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How the Urinary System Maintains Homeostasis

Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Learn about how the urinary system maintains the human body by influencing blood composition and volume and understand how the kidneys are influenced by hormones.

It's All About Balance

Imagine a man-made reservoir fed by a small river and drained by a man-made dam. This reservoir provides drinking water for nearby communities, but also serves as a source of recreation with a thriving community around it. Throughout the year, the reservoir management office wants to keep the water at a relatively constant level. Not letting it get too high to where it spills over its banks, but not too low so that there isn't a sufficient supply of drinking water. The management office can't really control the rainfall but it can control the opening and closing of the dam, opening it to release excess water or closing it to retain water. This system is all about balance, it requires the dam to physically control the flow of water, and it requires the management team to decide how to control the dam.

The Urinary System

Now, let's think about the urinary system, and let's use the reservoir as an analogy for how the urinary system works. I hope you aren't too surprised to find that the function of the urinary system is to filter waste out of the blood and produce urine.

Within the urinary system the key players are the kidneys (there are two of those) where the bloodstream and the urinary system meet. The kidneys contain millions of tiny tubes called nephrons where the blood is filtered, the good stuff (like nutrients) is reabsorbed back into the body, waste is secreted, and water can be secreted or reabsorbed depending on conditions within the body. The fluid produced by the nephrons is urine. The urine is collected into two tubes called the ureters that lead to the bladder. The bladder is the storage compartment for the urine. The last part of the urinary system is the urethra, which is a single tube that leads directly from the bladder to the outside of the body.

components of the urinary system

Homeostasis & the Urinary System

When we think about how the human body works, frequently we invoke the idea of homeostasis. Homeostasis is when the body constantly regulates a variable (blood pressure, temperature, etc.) to keep it close to a specific set point. When a homeostatic variable changes (gets too high or too low) the body will kick in mechanisms to reverse the change and get back to the set point. In our reservoir example, the management team worked to keep the level of water in the reservoir constant despite fluctuations in rainfall. The urinary system influences homeostasis by regulating the amount (volume) and the makeup (composition) of blood.

The makeup of blood is regulated in a couple ways. The kidneys act to filter out metabolic wastes, like urea, that are picked up by the bloodstream. The kidneys prevent the buildup of this waste by removing it from the body. The kidneys also help keep the pH of the blood right where it's supposed to be, which is about 7.4. They do this by controlling the rate of removal of hydrogen ions (H+), which are responsible for the acidity of fluids.

The regulation of blood volume is where our reservoir analogy really comes into play. On a daily basis, your body maintains fluid balance, where fluid intake approximately equals fluid loss. Your diet is the major source of fluid input, and the urinary system is the major route for fluid loss. Fluid balance is an example of homeostasis and many systems in the body influence fluid balance. The kidneys can respond to changes in fluid balance by increasing or decreasing the amount of water in the urine. You experience this relationship in a very direct way. When you are sufficiently hydrated, excess fluid is directed to your urine so the volume is higher, and you can see that it is more dilute (it is a lighter color). When your fluid intake is lower, the body retains water, your urine volume decreases, and the urine is a darker color. But how do the kidneys know whether to keep water or excrete water? In our reservoir example there was a management office that controlled the opening and closing of our dam. In the human body, this control comes in the form of hormones.

Hormones

Hormones are chemical messengers that circulate in the bloodstream and can interact with specific cells to change their activity. Hormones allow bodywide chemical communication. An organ can produce a hormone and release it into the bloodstream to directly influence cell and organ activity. In our reservoir analogy, the management office is like the hormones, the dam can't open or close itself, the office has to make a decision about how to control the dam. Let's take a look at how some different hormones influence fluid balance and how that directly relates to the urinary system.

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