11.05 Renal (Kidney) Fluid & Electrolyte Balance

Alright, in this lesson, we’re going to talk about the kidney’s role in fluid & electrolyte balance.
Remember that there are three main processes involved in urine formation in the kidneys - glomerular filtration, which is the initial filtration of blood - tubular reabsorption, where we bring substances back into the blood - and tubular secretion, where we put substances into the urine to be excreted out of the body. At each of these phases, water and electrolytes are moved back and forth to regulate our body’s fluid & electrolyte balance. So let’s look in more detail at the reabsorption and secretion of fluid & electrolytes.

The majority of our electrolytes are actually reabsorbed here in the PCT by active transport. We see 65% of the sodium come out of the filtrate, as well as 90% of the potassium and most of the calcium. Active transport means it involves some sort of carrier protein or pump. We also see our chloride and bicarbonate reabsorbed by diffusion - this is usually because they are following these positive ions. These ions are positive, and these are negative. We know the body is always trying to balance out charges, so the negative will follow the positive out of the filtrate and back into the bloodstream. Then we see that 65% of the water in the filtrate is also reabsorbed here in the PCT by osmosis. This is called obligatory water reabsorption. That just means it is be reabsorbed because it is following all of these ions based on a concentration gradient. It’s obligated to go, it has to follow those other substances.

Now, what’s left in the filtrate goes to the loop of henle - remember we talked in the urine formation lesson about the loop of henle having a descending and ascending loop - this comes into play quite a bit here. In the descending loop, we see a few ions move out of the filtrate by active transport, but the majority of what happens here is that water is pulled out of the filtrate. This loop is only permeable to water, so we see a good bit of the water being pulled out. However, the ascending limb is not permeable to water, so instead we see more sodium, potassium, and chloride coming out of the filtrate. Now - if we get rid of a bunch of our water by the time we hit the bottom, then the bottom of this loop is highly concentrated. So follow me here - the loop is super concentrated, AND we’re kicking a bunch of ions out on the way up - so this general area is all highly concentrated with a lot of ions and not a whole lot of water. Keep that in mind because this is going to come into play with our water reabsorption in a minute.

Now, we’ve talked about the PCT and the Loop of Henle - let’s look at what happens to our electrolytes in the DCT. This is where we see the final regulation of electrolytes based on whatever the body needs. So, for example - if we have hyperkalemia or high potassium levels or hyponatremia - low sodium, the body will secrete aldosterone. Aldosterone then comes here to the DCT and causes active reabsorption of sodium back into the blood. When that happens, we’ll see chloride and water follow it. Aldosterone will also cause secretion of potassium. So now we’re seeing our sodium levels come back up and our potassium levels go back down. Now - we mentioned that water is following the sodium here - let’s look closer at how the kidneys specifically regulate water balance as well.

We’ve talked about this type of water reabsorption a few times now. Obligatory water reabsorption is reabsorption that happens by osmosis. The water is following reabsorbed substances or moving out of the filtrate by concentration gradient. 65% of that happens in the CPCT and 15% happens in the descending limb of the loop of henle that is permeable to water and the DCT where it follows the sodium reabsorbed because of aldosterone. So that’s obligatory water reabsorption in the nephron.

But we also have a type called facultative water reabsorption. This type requires the hormone ADH which stands for antidiuretic hormone. Diuretic means to urinate or excrete water - so if it’s ANTI diuretic, it prevents water from being excreted, right? So what happens is that the pituitary gland senses that we need to hold onto more water and it releases ADH. ADH comes here to the collecting duct and opens up pores on the surface. Now, remember we talked about how super concentrated this area is here? The bottom of the loop as well as these capillaries where all the ions are being reabsorbed makes it highly concentrated. So - when those pores open up, guess what’s going to flow towards this area? The water, right!? It’s going to try to help balance out this concentration. So this is our last effort to pull water out of the urine and back into the bloodstream before it gets excreted.
Let’s recap quickly - we see reabsorption of the majority of our electrolytes in the PCT by diffusion and active transport. In the DCT we see some reabsorption and some secretion based on the body’s needs - and usually influenced by aldosterone. And we have two types of water reabsorption - obligatory, which follows other substances by osmosis, and facultative which is facilitated by antidiuretic hormone opening up pores on the collecting duct.
Now, guys I’m gonna be totally honest with you - when I was in this class during undergrad, this concept was one of the hardest for me to get. So I hope I’ve done a good job of breaking it down for you. Watch this video as many times as you need to until you’ve really mastered it! Now, go out and be your best selves today. And, as always, happy nursing!