MCAT Biology: Kidney Essentials

[@iZxpd]

Everything we need to know about the nephron in less than 20 minutes. Thank you so much, learned so much from your video.

[@bremmethod]

I'm so glad you found it useful!

[@09zboy]

Your videos are saving me on my MCAT journey! Thank you so much!

[@bremmethod]

I'm so glad they're helping!

[@abbyregacho3188]

I've been looking for a video that helped me understand and this saved me!

[@bremmethod]

I'm so glad it helped!

[@rachaelsennett8035]

I struggled understanding the nephron, but this video cleared it up in 20 min! I love the way you connect anatomy to physiology for better understanding :) I will definitely be checking out your other videos for MCAT prep!!

[@bremmethod]

Thank you so much! I'm glad it helped!

[@charlize-m9s]

thank you SOO MUCH!!! You have no idea how much you've helped me with this. this gives me hope! :)

[@bremmethod]

You're welcome!! Keep that hope up - you've got this!

[@CyrenSounds]

Wow, this was so helpful. Thank you so much for approaching MCAT studying with such a positive and encouraging attitude!

[@bremmethod]

I'm so glad it was helpful!

[@eesahuq942]

Amazing video! Thank you so much. Renal system is a topic I struggle with, and this made it much clearer. I just wanted to mention one thing: Vasopressin/ADH is produced by the hypothalamus and released by the posterior pituitary.

[@bremmethod]

Glad you liked it! And great catch - I should have said "release" in the video instead of "produce", since the paraventricular nucleus cell bodies that do the production are in the hypothalamus. Thanks for adding this clarification!

[@sneha5359]

u just saved my life it helps sm to see how everything connects to other mcat concepts

[@bremmethod]

Glad it helped! I find it so useful as well - Physiology made so much more sense to me when I started looking at the big picture.

[@amritnasiruddin3940]

Absolute legend behavior

[@bremmethod]

haha thank you!

[@gabrielbarton2424]

your smiling face and positive attitude really helped me retain the info. Testing tomorrow!

[@bremmethod]

Good luck today!

[@mikaylascott8254]

girl you ate that

[@bremmethod]

Thank you!

[@cja12345]

wow, what an amazing explanation. I was doing the anki card and was straight memorizing without really understanding but now that I watched your video, I understand it fully.

[@bremmethod]

I'm so happy to hear that! That's exactly why I make these

[@ethanchrist5161]

Best kidney video I have watched by far, thank you!

[@bremmethod]

I'm so glad you found it useful! You're welcome!

[@StevenChit]

Wow, thank you so much for your videos! You explain it in such a articulate and succinct way.

[@bremmethod]

Thank you so much! I'm glad they're helping

[@harrisonbogursky6535]

YOU ARE SO GOATED

[@bremmethod]

Thank you!

[@jessie3389]

SO UNDERRATED thank you so much!!

[@bremmethod]

You're welcome! So glad you found this useful!

[@adventurevloggeradnanahmedusa]

Thank you. This is excellent.

[@bremmethod]

I'm glad you're finding it useful!

[@mominakhalid9152]

Really helpful. Thank you

[@bremmethod]

I'm glad you found it helpful!

[@ihavenospoon7]

that was amazing 2 days out from test day ty sm

[@bremmethod]

Hope it went well!!

[@AverageTennisGuy]

Loved the loop of henle connection with the collecting duct, it made it immensely easier to grasp this topic! Quick question though - if the concentration is increasing down as it shows, then how does the descending loop of henle contribute to that? If water comes out from descending, then shouldn't it decrease concentration? And then if sodium ions are being pumped out for ascending, doesn't concentration increase up as well because now we have more solute outside?

[@AverageTennisGuy]

I think I better understand it now, would you mind letting me know if I got this correct? The thin loop of henle pasively diffuses water to concnetrate the filtrate,then the ascending loop actively sends the ions out. And the concentration is greatest at the bottom. This means the greatest active transport for increasing osmolarity of the interstitium is occurring at the very bottom and decreases as it is close to the cortex(?)

[@bremmethod]

You've got it! and because of this, as fluid moves down the collecting duct, the interstituim has a higher solute concentration, allowing for passive osmotic movement of water out of the aquaporins :)

[@AverageTennisGuy]

@@bremmethod this makes so much sense!!! All thanks to you!!!

[@faridshahid1833]

Thank you this was so helpful!!

[@bremmethod]

You're welcome!!

[@03yankeematt]

great video

[@bremmethod]

Thank you!

[@ashdawg721]

Amazing!

[@bremmethod]

Thank you!

[@graceforbes9139]

You are awesome!

[@bremmethod]

Thank you! So are you :)

[@DSnowaws]

The best kidney video I’ve seen. Please do more PPP glycolysis insulin connection stuff. If you had videos playlist on ur website I would insta purchase it as a resource

[@bremmethod]

I'm so happy that you enjoyed it, and I'll add that to the list! We're actually building a fully prerecorded course right now. I want it to be amazing so it should be ready early 2025!

[@simerjeetkaur8072]

Hi! Big fan of your work! I have a question. if too much energy is used for the reabsorption of sodium, what's the point of the sodium/potassium ATPase? I did some research and this is what I came up with but I'm not sure if it's right: sodium/potassium pumps take out sodium so the descending loop can take out water and the chain of facilitated diffusion can work. But, at the same time, I'm a little confused as to why the pump doesn't finish the job. Why set up the environment for the rest of the nephron? Do the pumps take out as much sodium as it can and the rest is taken out by the loop/collecting ducts with facilitative diffusion? Also, does the distal convoluted tubule have the same job as the proximal convoluted tubule? If that's the case, does it impact the environment for the facilitated diffusion just like PCT? (take out sodium so water wants to leave and the chain continues) THANK YOU :)

[@bremmethod]

Great questions! Let me clarify a bit: The PCT is our major site of reabsorption, and the DCT more or less is the same (they have slight differences but in general maintain homeostasis via reabsorption and secretion of ions, water, and molecules). The reason we have the Na+/K+ pumps is to reabsorb Na+ (it's very important to have high extracellular Na+ for our neurons/action potential), and to secrete excess K+. The pump is an antiporter, so it moves 3Na+ back into the body for every 2 K+ secreted into the nephron. This results in a NET movement of ions being reabsorbed into the body. The PCT (and DCT) are semipermeable to water, so water will follow the net direction of ions, and get reabsorbed into the body. This is great, as we generally want to retain as much water as possible. We will upregulate these pumps with aldosterone when we need to increase blood volume/blood pressure, so they aren't always working at max capacity (this conserves energy). The bigger energy issue is that pumping water out of the nephron would be too costly, so we use other systems to move water passively. Now, when we are talking about the PCT and DCT, they are in the cortex of the kidney (where the PCT and DCT are), the osmolarity is only ~285 mOsm, about the same osmolarity as blood. Because water is following it's osmotic gradient in the PCT/DCT, as soon as it equalizes with the blood and surrounding interstitium, water will no longer be reasborbed into the body. If our urine was to stay at ~285mOsm, we'd be excreting ridiculous amounts of water every day, which would be both inefficient and dangerous to our system. So the nephron does a cool thing, where it creates a concentration gradient that gets more concentrated the deeper you go into the kidney. In the center (medulla), the osmolarity of the surrounding tissue is ~1200mOsm (4x as much as the osmolarity of the blood!), which means water will pretty much always want to move towards the interstitium. This gradient is created by the loop of Henle, which has Na+ pumps on the ascending loop to put more salt into the interstitium, particularly deeper in the medulla. The ascending loop is NOT permeable to water, so that water doesn't immediately follow the sodium. Instead, the water in the nephron will continue through the nephron until it travels back down into the medulla via the collecting duct, where it will then be able to move into the interstitium via aquaporins. The descending loop is slightly permeable to water, so that as fluid on the inside of the nephron travels down into the medulla for the first time (after the PCT), the fluid will be able to increase in osmolarity as it also travels into the medulla, and therefore maintain that higher gradient in the center of the nephron. I hope this helps!

[@rayvenhickman1894]

Thank you! This video truly helped

[@bremmethod]

I'm so happy to hear that!

[@backrub747]

Thank you! Your videos have been so helpful

[@bremmethod]

Hi! Thanks for posting this - you are totally right. It's hard to depict aquaporins at the nephron level because they are so small - but they are just little channels in indivudal cell membranes! On the MCAT, you won't need to worry too much about identifying features on drawings, so as long as the concept of aquaporins makes sense, that's all you need in your notes! However, HistologyGuide is a super cool free resource where you can look at microscope images of all the different organs, including the nephron, and zoom in on the individual cells of the collecting duct! It's out of scope for MCAT but super cool for future doctors: histologyguide.com/slidebox/16-urinary-system.html#nephron

[@akfresh12]

Blown away by how great this video was. Na is reabsorbed and K is secreted in the DCT correct? Guessing there are other ions that are also involved here.

[@bremmethod]

Yes! Na+ and K+ are the most highly tested ions because their equilibriums are super important for our neural function (it's a great content connection to the action potential!). Cl-, H+, and HCO3- are also exchanged in the PCT and DCT.

[@DanielaMonteys]

Hi !! thank you so so much this was incredibly helpful!! I am a bit confused though because I just took a practice passage and I said aldosterone works in the proximal and distal convoluted tables, but apparently that's wrong and it only in the distal convoluted tubule (and not the proximal). I could have understood this wrong but it seems in your video that aldosterone acts on both proximal and distal. Just wanted to clarify. Thanks sm for your help though!

[@bremmethod]

Hi and thank you so much for your question and patience! I wanted to dive through some content resources to make sure I had all the details before responding. Full disclosure, I base my videos from the AAMC practice question content and the AAMC Content Guide, and use their linked textbook chapters as a reference for how much depth I should go into. The relevant chapter here states: "Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone, a steroid hormone that is produced by the adrenal cortex. In contrast to ADH, which promotes the reabsorption of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from extracellular fluid of the cells in kidney tubules." And the practice questions in the QBanks have answer choices that simply say "increase/decrease reabsorption in the kidney tubules". Based on this language, it seems as if the AAMC materials only include aldosterone action in general terms of "tubules", and since I spent more time on the PCT in my video, I described the mechanism there. HOWEVER!! Thanks to your question, I did a deeper dive into other textbooks and papers on aldosterone mechanism, and many textbooks do say that the site of aldosterone action is in the DCT (and even the collecting duct, to some extent!). However, I've also found papers that indicate that there is also some PCT action, especially in the renin-angiotensin pathway. So, what do we do with this as MCAT students? I usually base my decisions off the AAMC questions, which seems to tell me that they will not specifically ask you to compare PCT vs. DCT, and instead will use the general term "tubules". However, now we know that if we have to choose, DCT is better! I will make sure to update that information in the video. I looked through the AAMC materials but was unable to find a question like the one you referenced. If you let me know where to find it, I can research it and provide you with more insight!

[@woodprofil5654]

the goat

[@bremmethod]

🤭🐐