Studying for the MCAT: The Digestive System
January 12, 2012
Many test takers look forward to the biology section of the MCAT. After laboring through the Physical Science and Verbal Sections, it’s nice to finally tackle some passages on topics with which many pre-meds are familiar and comfortable. It also happens to be the last section of the lengthy exam, and after over 3 hours of test-taking, you might hear a grumbling coming from your stomach; a grumbling might grow even louder if you happen upon a passage focusing on the commonly tested topic that is the digestive system.
As with many biology topics, it’s important to think about this topic from both a structural and functional perspective (this is why medical students focus so much on anatomy/physiology). When it comes to structure, the digestive tract is easier to “digest” (pun completely intended) if you visualize the entire path that food must travel from the moment it’s ingested until it is excreted.
Mouth > Esophagus > Stomach > Small Intestine* > Large Intestine > Rectum
I’ve made special note here of the small intestine because there are 3 additional anatomical structures (the liver, the gallbladder, and the pancreas) that play functional roles in the process by releasing digestive aids into the small intestine – more on that soon.
After understanding each organ’s anatomical location, an area of greater difficulty—which inspires most of the test questions on the topic — revolves around function. Here, again, a test taker can divide the process into two parts: ingestion and digestion. Ingestion is when structures work to move food down the tract; this can be done by chewing, swallowing, flexing and relaxing muscles, and breaking down particles with bile, enzymes, and acids. As this ingestion proceeds, the process of digestion can then take place; here, the body absorbs necessary nutrients (i.e. sugars, fats, and amino acids) and leaves the remaining waste products to continue down the digestive tract.
Luckily, this is an autonomic process so you can take a nice long nap and let your body take care of transporting the food (try not to nap during the MCAT test though). The muscles of the digestive system are mainly smooth muscle that contract in a process called peristalsis, which is a “wave-like” alternating contraction and relaxation of smooth muscle in front of and behind the food particles that pushes everything down the tract. There are also sphincter muscles in place to prevent food from traveling back up the tract. One of these sphincters, the lower esophageal sphincter, connects the esophagus to the stomach. If you eat too much or if there is too much pressure on this sphincter, the acidic contents of the stomach can rise back into the esophagus leading to heartburn (another name for the lower esophageal sphincter is the ‘cardiac sphincter’ because of its proximity to the heart).
While there is some digestion taking place in the mouth and the stomach – this occurs because the enzymes amylase and pepsin are available for sugar and protein breakdown, respectively – the bulk of digestion occurs in the small intestine as bile from the liver and gallbladder help to emulsify fats. A variety of enzymes released by the pancreas (many of which you’ll want to know for test day) break down food particles into their respective nutrient byproducts. The remaining undigested materials continue to travel down the tract where water is reabsorbed and mainly waste products remain.
This highly efficient process allows us to reap the benefits of letting our involuntary muscles and our excretory organs work while we focus on more important things, like studying for and taking the MCAT. Of course, what we’ve covered here is just the core of your studies on the digestive system, but knowing this system well will help you also understand similar biology topics like the endocrine, respiratory, and reproductive systems, which are also commonly tested.