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Don’t Eat the Yellow Snow: 4 Misconceptions About the Excretory System

  • by Sam
  • Jan 30, 2018
  • MCAT Biology, MCAT Blog

by Clara Gillan

As a prospective doctor, you likely spend significant time thinking about how our bodies work. Some aspects of human organ systems may feel almost like second nature by now – how an action potential propagates, perhaps, or the parts of the digestive tract. But if you ask even the most physiology-savvy MCAT students, they will overwhelmingly describe one aspect of physiology as difficult: the excretory system (most notably the kidneys), and in particular the regulation of urine concentration and water balance.

The full scope of the excretory system is much too large to fit in a single blog post, so we’ve gathered several of the most prevalent kidney-related myths – or FALSE ideas – to explain here. We hope you enjoy!

Myth #1: Aldosterone and antidiuretic hormone are interchangeable

It’s true that aldosterone and ADH both promote increased water retention in the body. However, these hormones differ in several important ways. Perhaps most notably, aldosterone is a steroid hormone, while ADH is a peptide. (The MCAT often tests concepts related to this distinction, such as polarity and the ability to diffuse through the cell membrane – so have this info down!)

The two hormones also have very different mechanisms of action. Among other effects, ADH promotes the insertion of aquaporins (water channels) into the membrane of the nephron, largely along the collecting duct. In contrast, aldosterone upregulates sodium/potassium pumps along the nephron membrane, causing sodium to be retained in the body while potassium is excreted. The net effect of both mechanisms involves retention of water, but aldosterone acts more indirectly than ADH.

Myth #2: Urine gets concentrated as it moves through the loop of Henle

Interestingly, the opposite is true – the filtrate actually becomes slightly more dilute between the beginning of the loop and the end. So why do many sources say that the loop of Henle’s function is to concentrate the filtrate? The loop dives deep into the medulla of the kidney, maintaining a very solute (salt)-rich environment through its countercurrent multiplier system. The collecting duct of the nephron also passes through this medulla. So, when the collecting duct is permeable due to the action of ADH, water passively flows out of the nephron and into the salty environment. As such, the loop promotes the concentration of urine – just not in the interior of the loop itself.

Myth #3: Glucose is not filtered into the nephron

You’ve probably seen that in healthy individuals, no glucose is lost in the urine. (After all, glucose is an important energy source, so we need to hold onto it!) However, it is incorrect that no glucose makes it into the nephron in the first place. Instead, glucose is filtered into the Bowman’s capsule at substantial concentrations, but is then entirely reabsorbed in the proximal convoluted tubule. Of course, if too much glucose is present in the nephron – as in diabetic individuals – then the PCT might be unable to reabsorb all of it, leading to sugary urine.

Myth #4: Hormonal regulation only occurs in response to dehydration

In reality, overhydration can trigger a hormonal response as well. The hormone in question is atrial natriuretic peptide (ANP), a peptide released from (of all places) the heart! When an individual overhydrates, blood volume increases, stretching the atrial walls and promoting the release of ANP. The hormone then exerts several effects to counteract the raised blood volume.

Now, it may be a good time to revisit your notes on the excretory system and put it all together! Good luck.

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