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Demystifying Fluid Regulation on the PCAT

  • by Sam
  • Mar 27, 2018
  • Biological Processes, PCAT Blog

by Andrew Dombrowski

Quick! Think of some hormones! For many PCAT students, insulin (and its counterpart glucagon) may be the first to come to mind—and rightly so, since blood glucose regulation is a critical physiological function and the management of type I and II diabetes mellitus is a mainstay of the daily practice of pharmacologists. However, for the PCAT, you need to be aware of some other key hormones in the body. In particular, fluid regulation, another high-yield topic on the exam.

One reason why fluid regulation is important on the PCAT is because you will need to understand it when you become a pharmacist – medications affecting fluid regulation (especially diuretics, or so-called “water pills”) are often prescribed to patients with conditions such as hypertension and heart failure. Another, more directly PCAT-related reason, is that fluid regulation connects the endocrine system with the renal system, both of which are highly testable. In this blog post, we’ll review the key physiological relationships underlying fluid regulation and the three main hormones that you must be aware of for the PCAT.

The first step in studying fluid regulation is to thoroughly understand the physiology of fluid balance. Students often rush through this step in a hurry to memorize the relevant hormones, but a solid grounding in the background knowledge will help consolidate your knowledge of this topic for the long term.

Let’s start by asking, what happens when you have too little fluid in your system? First, you will have reduced blood volume, because relatively little water will be present in the blood plasma. This in turn means that you will have reduced blood pressure, because less liquid is available to exert pressure against the walls of the blood vessels. Another consequence of having too little fluid is that the solutes present in the blood will be dissolved in less solvent, meaning that you will have increased blood osmolarity.

When you have too much fluid in your system, the opposite holds. You will have increased blood volume and increased blood pressure because there is more fluid in your blood. This also means that your blood will be more dilute, so you will have decreased blood osmolarity.

These relationships are summarized in the image below. The key point when studying for the PCAT is that you want to immediately recognize that all of these parameters are synonymous when it comes to fluid regulation. That is, if you’re given information about one of these parameters, you should be able to fill in the blanks with the others.

There are two major hormones that increase fluid retention in response to low fluid levels: vasopressin (also known as antidiuretic hormone [ADH]) and aldosterone. Although these hormones have the same effect, they work through different mechanisms. Vasopressin (or ADH) works by increasing the permeability of the collecting duct to water, which increases water absorption. In contrast, aldosterone increases sodium absorption in the distal convoluted tubule and collected duct of the nephron, which promotes water absorption. (The reason for this has to do with osmosis: water will diffuse across a semi-permeable membrane from an area with a lower solute concentration to an area with an initially higher solute concentration. Another way of thinking about this is that vasopressin/ADH acts directly, whereas aldosterone acts indirectly.

In addition to the two ways that the body has to deal with low fluid levels, it can also handle excess fluid levels. The main hormone responsible for reducing fluid levels in response to high blood volume is known as atrial natriuretic peptide (ANP). ANP is essentially the opposite of aldosterone: it decreases sodium reabsorption in the distal convoluted tubule and the collecting duct, as well as increasing the glomerular filtration rate and inhibiting aldosterone release. ANP is also a great example of how the name of a substance can incorporate information about its function. “Atrial” tells you that it’s secreted from the heart muscles, and “peptide” tells you that it’s a peptide hormone. The “natriuretic” part of the name contains two elements: the “natri” part should remind you of sodium (Na) and the “uretic” part relates to urine. In other words, atrial natriuretic peptide is a peptide hormone released from the heart muscle that increases the amount of sodium in urine, thereby increasing its volume and decreasing the level of fluids in the body.

Hopefully this post has helped demystify one of the perennially tricky aspects of the endocrine system! To put your endocrinology knowledge to the test with some realistic PCAT practice, try our PCAT practice tests, or our free PCAT practice bundle. Need a more guidance or looking for more in-depth PCAT prep? Check out our online PCAT course or our private, one-on-one PCAT tutoring.

Best of luck on your journey towards pharmacy school!

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