Mouth and Pharynx
This is a tutorial to introduce you to the digestive system. The digestive system is essentially a long tube, which is about six meters long and it runs all the way from the mouth right down to the anus. You can see how this tube is folded in this abdominal area. This increases the surface area for absorption of nutrients.
As well as this long tube, which is referred to as the gastrointestinal tract or the alimentary tract, you've also got accessory organs of digestion. You can see these other organs coming off the side of the tract. This large one here is the liver. You've got the pancreas sitting behind the stomach. And you've also got the gallbladder sitting behind the liver.
The digestive system consists of this long tube running from the mouth to the anus and that includes the stomach. And then you've got the three accessory organs of digestion – the liver, the gallbladder and the pancreas (which sits behind the stomach).
The purpose of digestion is to absorb nutrients from food into the body. Digestion involves breaking down the food so that it can be easily absorbed.
You've got two types of digestion. You've got mechanical and chemical digestion. Examples of mechanical digestion are chewing. And then you've got the stomach which can contract and churn. It can churn up the nutrients or the contents of the stomach. And then you've got chemical digestion which involves enzymes produces by various organs and glands. You've also got acid in the stomach and bile in the gallbladder, which help to chemically digest the nutrients from the food.
We’ll start proximally. We'll start at the mouth and work our way down and we'll take a look at some parts of the digestive system.
Digestion begins at the mouth. The mouth is formed by the hard palate and soft palate at the top. You've got the lips at the front, which you can see here. The side walls of the mouth are made up by a muscle called the buccinator muscle. If I just isolate that, you can see this muscle called the buccinator making up the side walls of the mouth. And then you've got the teeth and tongue inside the mouth.
You can see on this model, the upper teeth sit in the maxilla. This is this bone of the skull. And the lower teeth sit in the mandible.
The function of the mouth is to provide an opening for the food to enter the body. It's used for mechanically digesting the food by chewing. If you take a look at my tutorial, Muscles of Mastication, you can see how the muscles can bring about chewing movements.
And it also begins chemical digestion. You’ve got salivary glands which produce saliva obviously. The saliva consists mainly of water, but there's also some enzymes for digestion. You’ve got things like salivary amylase, which breaks down starches into sugar. You've also got lingual lipase and other antimicrobial enzymes.
You’ve got three major glands, salivary glands. You've got the parotid, submandibular and sublingual glands. If I just rotate this skull model around, I'll show you the location of these glands.
The parotid gland sits kind of like this over here. And then you've got the submandibular gland, so it lies underneath the mandible in this location. And you've got this sublingual gland, which lies under the tongue in approximately this location. You’ve got three main pairs of salivary glands to remember.
After food has entered the mouth, it passes into the pharynx. The pharynx is this muscular tube which joins onto the nasal cavity and the mouth. It connects them to the esophagus and the larynx.
The larynx is this structures anteriorly which are commonly referred to as the voice box. Behind, you've got this tube, which runs down at the stomach and this is the esophagus.
I just got rid of the larynx and you can see the trachea anteriorly (which goes into the lungs) and the esophagus, which runs behind it posteriorly. The pharynx is this bit, which I'm outlining here between the nasal cavities, the back of the oral cavity and just sitting above the trachea and the esophagus.
There are three parts to the pharynx. You've got the nasopharynx, the oropharynx and the laryngopharynx. Food passes out of the oral cavity into the oropharynx and then you've got various muscles, constrictor muscles, which contract and propel the bolus of food from the oral cavity through the oropharynx and into the laryngopharynx. And then it passes down into the esophagus below. During swallowing, the soft palate actually swings up and closes off the nasopharynx.
Oesophagus and Stomach
After the bolus of food passes from the pharynx, it enters into the esophagus, which I've mentioned before. The esophagus is another long, muscular tube. This tube is about 25 cm. long and it joins the laryngopharynx to the stomach. You could see the stomach below this sac here.
I've just brought in the muscle layer because I just want to remove it to show you where the esophagus passes through into the abdomen. I'll just gradually remove the muscle layer. Actually, the skeleton as well. I don’t know why I put that there. You can see that the esophagus passes through the diaphragm, this kind of dome-shaped muscle which helps in respiration. It passes through the esophageal hiatus and the diaphragm to enter into the abdomen.
I'll just remove that. You can see that the esophagus joins the stomach. You can see that sac-like organ sitting here. This is the stomach.
I'm just going to remove some of these accessory organs of digestion. To the right of the stomach and a little bit anteriorly, we've got the liver. I'm just going to remove that. We've got this little sac sitting under the liver called the gallbladder. I'll remove that as well.
And to the left of the liver, the stomach, you've got the spleen, which isn't an organ of the digestive system. It's an organ of the lymphoid system. I'll just get rid of that as well.
You can see the esophagus joins the stomach. The region where it joins is called the cardiac region of the stomach because I guess this is the part of the stomach that lies closest to the heart. I've just added the vessels in here and the cardiovascular system and you can see that it sits just above the stomach.
Something to point out is that at this junction between the esophagus and the stomach, you have a sphincter. This is called the lower esophageal sphincter or the cardiac sphincter because it lies in the cardiac region.
A sphincter is essentially a circular muscle in the body which keeps a passage closed. A sphincter relaxes to open and it contracts to close.
You've got the lower esophageal sphincter or cardiac sphincter at the junction between the esophagus and the stomach. The sphincter is formed by circular muscle within the walls of the esophagus. And you've also got an upper esophageal sphincter at the other end of the esophagus.
The muscular wall of the esophagus contracts in waves, which is called peristalsis. This propels the bolus of food from the laryngopharynx down the esophagus into the stomach.
Next, we've got the stomach which I'm sure you're well away of and I've pointed out a few time already. The stomach is a muscular sac and it sits in the upper left abdomen. You can see its position here relative to the surface of the body.
It also lies under the diaphragm. If I just bring in the muscles, you can see the diaphragm sitting over the stomach.
And if you remember, we've got the liver sitting to the right of the stomach. You can see that the liver has these two parts. The left part of the liver called the left lobe lies in front of the stomach. And then to the left of the stomach, you've got the spleen, which isn't a digestive organ. It's part of the lymphoid system.
This end, the proximal end of the stomach is called the cardiac region. And then at the distal end, you've got the pyloric region. And you've got another sphincter at the end of the stomach called the pyloric sphincter.
The sphincters help to keep the contents of the stomach contained. The upper sphincter, the cardiac sphincter or lower esophageal sphincter prevents contents of the stomach refluxing back up into the esophagus. You've got things like acid and other enzymes and things in the stomach that you don’t want coming back up there into the esophagus. And if you were to stand in your head or flip upside-down, you don't want all the contents of your digestive system to come exploding out of your mouth. This is the purpose of sphincters, to keep things contained.
The function of the stomach is to mechanically digest the food by churning it. The muscular walls of the stomach churn up the food and mix it with digestive enzymes. And you've also got some cells in the walls of the stomach producing acid which kills bacteria. In this way, it serves as an infective barrier.
The partially digested food of the stomach through this chemical acidic and mechanical digestion passes into the duodenum, which is the next part we're coming onto, this part here of the digestive system. This partially digested food is called chyme.
That’s the stomach and esophagus.
Intestines and Beyond
After the partially digested food known as chyme exits the stomach, it enters into the small intestine. The small intestine extends from the pylorus of the stomach all the way to the cecum, which is this little pouch at the beginning of the large intestine. It’s this little bit here.
I'll just isolate the small intestine by fading away the large intestine. You can see how it's this long, convoluted tube. The small intestine consists of three parts. The first part is known as the duodenum. It's this short little c-shaped part here. And then you've got the second part called the jejunum and the distal part is called the ileum.
It's not really important to clearly mark out the jejunum and the ileum, but you just need to know that the jejunum makes up the second part of the small intestine and the ileum makes up the distal part, so the third part of the small intestine. The jejunum roughly lies in the left upper quadrant and the ileum roughly lies in the right lower quadrant and it makes up the distal three-fifths, whereas the jejunum makes up the proximal two-fifths.
But the duodenum is this first part. It's this c-shaped part which emerges at the end of the stomach, at the pylorus of the stomach. It curves around the head of the pancreas.
The pancreas is one of these three accessory organs of digestion that I mentioned in the first part of this tutorial. This organ sits behind the stomach as you can see here. If I just fade away the other organs, you can see the pancreas. You can see how it sits in this c-shaped part of the duodenum. The head of the pancreas nests itself into this curve of the duodenum.
You've got ducts from the pancreas and ducts from the gallbladder which open up into the duodenum. This allows bile from the gallbladder and pancreatic secretions from the pancreas to enter into the duodenum and this helps in digestion.
The gallbladder is this little structure here, which sits underneath the liver (if you remember that triangular-shaped liver which sat on the right side and covered a bit of the stomach).
The duodenum ends at about this point where it then becomes the jejunum. The jejunum forms the proximal two-fifths of the small intestine and then the distal, the last three-fifths are called the ileum.
As you can see, the small intestine is highly convoluted. There's all these folds and it's really long and folded. This really serves to increase the surface area. By having a larger surface area, there's much more possibility for absorption.
The small intestine is a major site for chemical digestion and absorption. And as I showed you just now, the duodenum receives bile and pancreatic secretions to help with this chemical digestion.
As I told you before, the small intestine starts at the pylorus of the stomach and it ends at the cecum. The cecum is this little pouch and it marks the beginning of the large intestine.
The large intestine lies around the edges of the abdominal cavity. We actually can see it in relation to the skin.
The function of the large intestine is to absorb water. First, you've got this cecum, which I showed you. It's this little pouch here. This is where the ileum joins the large intestine. At this junction between the end of the ileum and the cecum, you've got the ileocecal valve.
And just below, you can see this little worm-like structure. This is called the vermiform appendix. 'Vermiform' in Latin just means 'worm-shaped'. It kind of looks like a little earthworm. This is the appendix.
And then the colon is separated into four parts. You've got this ascending colon, which is this first part on the right, which ascends. And then you've got transverse colon, which runs horizontally, so it's called 'transverse' colon. And then you've got the descending colon, which descends down into the last part called the sigmoid colon, which is this curved part of the colon.
The sigmoid colon gets its name because it's kind of s-shaped. The greek symbol, sigma is the letter s. It's kind of s-shaped, so that's the sigmoid colon.
Where the ascending colon meets the transverse colon, there's this kind of bend. This bend is called flexures. The liver sits over this right side. This bend is given the name the hepatic flexure (hepatic referring to the liver).
And on the left side, you've got this other bend where the transverse colon meets the descending colon. This is called the splenic flexure because this is where the spleen lies on the left side.
You can see that here with the liver and the spleen, so they're the hepatic flexure and the splenic flexure.
After the colon, after the sigmoid colon, you've got the rectum. And then you've got the anal canal and the anus right at the end.
The rectum serves as a temporary holding site for feces. And then the feces is expelled out through the rectum into the anal canal. And then it exits the body through the anus. Intra-abdominal pressure is increased to force out the feces from the rectum and into the anal canal and finally out of the anus. The anus is the opposite end of the mouth. It's the exit of the digestive tract.
I'll just quickly talk about the accessory organs of digestion. We've got three accessory organs of digestion excluding the salivary glands which I've talked about in the first part on the mouth and pharynx. In this tutorial, we're going to talk about the three accessory organs of digestion in the abdomen.
The three accessory organs we've got in the abdomen are the liver, the gallbladder and the pancreas.
The liver lies in the upper right quadrant. You can see this organ here, this triangular-shaped organ, which sits underneath the diaphragm. I just added in the diaphragm here and you can see it lies over the abdominal viscera.
The function of the liver is metabolic. It receives nutrient-rich blood from the gut via a vein called the portal vein, the hepatic portal vein. Blood which comes from all the other gut structures enters the liver via portal vein. The liver then processes the products of digestion. It also produces bile.
Just sitting underneath the liver, you've got this little structure, this little sac called the gallbladder. The liver which produces bile stores and concentrates its bile in the gallbladder. It passes from the liver via various ducts into the gallbladder. You can see this little sac sitting underneath the liver. You can see these various ducts (they're kind of faded out). This forms the biliary system.
When food enters into the stomach, a hormone is released which makes the gallbladder contract and the gallbladder then releases bile. And as I mentioned in a previous part, the gallbladder has various ducts which empty into the duodenum. The bile enters into the duodenum to assist in the digestion of the chyme, which enters from the stomach.
Lying behind the stomach, we've got the pancreas. You can see this structure here. The head of the pancreas sits in the curve of the duodenum and the tail extends to this organ, the spleen.
The pancreas has endocrine and exocrine functions. The endocrine functions are producing insulin and glucagon. Its exocrine function is to produce pancreatic juice, which is secreted into the duodenum and this aids digestion.
That’s the end of my introduction to the digestive system. We've gone through the alimentary tract. We've gone through the tube from mouth to anus talking about the mouth, the pharynx, the esophagus, the stomach, the small intestine consisting of its three parts, the large intestine consisting of the various parts of the colon, the cecum and then you've got the rectum and anal canal finally ending in the anus. And then you've got these accessory organs which aid digestion. You’ve got the liver, the gallbladder and the pancreas.