The Excretory System

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Welcome to MCAT Basics, your ultimate guide to the essential topics you need to master for the MCAT. Brought to you by the physicians at MedSchoolCoach. Every week, Sam Smith breaks down high-yield MCAT topics, ensuring you're primed for success on test day. Join Sam as we explore the most crucial subjects outlined by the AAMC, pulled from official practice materials and third-party resources. Get ready to elevate your MCAT game with topics tailored to maximize your score potential.

Sam Smith (1:03)

Hello, I'm Sam Smith. In this podcast, I cover the excretory system. So I talk first about the difference between the excretory system and related organ systems like the urinary system and the digestive system. Then I talk about some important anatomical structures that are part of the excretory system. Then last, I get into some physiology where I discuss the physiology of a few of these components of the excretory system. And this material is going to show up on one of the four MCAT sections that's going to be BioBioChem. I hope this podcast helps in your studies, and as always, good luck to those of you who are taking the MCAT soon.

The main function of the excretory system is to remove excess, unnecessary materials from different bodily fluids in order to maintain homeostasis. These excess unnecessary materials include things like metabolic breakdown products of sugars, fats and proteins, excess electrolytes like sodium, ingested toxins and drugs, and then also gases, especially carbon dioxide, which builds up as a result of cellular metabolism.

The excretory system is not to be confused with the digestive system or the urinary system. They play similar roles, but I think it's important to understand the differences between those three systems. So the role of the digestive system is to break down our food into small bits that the body can then absorb. So one way to think about it is the digestive system is what's going in. Now, as I said, the excretory system is removing excess unnecessary materials. So you can think about the excretory system as taking things out. So digestive system is in, excretory system is out. Now the urinary system is contained within the excretory system. The urinary system plays a role in producing, storing, and eliminating urine. And it's also known as the renal system. And this has a lot to do with the kidneys. I already talked about the renal system and renal physiology in another podcast. So if you want to learn more about the kidneys and the nephron and what goes on in the kidneys, then go listen to that podcast. But this is all to say that the urinary system is only part of the excretory system, right? That is the elimination of urine specifically. There's still many other ways that our body eliminates substances from the body. So to recap that all, digestive system is absorption of nutrients. The excretory system is the excretion of substances. And then the urinary system falls under the excretory system, and that is the excretion of urine.

Next, I want to get into some of the anatomy of the excretory system. And there's some important organs to know. That's going to be the bladder, the kidney or kidneys, the liver, skin, lungs, and large intestine. And these all play different but important roles within the excretory system. So let's start with the kidney. So as I said a little bit earlier, I already did a whole podcast on the renal system. The kidney is an extremely important organ, especially on the MCAT. You're going to see that come up. In terms of their anatomy, we have two kidneys and they're being shaped, and they're located at the middle back of your trunk. An easy way to remember where these are at is to think about a box for doing a kidney punch. If you've ever seen it, it is to the lower back kind of on each side. So that's just kind of how I remember where the kidneys are located. Now, right on top of each of the kidneys is a white glob of cells. And this glob of cells looks a little bit like melted ice cream that's kind of starting to drip down the kidney. And these globs, one on each kidney, are the adrenal glands, and they produce three types of steroid hormones. Mineral accordicoids, glucocorticoids, and androgens. But of course, since this is a podcast on the excretory system, I'm not going to get into those hormones. And if I was to slice the kidney in half, what I would see is a bunch of little tubes. And these little tubes are the functional unit of the kidney, and they're called nephrons. And your kidney contains about a million of these little nephrons, and they're capable of filtering toxins, salts, and water out of your blood. And if you want to know exactly how that works, go listen to the Renal Physiology podcast I put out a while ago. Now, the bladder is an organ that is connected to the kidneys by a tube that's called the ureter. And a little aside about the ureter is there's two of them because there's two kidneys, and they're actually really long. They're about a foot long. So the connection between your kidneys and your bladder is like this foot long tube that is in the middle of your abdomen, which is a bit strange. But anyways, the bladder itself is a triangle shaped hollow organ that is located in the lower abdomen. And its purpose is to store urine that is created by the kidneys. And in terms of its structure, there is a layer of muscle surrounding an inner lining. And this layer of muscle helps push out the urine when it is time to pee. So again, urine is produced by the kidney. It flows into the ureter to the bladder. And from the bladder, it travels through the urethra and is expelled outside the body, hopefully into a toilet. And this is a dumb little mnemonic, but I always remember that the urethra is after the bladder because when you really, really, really need to pee, it comes out of the urethra and you're like, ah, dumb, but it works for me. The next organ here is the liver. And the liver is a large reddish brown organ. And you know, if you've ever eaten liver meat, first of all, I'm sorry if you have eaten liver meat because it's nasty. But second of all, you know, it's a very deep dark red color. And I should also note it is the heaviest organ in the body, and it counts for about two to three percent of your body weight. Now the liver sits pretty high up in your abdominal cavity. It sits essentially right below the diaphragm, and it consists of two main lobes, a right lobe and a left lobe. And these lobes are separated by a ligament that's called the falciform ligament. And if you were to take a microscope and zoom in on this tissue, this liver tissue, you would see that it's made up of these really small units. And these small units are called hepatic lobules. And these lobules are hexagonal in shape, so they kind of look a little bit like honeycomb. And at the corners of this hexagon that is the hepatic lobule, sits what's called a portal triad. And the triad is made up of a portal venule, a portal arteriole, and a bile duct. So essentially, you have this hexagon, and then at each corner of the hexagon, you have three little tubes. In addition to those components, you also have a central vein and you have sinusoid. The central vein is just a vein that runs right through the center of these hepatic lobules. Sinusoids are enlarged capillaries through which blood travels from the portal triad, so portal venule and portal arteriole, into the central vein. So essentially, sinusoids just allow blood to flow from the portal triads into this middle central vein. And to me, this is where it gets interesting. So as I said, there are three components to this portal triad. There's a venule, there's an arteriole, and there's a bile duct. And as I also just said, the blood flows from these venules and arteries through what's called a sinusoid into this middle central vein. So blood flow in a hepatic lobule goes from the corners of the hexagon into the middle. Now, when we look at the bile duct, which remember was also part of this portal triad, the flow of bile is actually in the opposite direction. So bile is produced by the hepaticites in the liver and then moves from inside the lobule towards the outside, where it makes its way eventually to the bile duct. So why do I find this so interesting? Who knows? It's just kind of cool. The blood flows in one direction from the outside of this hexagon to the inside, and the bile flows from the inside to the outside. So you've essentially got these cross streams of blood and bile, and of course they're not in the same tube traveling together, like in opposite directions, but they run parallel to each other in opposite directions. Pretty sweet. So to recap all of that, the liver is broken down into two lobes, the right and the left lobe, and each of these lobes is further broken down into smaller functional units called hepatic lobules. And these lobules are hexagonal in shape, and the corners formed by this hexagon shaped hepatic lobule are what's known as the portal triad, which includes a venule, arterial and bile duct. And I'll get into the physiology of the liver and what its role is later in the podcast. For now, just understand its general structure. The skin is the next organ in the excretory system I want to talk about, and it's made up of three distinct layers. The epidermis, the dermis, and the hypodermis. And the main way that the skin is involved in the excretory system is through sweating. So I want to talk about sweat glands in the skin and kind of where they live and what they look like. Sweat glands look like a coiled hose that extends from below the surface of the skin all the way up to the skin, where the coiled part of this hose lives deep down in the skin, and then the hose extends up, up, up until it reaches the surface of the skin. And the coiled part that is way down in the skin is the secretory portion of the sweat glands. This is where the sweat is actually produced. And it usually sits all the way down in the dermis, but is sometimes found even deeper into the skin in the hypodermis. And each of these sweat glands is connected to a pore that is on the surface of the skin.