11.04 Formation & Excretion of Urine

In this lesson we’re going to talk about the formation in the kidneys and excretion of urine through the urinary system.
Let’s talk about what is actually in urine - once it actually leaves our body, it contains water, waste products like urea, uric acid, and creatinine, electrolytes, and a substance called urobilin or urochrome - this is a byproduct of heme breakdown that gives the urine its amber color. There are 3 major processes involved in urine formation - glomerular filtration, tubular reabsorption, and tubular secretion. When we talk about filtration - we are talking about pushing the blood through a semipermeable membrane so only certain particles can get through. When we talk about reabsorption - we’re talking about substances being reabsorbed or brought back into the bloodstream. So any time you hear reabsorb, think back into the blood. Then secretion is when we push substances out of the blood - in this case, we’re pushing it out of the blood and into the urine. So we filter the blood, reabsorb important things, and secrete things we want to get rid of. So let’s look a little closer at this process.

Let’s remind ourselves of the anatomy of the nephron - remember that is the functional unit of the kidneys - this is where the magic happens. Now, when I first learned about the nephron - I saw it drawn this way - with everything stretched out in kind of a linear fashion. So when I started learning about this process, I couldn’t picture how it all worked. But then, I saw this picture and it make so much more sense. You see, the nephron is much more bunched up, like this, and there’s a full network of capillaries all the way around all of these tubules. So any time we talk about something leaving the tubules, it goes right into the bloodstream in these capillaries. And any time we need to secrete something, it diffuses across from the capillaries into the tubules. Okay - let’s look at these individual processes.

First is glomerular filtration. Remember this is the glomerulus - it’s a tuft of capillaries that enters the nephron at Bowman’s capsule. These capillaries are so tiny that the pressure coming into the glomerulus from the afferent arteriole is super high. That super high pressure forces fluid and small particles through the glomerulus. The larger particles and cells like the blood cells are left behind in the capillaries. The stuff that got filtered into Bowman’s capsule is now called the glomerular filtrate and it gets absorbed into the proximal convoluted tubule. Again, the large cells get left behind, but a LOT of the components from the blood are actually sent into the filtrate. There’s a bunch of water, electrolytes, glucose, proteins including amino acids and albumin, vitamins and hormones, and waste products like urea, uric acid, and creatinine. Now - while our body filters a lot of this into the nephron, it’s important to realize that the majority of this volume will actually be reabsorbed back into the blood. The body can filter about 180 liters in 24 hours, but it only excretes about a liter and a half in that time. So the rest is actually absorbed back into the body.

Now, let’s look at the reabsorption process. Certain things are reabsorbed in each portion of the nephron - first we’ll look at the PCT or proximal convoluted tubule. Big thing to know here is that the majority of the filtrate is actually reabsorbed as soon as it gets into the PCT, but it’s regulated by concentrations and things like that. So we see the majority of our electrolytes reabsorbed here in the PCT. We also see that 100% of the glucose pulled into the filtrate is reabsorbed back into the blood to prevent us from getting too hypoglycemic. However, glucose is what’s known as a threshold substances - that means that if our blood glucose levels are too high, we actually will see some of it start to spill into the urine. Generally speaking, if the blood sugar gets over 180 mg/dL, we’ll start seeing sugar in the urine. We also see amino acids and proteins like albumin being reabsorbed here, as well as most of our water soluble vitamins. And we also see reabsorption of the majority of the water in the filtrate by osmosis. We’ll talk about this more in the fluid & electrolyte balance lesson, but this process is called obligatory water reabsorption, meaning the water has to follow these electrolytes because of osmosis and the concentration gradient.

From there, the filtrate moves into the Loop of Henle. Remember the loop of Henle has a descending loop and an ascending loop. What you need to know here is that the descending loop is permeable to water, but the ascending loop is not. So - what we see is that as the filtrate moves down the descending loop, water is able to move out of the filtrate and be reabsorbed into the blood. Then once we hit the ascending loop, water can’t move anymore, so in order to try to balance those concentrations back out, we see some of our ions move out of the filtrate. Big thing to know here is that because all this water leaves as it descends - the bottom portion of the loop of henle is highly concentrated. This comes into play when we talk about water reabsorption in the fluid & electrolyte balance lesson.

Then the filtrate moves into the distal convoluted tubule where the body performs a final regulation of electrolytes based on the body’s needs. We’ll talk in more detail about this, again, in the fluid & electrolyte balance lesson.

The third process we see during urine formation is tubular secretion - this is when substances are moved from the blood into the urine. In the PCT we see waste products, drugs and their metabolites, hormones, hydrogen, and ammonia secreted into the blood. Urea is mostly secreted in the loop of henle. And in the DCT we see secretion of potassium, hydrogen and ammonia. This image also gives you a little recap of what is reabsorbed in the various areas of the nephron as well.

Once the filtrate gets past the nephron and leaves the kidneys, it’s considered urine and heads to the bladder. As the bladder fills, it stretches and at about a quarter full we’ll start to get the urge to go. When it’s time to go, the internal sphincter opens, the detrusor muscle contracts, and we’ll open the voluntary external sphincter in order to actually excrete the urine.
So let’s recap the urine formation process. Urine is composed of water, electrolytes, waste products, and urobilin, which is what gives it its amber color. The processes involved are glomerular filtration, where the blood is filtered through the glomerulus - tubular reabsorption, which is when we bring the important substances back into the bloodstream - and tubular secretion, which is when we push substances into the urine to get excreted out of the body.
Make sure you also check out the renal fluid & electrolyte balance lesson to learn more details about how the kidneys participate in that regulation. Check out all the resources attached to this lesson as well. Now, go out and be your best self today. And, as always, happy nursing!