the fourth and final part of the duodenum, the ascending part. As the name suggests, this part courses upward until it reaches the inferior border of the body of the pancreas terminating at the duodenojejunal flexure after which the small intestine is now known as the jejunum.

So as we just saw after the duodenojejunal flexure, the small intestine becomes known as the jejunum and this part of the small intestine is approximately only zero point nine meters long. In comparison to the ileum, the jejunum is more highly vascularised, has a thicker wall, and has more developed mucosal folds in the lumen, which supports the mainly absorptive role of this section of the small intestine. And, interestingly enough, the word jejunum means empty in Latin. It was given this name by the ancient Greeks who had observed that the jejunum was always empty at death which is pretty weird, right?

After the jejunum ends, the ileum which is the third and the most distal part of the small intestine begins. And this final section of the small intestine measures about one point eight meters in length. The ileum courses until its termination at the ileocecal junction which is the junction between the terminal end of the ileum and the cecum of the large intestine.

So now as we get to the large intestine, we're going to be drawing closer to the end of the alimentary canal, and the large intestine, also known as the colon, extend from the cecum to the rectum measuring about one point five meters in length and it is here in the large intestine that the absorption of water and vitamins from ingested food takes place. It’s also in the large intestine that undigested food is stored in the form of fecal matter before it is voided from the body through defecation.

Alright, so now let's take a look at parts of the large intestine. So, the first part of the large intestine is this part that we have already mentioned today, the cecum, and if you look at it, it kind of looks like a pouch or a sac. And this part of the large intestine is situated in the right lower quadrant of the abdomen and it connects the small intestine to the large intestine via the ileocecal valve.

At its inferior end is an outgrowth or elongated structure known as the vermiform appendix – and we're going to be discussing the vermiform appendix a little bit later in the tutorial – but for now, let's move on to the next part of the large intestine which is the ascending colon.

So as its name suggests, this part of the colon courses superiorly on the right side along the posterior abdominal wall. If we dissect the large intestine out and have it as a standalone organ, it would look something like what we see in this illustration. So here we can see the ascending part of the colon highlighted in green and this part of the colon is retroperitoneal and continues upwards until the right colic flexure which is also known as the hepatic flexure, due to the fact that it lies posterior to the liver and after which it becomes the transverse colon.

The transverse colon begins at the right colic flexure and courses horizontally within the peritoneal sheath to the left colic flexure which is also known as the splenic flexure. And even though it is attached to the posterior abdominal wall via its own mesentery, this is the most mobile part of the large intestine. At the left colic flexure, the large intestine turns and courses downwards and this part is rightly known as the descending colon.

At this point, the colon loses its mesentery and is once again retroperitoneal. It courses down on the left side of the posterior abdominal wall for about twenty five centimeters after which it curves medially entering the pelvis to continue as the sigmoid colon, and you can see the sigmoid colon highlighted in green. The sigmoid colon gets its name because it makes an S-shaped loop and, unlike the descending colon, this part of the colon is considered intraperitoneal as it is covered by its own mesentery known as these sigmoid mesocolon.

Situated at the terminal end of the large intestine is the rectum and the rectum is a continuation of the sigmoid colon which we just looked at, however, unlike the sigmoid colon, the rectum is not covered in its own mesentery making it retroperitoneal. The rectum descends into the pelvic floor and it has a dilation at its distal end known as the rectal ampulla. After the rectal ampulla, the rectum continues as the anal canal and the anal canal is the terminal part of the alimentary canal.

If you stuck with me so far, then we've totally made it.

The demarcation between the rectum and the anal canal is made at the anorectal junction which is at right angle formed by the rectum at the levator ani muscle and the anal canal is also a muscular tube measuring about four centimeters in length and whose wall is made up of circular muscle. So we've got two sphincters which are formed by the circular muscle. The internal anal sphincter and that's made up of visceral muscle and it's permanently contracted as a result of sympathetic tonus and it relaxes under parasympathetic influence.

We also have an external anal sphincter that is made up of skeletal muscle, and the external sphincter is under voluntary control and can be made to relax and open during evacuation of fecal matter known as defecation, otherwise, known as pooping.

Alright, so that covers the main organs of the alimentary canal. Now, let's move on to look at some of the other organs that do not directly come into contact with the food bolus but do still aid in the process of digestion, and these are the accessory digestive organs.

So there are a number of accessory digestive organs, but we are only going to be covering some of these in this tutorial. The first accessory digestive organ that we're going to be looking at is the tongue. The tongue is a muscular organ of the oral cavity and it's comprised of both extrinsic and intrinsic muscles. This is because it aids in mastication, taste, swallowing, and oral cleaning. It also aids in the formation of speech.

Anatomically, the tongue can be divided into two parts – the anterior two-thirds of the tongue which is known as the body of the tongue and the posterior third of the tongue which is known as the root of the tongue – and here we can see the root of the tongue from two different perspectives and this part of the tongue is part of the oropharynx.

The next accessory organs were going to be looking at are the salivary glands, and there are three main glands that produce and secrete saliva into the oral cavity – the submandibular glands, the sublingual glands, and the parotid glands. But today, we're only going to discuss the first two.

Okay, so you may be thinking, “Oh gosh, we just went from talking about fecal matter to saliva.” Well, the production of saliva plays an important role in the digestive process because it contains enzymes such as amylase which catalyze the breakdown of starch as well as lubricates the oral cavity thereby aiding in chewing, swallowing, and taste. As I said, we'll only look at two of the three salivary glands – the submandibular gland and the sublingual gland.

The submandibular gland is the second largest of the three salivary glands, but in spite of that, this gland produces the largest amount of saliva of all three glands. The saliva produced by this gland is secreted into the sublingual space through the submandibular ducts that are also known as Wharton's ducts. The sublingual gland is the smallest of the three major salivary glands and, unsurprisingly, it secretes the least amount of saliva of the three glands. It empties its secretions by numerous ducts and the largest sublingual duct is known as Bartholin’s duct and the minor sublingual glands which can number between eight and twenty are known as the ducts of Rivinus. And these empty the secretions from the sublingual gland into the floor of the oral cavity.

Next, we'll look at the palatine tonsils. So, tonsils are masses of lymphoid tissue that play an important role in our immune system. For the purposes of this tutorial, however, we're only going to be looking at the palatine tonsils. And the tonsils are located between the palatoglossal and palatopharyngeal arches, and here you can see them from an anterior view with the mouth open wide to reveal the tonsils.

The palatine tonsils are part of Waldeyer’s ring which is a collection of four types of tonsils arranged in a ring around the oropharynx and the nasopharynx, and these serve as a type of surveillance system for all kinds of bugs entering the body.

The next accessory digestive organ that we'll look at is the liver, and the liver is situated in the upper right quadrant of the abdomen. It is a multifunctional organ that carries out detoxification, glycogen storage, protein synthesis, and hormone production, to name a few. And the liver is an intraperitoneal organ that is invested in visceral peritoneum except for the portion of the liver that is in contact with the diaphragm known as the bare area.

In addition, the liver possesses four anatomical lobes and these can be subdivided into smaller segments. It also comes into contact with the duodenum, the hepatic flexure of the colon, the transverse colon, the right kidney and the suprarenal gland, the diaphragm, and the gallbladder.

The gallbladder seen here highlighted in green is also an accessory digestive organ as we just saw and it's situated in a fossa between the left and the right lobes of the liver. And the gallbladder stores and concentrates bile that is produced by the liver and it systematically releases the bile into the lumen of the descending part of the duodenum via the bile duct. The bile aids in the digestion of the food bolus in the small intestine and also facilitates fat absorption as well as elimination of certain products such as excess cholesterol and hemoglobin from damaged red blood cells.

The pancreas is another accessory digestive organ, and here you can kind of make out the pancreas highlighted in green, although our view is obstructed by the stomach and the liver. In this illustration, the stomach has been removed and the liver retracted to give us a clearer view of the pancreas. So the pancreas is a retroperitoneal organ that possesses both exocrine and endocrine glands. The exocrine glands of the pancreas produce digestive enzymes that are secreted into the lumen of the duodenum via the main pancreatic duct and the accessory pancreatic duct to act on the food bolus.

So if you remember when we looked at the duodenum, we saw the major duodenal papilla and the minor duodenal papilla in the wall of the descending duodenum and these openings for the main and accessory pancreatic ducts respectively. The endocrine function of the pancreas also plays an important role in the maintenance of normal blood sugar levels through the production and the release of insulin and glucagon.

The final accessory organ we'll look at is the vermiform appendix, and we saw this structure briefly when we looked at the cecum. The vermiform appendix is a blind-ended muscular tube attached to the dorsomedial end of the cecum measuring from between two to fifteen centimeters in length depending on the individual. Initially, the purpose of the vermiform appendix was not known and it was thought to be an evolutionary organ that no longer had any relevant function, but now, we know that it's actually part of the gut-associated lymphatic tissue and, as such, it fulfills immunological functions.

Okay now that we've looked at the main and accessory organs of the digestive system, let's have a look at some of the diseases of the digestive tract that affects hundreds of thousands of people worldwide – gastroesophageal reflux disease.

Gastroesophageal reflux disease or GERD is the chronic irritation of the esophagus, pharynx, and, sometimes, the larynx as well. And this irritation is caused by the flow back of gastric acid from the stomach into the esophagus and back up the alimentary canal, a phenomenon known as reflux. And this condition is also more commonly known as acid reflux.

So, there are a number of factors that can cause GERD including gastrointestinal hernia, weakness in or an insufficient lower esophageal sphincter, obesity can also be a contributing factor, pregnancy is also another factor, and delayed stomach release or even connective tissue disorders like scleroderma can play a factor. Certain habits can also exacerbate or increase the risk of GERD such as eating certain foods that may trigger or aggravate the condition like foods that are heavy in fats, smoking, drinking certain beverages that may also aggravate the condition such as coffee or alcohol, certain medications can also have an effect for example aspirin, and even overeating or eating late at night.

The patients suffering from GERD often experience difficulty swallowing, chest pain or a burning sensation commonly known as heartburn in the chest that usually occurs after eating or late at night, feeling like that they've got a lump in their throat, and regurgitation of stomach contents or a sour liquid. But now for some good news. These symptoms can usually, if not too severe, be treated using over-the-counter medications such as antacids or PPIs, otherwise, known as proton pump inhibitors or sometimes with the help of prescription medications.

In severe cases, surgical measures may have to be undertaken using procedures such as fundoplication to strengthen the lower esophageal sphincter by wrapping the top part of the stomach around it or a small magnetic-beaded ring known as a LINX device can also be placed at the gastroesophageal junction to prevent the stomach contents from being pushed back into the esophagus.

So that brings us to the end of this tutorial. Let’s quickly recap what we've covered so far.

So we talked about how the digestive system consists of digestive organs and accessory digestive organs. We first talked about the digestive organs starting with the oral cavity proper then we move down the digestive tract to the pharynx where we talked about two of the three parts of the pharynx that are associated with the digestive tract – the oropharynx, and the laryngopharynx.

Then we went on further to look at the esophagus followed by the stomach and continues to move down the alimentary canal by talking about the small intestine and its three anatomical parts namely the duodenum, the jejunum, and the ileum. Next, we moved on to the terminal part of the alimentary canal which was the large intestine and we discussed the parts of the large intestine in some detail, and these included the cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, the rectum, the anal canal, and the anus.

And then we looked at some of the accessory digestive organs such as the salivary glands, the tongue, the tonsils, the liver and the gallbladder, the pancreas, and the vermiform appendix.

I hope you enjoyed this tutorial. Don't forget to check out more of our fun anatomy videos just like this one. Thanks for watching and happy studying!