Urinary System – Waste Elimination and Water Balance

What's the Point?

By the end of this lesson, you will be able to describe the structure and function of the urinary system, explain how the body filters waste and maintains water balance, and connect this system’s role to other systems that regulate homeostasis.

Your body is constantly sending and receiving information. The nervous system is your body’s control and communication network—it helps you think, feel, move, and react to your environment. It allows instant communication between the brain, spinal cord, and the rest of the body.

Glass of water with text below. Full text available below image

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Jordan’s Story:
After physical therapy, Jordan always drinks water to stay hydrated. His kidneys work hard to remove any waste from cell repair and flush out what his body no longer needs. Keeping water and electrolytes balanced helps him heal efficiently.

Maya’s Story:
During a long run, Maya sweats to cool her body. Her kidneys must adjust by conserving water so she doesn’t become dehydrated. After the race, when she drinks more fluids, her urinary system increases urine output to remove the excess. Her body constantly adjusts to find the right balance.

Main Structures of the Urinary System

Urinary System
Structure	Functions
Kidneys	Filter blood, remove waste, and regulate water and salt balance.
Ureters	Carry urine from the kidneys to the bladder.
Bladder	Stores urine until it is ready to leave the body.
Urethra	Carries urine out of the body.

Diagram labeling kidneys, ureters, bladder, and urethra.

How the Urinary System Works

Every minute, your kidneys filter about one liter of blood. Here’s what happens during that process:

Filtration: Blood enters the kidneys and passes through millions of tiny filters called nephrons.
Reabsorption: Useful substances like water, glucose, and certain ions are reabsorbed into the blood.
Secretion: Extra waste and water are moved into collecting ducts, forming urine.
Excretion: Urine leaves the kidneys, travels through the ureters, and is stored in the bladder until released through the urethra.

Diagram showing kidney cross-section with labeled nephron, collecting duct, and urine pathway.

Watch the video below to learn more.

Maintaining Balance

The kidneys constantly adjust to maintain homeostasis. For example:

When you drink a lot of water, the kidneys remove extra fluid.
When you sweat or don’t drink enough, the kidneys conserve water.
Hormones like antidiuretic hormone (ADH) and aldosterone signal the kidneys to keep or release water and salts.

This careful regulation helps keep your blood pressure, hydration, and chemical balance stable.

My Body Tracker – Urinary System Entry

You will add information about the urinary system to your ongoing tracker. This entry should include:

- - The main organs and their functions
  - A description of how the nephron filters blood
  - A short paragraph explaining how the urinary system maintains homeostasis and interacts with other systems

Connections to Other Systems

Circulatory System: Delivers blood to the kidneys for filtration.
Endocrine System: Sends hormones that control kidney function.
Digestive System: Provides nutrients and fluids that enter the bloodstream.
Integumentary System: Works with the urinary system to remove waste through sweat.

Test Yourself

Review what you have learned by completing the activities below.

Kidney Connection

Read the following short answer questions. Consider your response, then choose the arrow to reveal a possible answer and see if you are correct.

Explain why your kidneys need to filter your blood even though the liver also helps remove toxins.

The kidneys remove water-soluble wastes and control fluid balance, while the liver processes chemicals and toxins. They work together to keep the blood clean.

Describe what happens in the nephron during reabsorption.

The nephron pulls useful materials like glucose and water back into the bloodstream so they aren’t lost in urine.

Explain how sweating and urination are both ways your body maintains homeostasis.

Releases heat and small amounts of salt and water, while urination removes waste and balances fluid levels. Both help regulate the body’s internal environment.

Your urinary system is the body’s natural filtration and balance center. It works quietly but tirelessly, keeping every cell healthy by maintaining the right levels of water, salts, and waste. When Jordan hydrates after therapy or Maya sweats during a race, their kidneys are constantly adjusting behind the scenes to protect their health.

Consider the following...

Why do you think dehydration can affect both the urinary and cardiovascular systems at the same time?

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