Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be discussing the organs of the digestive system. So what is the digestive system? The digestive system is comprised of an array of organs that work together to breakdown large molecules which are in turn ingested into smaller soluble molecules that can be absorbed into the body. This process is known as digestion. Throughout this tutorial, we'll discuss the main organs of the digestive system also known as the organs of the alimentary canal and their accessory organs. Let's begin first with the organs of the alimentary canal.
The oral cavity is the entrance to the alimentary canal and in the image on the right, you can see its inferior surfaces highlighted in green. The anterior border of the oral cavity is formed by the anterior dental arches which are around about here. Its posterior border is formed by the isthmus of fauces which is a small section of the oropharynx connecting the oral cavity to the pharynx. You can see me pointing it out just here. It's in the oral cavity that the process of digestion begin to the process of mastication. Mastication or chewing is the process of crushing or grinding food by the teeth. The oral cavity is made up of four parts – the maxilla or upper jaw, the mandible or lower jaw, the tongue and the teeth.
Situated just behind the oral cavity is the next part of the alimentary canal known as the pharynx. The entire pharynx is a 5-centimeter hollow tube which runs behind and connects the nasal cavity and the oral cavity to the esophagus and the upper respiratory tract. The pharynx which functions as a passageway for air and food as well as a resonator for the voice can be divided into three regions – the nasopharynx which is involved with respiration and houses the opening of the Eustachian tubes, the oropharynx which is the middle section of the pharynx behind the mouth and is bounded laterally by the superior and middle pharyngeal constrictor muscles, and the laryngopharynx which along with the oropharynx is a passageway for both food and air.
The next organ of the alimentary canal that begins after the pharynx is the esophagus which you can see at the base of this image. In this ventral image of the thorax and the abdomen with the thoracic viscera cut away, you can see the esophagus as a fibromuscular tube. The esophagus runs the length of the thoracic cavity anterior to the vertebral column and enters the first part of the abdomen. The esophagus can be divided into three sections – the cervical part which extends from about the level of the sixth cervical vertebral body to the level of the first thoracic vertebral body; the thoracic part which is the longest part of the esophagus extends from the level of the first thoracic vertebral body to the esophageal hiatus at the diaphragm at approximately the level of the eleventh thoracic vertebral body, this part of the esophagus is found in the superior and posterior mediastinum; and the third part of the esophagus which is the abdominal part, it is the shortest section of the esophagus and it extends from the diaphragm to the cardiac orifice of the stomach.
The musculature of the esophagus is comprised of both longitudinal and circularly running muscle fibers both of which you can see in this dorsal view of the pharynx as it leads into the esophagus – the inner muscle layer being the circular layer and the outer layer being the longitudinal one. In this image also note the epiglottis and the trachea, otherwise known as the windpipe, and their positions anterior to the esophagus. The food bolus moves down through the esophagus with the aid of a series of wave-like muscle contractions known as peristalsis.
The stomach is a hollow organ of the gastrointestinal tract located in the abdomen just inferior to the diaphragm. It can be divided into four parts – the cardiac part which is where the esophagus opens into the stomach at the cardiac orifice, the fundus which is the highest part of the stomach when we are standing in the upright position, the body or the corpus which is the largest part of the stomach, and the pyloric part which consists of the pyloric antrum and the pyloric canal. This is the part of the stomach that opens into the duodenum which is highlighted on the right in green.
The stomach is an important part of the digestive system and is essential for nutrient supply to the body. The inner surface of the stomach is lined by mucosa which contains gastric pits and glands which I'm pointing out with my arrow just here. These glands are formed by different exocrine cells which secrete various substances depending on the part of the stomach.
The small intestine is the part of the alimentary canal where most digestion and absorption takes place. It is divided into three parts – the duodenum, the jejunum and the ileum. Let's begin by looking at the duodenum.
The duodenum is the first section of the small intestine and is situated between the stomach and the jejunum. It is a C-shaped hollow loop and can itself be divided into four parts. The superior part of the duodenum which is connected to the liver via the hepatoduodenal ligament. The superior part ends at the superior duodenal flexure and lies intraperitoneally while the rest of the duodenum lies retroperitoneally. The descending part of the duodenum which begins at the superior duodenal flexure extends to the inferior duodenal flexure.
The major duodenal papilla or papilla of Vater is an elevation in the mucosa of the descending duodenum formed by the opening for the hepatopancreatic duct into the duodenum. It's around about here in this image. In many individuals, a second opening known as the minor duodenal papilla or the papilla of Santorini is also found in this part of the duodenum and it is an opening into which the accessory pancreatic duct empties. The minor duodenal papilla is around about here.
The horizontal part of the duodenum seen here in green runs from right to left ventrally from the inferior duodenal flexure and passes in front of the abdominal aorta and the inferior vena cava. The final part of the duodenum is the ascending part. This part of the duodenum runs cranially along the left side of the vertebral column and ends at the duodenojejunal junction.
The jejunum which lies intraperitoneally begins at the duodenojejunal junction and is mostly situated in the upper left quadrant of the abdomen. On the other hand, the ileum is mostly found in the lower left quadrant of the abdomen and the ileum is thinner and its walls are less vascularized than those of the jejunum. The ileum extends to the ileocolic junction. Together, the jejunum and the ileum can reach a length of approximately 6 meters.
The large intestine is the terminal part of the alimentary canal. It is here that the final absorption of nutrients and water takes place before excretion. The large intestine is made up of 8 parts – the cecum, the appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, the rectum and the anal canal. Let's now go through the structures over the following slides.
The cecum is an intraperitoneal section of the large intestine. This pouch-like section of large intestine connects the large intestine to the ileum of the small intestine via an invagination known as the ileocecal valve which is this structure just here. This part of the large intestine is bound to the abdominal wall via the cecal folds of the peritoneum. Arising from the posteromedial side of the cecum is a 4 centimeter tubular structure known as the vermiform appendix. It is not uncommon to have this small structure surgically removed when it becomes inflamed due to a condition known as appendicitis. The surgical removal of the appendix is known as an appendectomy.
The next part of the large intestine is the retroperitoneally-situated ascending colon. It extends cranially from the cecum to the right colic flexure otherwise known as the hepatic flexure due to its proximity to the liver. The next part of the large intestine is the transverse colon. This horizontally running part of the large intestine extends from the right colic or hepatic flexure to the left colic or splenic flexure of the large intestine. The transverse colon is intraperitoneal. It is the largest of all the sections of the large intestine as well as the most mobile. The descending colon begins after the splenic flexure and extends caudally to the sigmoid colon. The descending colon runs through the retroperitoneal space. In contrast to the descending colon, the sigmoid colon is intraperitoneal. This S-shaped loop of the large intestine is suspended via the sigmoid colon and extends from the end of the descending colon to the rectum.
The rectum is also intraperitoneal but becomes retroperitoneal as it descends into the pelvic floor to the anal canal. The anal canal begins at the lower end of the rectum. This is marked by the anorectal junction after which the anal canal extends from the pelvic floor to the anus.
So now that we've seen the organs of the alimentary canal, let's look at some of the accessory organs that aid in digestion. There are several accessory organs that are related to enzymatic activity. The salivary glands are accessory organs of the alimentary canal that are positioned in and around the oral cavity. These exocrine glands secrete their salivary contents into the oral cavity in order to lubricate and protect the mucosa of the oral cavity as well as help with the initial stages of digestion during mastication.
There are three types of salivary glands – the parotid glands, the submandibular glands and the sublingual glands. In addition to the salivary glands, the liver also releases several enzymes as it processes nutrients absorbed into the blood from the food bolus. The liver also stores nutrients and filters the blood as well as reduces bile that is stored in the gallbladder – another accessory organ of the alimentary canal. In this image of a ventral view of the abdominal cavity with the liver pulled away, you can see the gallbladder highlighted in green as it sits on the inferior aspect of the liver. The bile stored in the gallbladder is secreted into the duodenum when it is needed for lipid breakdown in order to increase the absorptions of fats.
The pancreas also secretes digestive enzymes and basic enzymes such as those that neutralize chyme into the duodenum. In addition, it secretes insulin into the bloodstream to increase glucose uptake by the cells. In this image, you can see a ventral view of the pancreas highlighted in green as it sits within the inner border of the duodenum. The taupe-colored tube visible on the left of the image is the bile duct. The liver, gallbladder and the pancreas are all located within the upper right quadrant of the abdomen.
Lastly, there are three accessory organs that provide immune protection to the alimentary canal. Starting at the top, the tonsils provide immune protection against noxious substances and the byproducts of the food bolus. In the images on our right, you can see the palatine tonsils on the left and the lingual tonsil on the right. Similarly, the spleen also provides immune protection against noxious substances. On the image on the right, you can see the spleen from our lateral right view, its superior aspect being along this side and its inferior aspect along this side. The anterior extremity of the spleen is here. In the second image of the spleen which is a ventral view of the abdomen with the liver pulled back, you can see the spleen's superior margin highlighted in green as it sits behind the body of the stomach.
Finally, the lymph nodes of the digestive tract also act as accessory organs of the alimentary canal by governing the lymphatic drainage of the gastrointestinal tract. In this image, you can see the lymph nodes of the duodenum highlighted in green.
Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.
Now, good luck everyone, and I will see you next time.