How (and Why) Does Your Body Create Urine?

This is the first post in a two part series on the Urinary System. Read Part 2 (When The Urinary Tract Doesn’t Work Right: Urine Tests, Urinary Tract Infections and Kidney Stones).

The Urinary System is one of the most important players in filtering waste out of the body. It influences so much of your body’s overall health. Not only does it work as a filter, but it also creates hormones. When it isn’t working right as a filter, your body can end up hanging on to too much fluid. When your kidneys are not creating hormones correctly, then you can end up with low red blood cell production, muscle weakness and bone softening, and unhealthy blood pressure. Now that we know that this system does way more than just help you pee, let’s dive in to see how it actually works.

Your Urinary System is made up of your kidneys, ureters, bladder and urethra.

Kidneys

Your two kidneys are amazing and absolutely incredible in the work that they do. I remember being in my professor’s office hours several times trying to wrap my head around the intricacies of how this organ works. Your kidneys filter half a cup of blood every minute, this means they each filter about 37 gallons of blood every day. During this process they create 4 to 8 cups of urine.

How do they make urine?

Your renal artery delivers blood to your kidney (for the sake of simplicity I am going to only talk about one kidney, but the same process is occurring in the second). This artery branches into smaller segments until it reaches a nephron. There are about a million nephrons inside your kidney. This is where the waste removal process begins. Each nephron has two parts, a glomerulus and a renal tubule. When the artery enters the glomerulus it enters a network of capillaries. These capillaries keep the larger items like red blood cells and proteins inside your blood, while allowing smaller items to enter the glomerulus.

The glomerulus then sends these smaller items into a renal tubule. This is where the waste products in your blood gets filtered out. The tubule hangs onto things like urea and creatinine. Urea is a waste product left over by the liver after it breaks down proteins, it contains nitrogen and needs to get removed from your body. Creatinine is a waste product that is made by your muscles after the normal process of them using energy to help your body move. This renal tubule also hangs onto a portion of the acid that accumulates in your blood to prevent your blood from becoming too acidic which is very important for keeping your body functioning properly.

What about all of the good stuff that was in your blood? Well, there is a blood vessel that runs alongside the tubule. As the tubule recognizes the products that are still helpful, along with most of the water in your blood, it passes these items back into your blood vessel to stay in your bloodstream. Remember how we said your body only makes about 4 to 8 cups of urine each day? That means that your kidneys are returning about 36 and a half gallons of water, salts, and minerals (think sodium, potassium and calcium) back to your bloodstream.

This is where the waste products and the good products completely part ways. All the good stuff in your blood that your body still needs pass into larger and larger vessels until they leave the kidney through the renal vein. All of those waste products stay in the tubule and get passed into your ureter. All of this just took place inside ONE nephron. And each kidney has about a million nephrons.

But wait, there’s more!

Your kidney create hormones. One of the main hormones is what tells your body to create more red blood cells (your body’s cells can’t get oxygen without red blood cells). Another is the hormone that turns vitamin D into a form that the body can use (your body’s cells can’t get calcium without vitamin D). Your kidneys also creates an enzyme that keeps your blood pressure from becoming too low. This enzyme affects your adrenal gland that sits on top of each kidney. (This is part of your Endocrine System rather than your Urinary System). Ultimately this influences how the renal tubules excrete or reabsorb sodium and water. Talk about a hard working organ! Pretty amazing, right?

Ureters

Now let’s get back to all of that waste product the tubule identified as needing to get out of your body. The tubule sends the waste product through tubes to get to your ureters. These are two tubes (one coming from each kidney) that carry what is now your urine down into the bladder.

Bladder

As the waste product urine leaves the kidneys it travels through your ureters and ends up in your bladder. Your bladder is the organ that holds your pee. This is so important because if you didn’t have a way to store pee, as your body continually makes urine, you would just have a constant trickle of pee coming out of you. When your bladder is empty it is about 2 inches but it can stretch to be about 6 inches. Generally your bladder holds about 2 cups of urine. When your bladder has stretched to a certain point it sends a signal to your brain using your nerves and this is what gives you the “I need to go to the bathroom” thought. When your brain tells your bladder muscles to contract urine is forced out of your bladder and into a tube called your urethra. There is a sphincter at the base of your bladder where it attaches to the urethra that relaxes in order to release the urine. This first sphincter is an involuntary muscle meaning you can’t voluntarily keep it from relaxing. In order for you to pee, you need both your bladder muscles to contract and your sphincter muscles to relax in coordination.

Urethra

When urine exits the bladder it enters the urethra. As we just mentioned the first sphincter that relaxes to allow urine to exit the bladder is an involuntary muscle. The second sphincter which is just outside of the bladder and surrounds the urethra is the one that we can “voluntarily” contract to keep the urine from progressing through the urethra.

Incredible, Right?

When we see all that has to work in coordination for waste to first be filtered out of your body correctly. Then we see all of the nerves and muscles that must work in coordination to allow it to leave your body this really is an amazing system! It also helps us to understand how serious complications can be when parts of this system are not working right.