The digestive system forms part of the Biological and Biochemical Foundations of Living System section of the MCAT test. It’s made up of several anatomical structures, each with their own unique physiology. To help you prepare, we’ve produced a thorough overview of this key MCAT topic.
The digestive system is one of the largest organ systems that exist in the body, taking up nearly the entirety of the abdomen. It starts with the mouth, with food traveling down the esophagus into the stomach, then through the duodenum into the small intestine, which passes directly into the large intestine, and finally to the rectum and anus. The digestive system is responsible for the breakdown of food and the absorption of nutrients into the bloodstream to be used for different cellular processes throughout the body.
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When food first enters the body through the mouth, the first substance it comes across is saliva. This is an enzyme-rich fluid that is made of mixed mucus and serous components and is produced by several salivary glands in the oral area, the most predominant of which is the parotid gland. The main salivary enzyme is salivary amylase, which is responsible for initiating the original breakdown of carbohydrates when they enter the body. The saliva is also responsible for helping some of the components of the food dissolve once they are manually broken down by mastication (eating). The food can also become lubricated, resulting in smoother movement down the esophagus.
The esophagus is a long tube that connects the mouth to the stomach, separating it from the airway via the epiglottis. The esophagus is a transport organ and is the first part of the digestive system that works through a mechanism called peristalsis. This is the way in which the esophagus contracts and relaxes in order to push chewed-up food, now known as ‘bolus’, down to the stomach. The way in which this works is that the circular muscles downstream of the bolus will relax, thereby allowing for the expansion of the esophagus, and the circular muscles upstream of the bolus will contract, increasing the pressure behind the bolus and therefore propelling it forward. This action is increased by the longitudinal muscles of the esophagus which act almost as a conveyor belt to aid in the movement of the food.
The stomach is a muscular bean-shaped sac that consists of several circular, longitudinal, and oblique muscles. More internally to these muscles are the gastric rugae, which are folds of tissue that increase the overall surface area of the stomach to age in digestion. The esophagus enters the stomach at the cardiac sphincter, otherwise known as the lower esophageal sphincter. This passes into the cardiac area of the stomach. The top part of the stomach is known as the fundus, while the main part of the stomach is the body of the stomach. This goes into the pylorus of the stomach, which leads to the pyloric sphincter, which then passes into the duodenum. The lateral aspects of the stomach are known as the greater curvature on the left and the lesser curvature on the right, as a result of their respective lengths.
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The stomach is the initial point where food can be stored in the digestive system. The stomach is capable of storing a large portion of food and water (between 2-4L). The muscular layers of the stomach allow for the churning of the bolus to mix it with the components of the stomach. This is a way in which the stomach participates in both mechanical digestion as well as chemical digestion, and the rate of this is controlled by a large series of hormones. Once the bolus has been mixed with cardiac juices, it is known as chyme.
The stomach is most known for producing hydrochloric acid, of which it produces 2-3L every day. This hydrochloric acid has a few functions. Firstly, it breaks down the bolus more. This is because the high acidity causes different chemical bonds in the food to break. Secondly, it has an immune function. This is due to the same reason that it breaks down bolus: it breaks the bonds in pathogens, causing them to be degraded and preventing them from entering the bloodstream unless they are capable of surviving the high acidity levels.
The stomach is only responsible for the production of a precursor to one specific enzyme. This precursor is known as pepsinogen, and once it has been activated it becomes pepsin. Pepsin is a protease enzyme, meaning that it’s responsible for the breakdown of different proteins. This is a further way in which the stomach can help protect against different pathogens: several have protein cores or enzymes within them. If the acidity of the stomach can degrade the membranes of pathogens, pepsin can work to break down the proteins that help to make up the pathogen, causing lysis of the pathogen before it can enter the bloodstream.
The liver is a large organ weighing approximately 1.2-1.5kg that is situated in the upper right quadrant of the abdomen. It is composed of 8 different lobes, the most obvious of which being the left and right lobe. The liver contains several portal tracts that allow for the filtration of blood. Inferiorly to the liver is the gallbladder, and there is a tube that connects the two known as the biliary tree. Although bile is made in the liver, it is stored in the gallbladder, therefore it passes through this biliary tree.
The liver is responsible for the production of bile, which is an alkaline fluid that aids in the digestion of chyme after it has left the stomach. It is made originally by hepatocytes, which are specialized liver cells that can deal with the different stresses put on the liver. It is made up of salts, known as bile salts, as well as phospholipids, cholesterol, water and some electrolytes amongst others. Once it has been initially made by the hepatocytes, it is modified by cholangiocytes, which are epithelial cells that add more water as well as bicarbonate to the bile.
The liver is known for its role in the detoxification of blood. It removes ammonia from the blood, which will then go on to be carried in urea and secreted in the urine. It is also responsible for the metabolism of different drugs through a series of enzymes known as the P450 enzymes. Further, it can metabolize alcohol. Large amounts of these, however, will cause damage to the liver, hence why cirrhosis is a common complication of long-standing alcoholism. The liver is capable of regenerating itself to a larger extent than a lot of other organs, however.
Finally, the liver is responsible for regulating blood glucose levels. When levels of glucose in the blood are too high, glucose gets converted into glycogen, which then gets stored in the liver. When blood glucose levels dip, this glycogen will then get converted back to glucose after receiving signals from a hormone called glucagon, through a process called gluconeogenesis.
After being produced in the liver, bile flows down to the gallbladder where it is stored until it needs to be released. The release of bile is mainly regulated by a hormone called cholecystokinin, which is a peptide hormone produced by cells in the small intestine in response to acidity and fat. Once stimulated by cholecystokinin, the gallbladder will contract, forcing the bile into the bile duct, which will then join with the pancreatic duct to enter the major papilla of the duodenum at an opening called the ampulla of Vater.
Bile is a product of the liver which has an important role in the digestion of food. It is an alkaline fluid (pH ~7.5-8) that helps to neutralize chyme that has been released from the highly acidic stomach. The benefit of this is that it allows for the enzymes within the small intestine to work and prevents damage to the rest of the digestive system due to the acidity of the stomach. The other way in which bile helps with digestion is by emulsifying fats. The charge of bile salts means that they have a tendency to aggregate around fat molecules, preventing the molecules from clumping together. This means that the surface area of the fats is increased; therefore, lipases can work more efficiently to break the fats down.
The pancreas is one of the main organs responsible for the production of enzymes that aid in digestion. The pancreas produces amylases (for carbohydrates), proteases (for proteins), lipases (for fats), and ribonucleases (for RNA), as well as trypsinogen, a precursor for trypsin. These enzymes will be secreted by exocrine cells known as acinar cells in the pancreas, and their production and release are stimulated by hormones from the gastrointestinal tract.
The enzymes of the pancreas are released in the pancreatic juices, which are a mixture of the enzymes and a lot of bicarbonate. This serves the same role as the role of bicarbonate in bile: it reduces the acidity of the chyme to allow for optimal functioning of the digestive enzymes. As in the gallbladder, the release of pancreatic juice is regulated by the release of cholecystokinin. Pancreatic juices will leave the pancreas in the major pancreatic duct, which will then join the bile duct to form the common bile duct and enter the duodenum through the major duodenal papilla.
The small intestine is a long organ that can extend from 9 feet to over 18 feet, which connects proximally to the duodenum and distally to the large intestine, although there is some debate as to whether the duodenum should be included within the small intestine. The small intestine consists of two parts, the jejunum (first 2/5 of the small intestine) and the ilium (final 3/5 of the small intestine). The small intestine is connected to the posterior wall of the abdomen by mesentery, which is a double fold of the peritoneum that allows for the anchoring of different organs. The small intestine is functionally equipped to increase its surface area as much as possible, by having plicae circulares (circular folds) and many villi on the internal aspect.
The villi are evaginations of mucosa that are present throughout the small intestine to increase its surface area and increase absorption. Lining these villi are microvilli, with further assistance in this. The villi and microvilli have several functions that assist in the digestion of food. They have channels on their membranes that allow for the movement of molecules that have resulted from food digestion from the lumen of the intestine to the blood vessels to be transported to different bodily tissues. They also have cells that send signals to immune complexes in the small intestine to aid in the breakdown of pathogens. Further to this, the villi contain secretory cells which help to secrete mucus to protect the membrane and the movement of food through the intestines, as well as secreting different hormones to trigger the function of the gallbladder, stomach and pancreas.
The chyme that enters the duodenum and therefore the small intestine is very acidic as it has been blended with the stomach juices. This is detrimental to the small intestine as it does not have as thick of a mucous on its lining as the stomach does, therefore its membrane is very susceptible to damage from acids. Further, the enzymes that are secreted from the pancreas and the small intestine work optimally at a more neutral pH and will be denatured at a lower pH. This is why it is so important that the pancreatic and bile juices contain bicarbonate: it reduces the acidity of the chyme to allow optimal enzymatic function and prevent damage to the small intestine.
The small intestine itself only makes two types of enzymes: proteases and lipases, for breaking up proteins and lipids respectively. These are released from glands within the ilium of the small intestine in response to hormonal signals. Most of the digestion in the small intestine is completed by enzymes that are present in the pancreatic juices instead.
Most of the absorption in the small intestine occurs in the jejunum: this is the part where carbohydrates and proteins are absorbed. When they have been fully broken down, they are able to travel into the bloodstream from the intestinal lumen via diffusion. This is not possible for lipids due to their polarity. They are more likely to be absorbed in the ileum, in which vitamin B12 and bile acids are also absorbed. The lipid molecules are absorbed in micelles. The amount of absorption in the small intestine is massively increased by the presence of the villi and microvilli, as they work to increase the overall surface area of the small intestine.
The large intestine starts with the cecum and the appendix is an accessory organ of this. The cecum is on the right side of the body and starts ascending as the ascending colon. In the upper right quadrant of the body, the colon then turns at the hepatic flexure to become the transverse colon. This traverses horizontally to the upper left of the abdomen, where it turns again at the splenic flexure to travel in a downwards direction as the descending colon. In the lower left abdomen, the colon forms an ‘S’ shape and is known as the sigmoid colon before finally leading into the rectum. Along the colon there are 3 muscular strips, known as the taenia coli, that shorten to form the haustra.
The small intestine is not capable of absorbing water from the material that goes through it. This is done by the large intestine. The large intestine moves water against the osmotic gradient, absorbing approximately 400ml of water every day. The way that it can do this is because the colon is also responsible for absorbing different electrolytes, such as sodium, allowing the water to move across the membrane alongside the sodium.
The majority of the digestive system is aseptic, meaning it is not contaminated by any bacteria. The colon, however, does not follow this and contains bacterial flora. Flora describes ‘normal, healthy bacteria’, which is responsible for assisting in bodily functions. This can include lysing non-host pathogens that enter, phagocytosing waste products as well as neutralizing toxins amongst other roles. These bacteria are very healthy to have in the gut, however, if it spreads to other parts of the body, it can cause an infection.
The rectum is the final part of the digestive system that is responsible for the storage of feces before it is eliminated from the body through the anus. It can be distinguished from the anus as the anus is a very narrow tube whereas the rectum is wider. The rectum and anus are controlled by different sphincters that allow an individual to prevent defecation. The first is the internal anal sphincter, which is an autonomic sphincter that opens with pressure. When it opens, signals are sent to the brain which initiates the urge to defecate. The external anal sphincter is under voluntary control therefore the individual can choose when they defecate.
The digestive system moves bolus through a mechanism known as peristalsis, which is performed via smooth muscles. Longitudinal muscles will help to slide the food down the digestive tract whereas circular muscles will push the bolus forward by contracting behind it. This means that bolus, in most instances, travels in a unidirectional motion. Recently, it has been found that peristalsis happens in the absence of food in the intestines. Although the reasoning behind this is not yet concrete, it is theorized that this occurs so that if there are any pathogens in the intestines, they are unable to stick in one area and colonize, reducing overall infections.
The digestive system requires several different hormones for its correct functioning. Each of these acts in slightly different ways.
Gastrin is the main hormone to work on the stomach. It is stimulated by a higher pH within the stomach, usually as a result of alkaline foods, as well as by stomach expansion. It is released by the G cells of the stomach and increases the release of H+ (acidic) ions into the body of the stomach, thereby making the contents more acidic. At the same time, it increases the rate of growth of the mucosa of the stomach, therefore protecting the stomach lining against the excess acid. Finally, it increases gastric movement to encourage mechanical digestion.
Cholecystokinin is a hormone released by the duodenum and jejunum in response to acids in these tracts, either from fatty acids in the chyme or from the acid that is present in the stomach itself. It works to increase the function of the gallbladder and pancreas by increasing secretions, as well as slowing down the emptying from the stomach. This means that more of the chyme will get covered in the biliary and pancreatic juices, which contain bicarbonate. In turn, the chyme will become more neutral in its acidity.
Somatostatin is released by the pancreas and gastric mucosa in response to increased levels of acid and works to inhibit several hormones. These hormones include gastrin, insulin and glucagon, as well as decreasing secretions from the gallbladder and pancreas. In this way, it slows down the gastrointestinal system as a whole.
Secretin is released in response to duodenal acids and is released by cells in the duodenum. It works in a way similar to cholecystokinin, by increasing bicarbonate release from the pancreas, increasing the secretion of bile from the gallbladder and decreasing acid release from the mucosa of the stomach. As a result, the chyme becomes less acidic, allowing for more optimal functioning of the intestines and the enzymes inside them.
Motilin is the hormone that is responsible for the movement of the intestines in the absence of food. It is released from the small intestine when the intestines are empty and stimulates the migratory motor complex, which sends waves of peristalsis down the digestive system.
Vasoactive intestinal peptide is a peptide released by the digestive sphincters, the gallbladder and the small intestine. Its release is increased by expansion of the different lumen within the digestive tract. It works to increase the secretion of water and electrolytes within the intestines and relax the smooth muscles of the intestines as well.
Ghrelin is a hormone produced by the stomach in response to a lack of food. It stimulates someone’s appetite by initiating the ‘growling’ action of the stomach, encouraging the individual to eat.
The digestive system receives its innervation from its own section of the autonomic nervous system known as the enteric nervous system. This is further split into two categories: the myenteric plexus and the submucosal plexus. The myenteric plexus is mostly responsible for muscular control: it works on the intensity and frequency of muscle contractions for processes such as peristalsis. The submucosal plexus, however, works in the submucosa of the digestive system. This is the area in which the most glands exist, therefore this plexus is responsible for the secretion of different gastrointestinal hormones and the absorption of the contents of the digestive system.
We hope you’ve found this information helpful when revising the digestive system and preparing for the MCAT exam. We have blogs covering a wide range of MCAT topics – including membrane-bound organelles and defining characteristics of eukaryotic cells, the citric acid cycle, and the immune system – to further support your MCAT prep. For everything you need to know about the MCAT test visit our MCAT Guide.
