Earlier in the series we learned about the digestive system. This system is centered around the gastrointestinal tract, which runs from the mouth, where food enters the body, through the stomach and intestines, all the way down to the anus, where waste is expunged. Of course the purpose of this system is to digest food and extract nutrients from it, so as to generate energy for the body to use, and we previously studied some diagrams depicting all of the components of this system in reasonable detail. To quickly review, after food is chewed and swallowed it travels through the esophagus into the stomach, where the food is broken down by digestive juices in a highly acidic environment. It then moves into the duodenum, the first section of the small intestine, where the bulk of the nutrient absorption takes place, followed by the cecum, the first section of the large intestine or colon, where some additional absorption occurs. There, any remaining waste is compacted into stool for storage in the rectum and eventual expulsion through the anus. We also examined the accessory digestive organs, the ones which are not part of the GI tract, but nevertheless play an important role in digestion. Those would include the salivary glands, tongue, liver, gallbladder, and pancreas. But with all of this understood, it’s time to get a closer look inside the digestive system, just as we’ve done with the heart and the brain. What do these organs really look like? What is it like to travel through sections of the GI tract? Let’s start at the mouth, where chewing, or mastication, is taking place. This is where the teeth grind the food up, and chemical processing actually begins right away, thanks to enzymes contained in the saliva. Saliva is released by salivary glands, and allows for the breakdown of complex carbohydrates before food even reaches the stomach. We can also clearly see the jaw motion that is allowed by the tempromandibular joint, as well as the surrounding muscles and ligaments. Now let’s head inside the mouth for a closer look at the tongue. This is a muscular organ with taste buds for assessing the taste and temperature of food. The tongue performs a variety of movements when talking or eating, and will push a lump of food down the pharynx when swallowing. Here we can see the swallowing mechanism taking place, observing that each time, the windpipe, which is the entry to the respiratory system, is blocked by a flap of cartilage called the epiglottis. This is to ensure that food does not enter the larynx, and instead moves into the pharynx and on to the esophagus. Above the tongue we also notice the uvula, a small conical process of the posterior edge of the soft palate, which rises and closes the junction of the pharynx with the nasal cavity. At the other end of the esophagus we reach the stomach. The area where food enters is called the cardia, while the top region is the fundus, the bulk of the stomach is called the body, and the curvy section towards the bottom is the pylorus. We can look at the stomach from another view to see its proximity with important blood vessels, and we can also see that the stomach wall is covered in lymph nodes, which are part of the immune system. Now let’s look inside the stomach. During swallowing, the muscles that form the walls of the stomach relax, allowing the lump of food to pass inside. As a result of further undulating contraction, food is mixed, frayed, and loosened. We can see the four layers of the wall, those being the mucosa, submucosa, muscular layer, and serous layer. And we can also see the folds in the mucous membrane, which are called rugae. Let’s now head inside the stomach. Here the rugae are quite pronounced, and we can observe the secretion of gastic juices which are responsible for the highly acidic environment that makes digestion possible. From this other view we can see the opening up above where food enters the stomach from the esophagus. Then slowly pivoting we can see the pyloric sphincter, a circular fold which separates the stomach from the duodenum. Once food is sufficiently digested, it will pass through this barrier to enter the small intestine. The small intestine is the longest part of the gastrointestinal tract, at around five or six meters in length, which is why it must adopt this compact arrangement in order to fit in the abdominal cavity. It is split up into the duodenum, jejunum, and ileum, and we can see material being pushed along by peristalsis, which is the involuntary constriction and relaxation of smooth muscle. The primary function of the small intestine is the absorption of nutrients through the intestinal lining. We examined the villi and microvilli in the mucous membrane that makes this process highly efficient in the earlier tutorial. Once through the small intestine, material enters the large intestine at the cecum, visible on the lower left, with the tiny appendix protruding at the bottom. From there it travels up the ascending colon, across the transverse colon, down the descending colon, and then laterally through the sigmoid colon to reach the rectum. Just like with the small intestine, the motor function of the large intestine is carried out thanks to the smooth muscle present in the intestinal wall. The movements are very slow, as the muscles are poorly developed, but let’s zoom in for a closer look. As we approach, we can see the blood vessels that service the intestinal cells, and the longitudinal muscle bands that run along the wall of the colon, contracting and generating the swelling activity we observe. Now let’s enter the large intestine. Once inside, we can see that the mucous membrane of the large intestine has many folds, but we do not see the villi and microvilli that are characteristic of the small intestine. We also see the intestinal juice being produced by the intestinal glands present in the mucous membrane. As we already discussed, it is here that remaining waste material which is not absorbed will be compacted into feces for storage in the rectum and eventual expulsion through the anus during a bowel movement. To quickly review and visualize some of the accessory organs along the way, food enters the mouth and then travels down the esophagus until it reaches the stomach. Adjacent we can also see the liver, which filters blood coming from the digestive tract and produces bile, which aids in the digestion of fats. Continuing we see more accessory organs, the gallbladder, which stores bile for release into the small intestine, and the pancreas, which produces enzymes that aid in digestion, as well as hormones. And finally, the small and large intestines, finally seen together here, ending at the anus, which is the end of the gastrointestinal tract. And with that, you should have a much greater appreciation for what your body does with all of the food that you eat.