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B. Life Systems · Human Organ Systems

The Excretory System: Removing Wastes

B2.1 B2.2 A1.3 A1.5About 45 minutes
Learning goals
  • I can explain why my body must remove wastes.
  • I can name the organs of the excretory system and describe what each one does.
  • I can describe how the lungs and skin also help remove wastes.
  • I can design and test a filter that models how the kidneys clean blood.

Words to know

excretory system
The organ system that removes wastes made by the body's cells.
waste
Something the body has made or has extra of and cannot use, which must be removed.
kidneys
Two bean-shaped organs that filter wastes and extra water out of the blood.
urine
Liquid waste (pee) made by the kidneys: water with wastes and extra salts.
ureters
Two thin tubes that carry urine from the kidneys to the bladder.
bladder
A stretchy, muscular bag that stores urine.
urethra
The tube that carries urine from the bladder out of the body.
filter
Something that lets some materials pass through and holds others back.

Imagine nobody took out the garbage

What if nobody in your home ever took out the garbage or the recycling? After a few weeks, there would be no room left, and it would be smelly and unhealthy!

Your cells make wastes all day as they work. If the wastes stayed in your blood, they would build up and harm you. Your excretory system is your body's clean-up crew.

Wastes and the organs that remove them

Your body removes wastes in several ways:

  • Carbon dioxide (a gas) is breathed out by your lungs.
  • Sweat comes out through your skin. It is mostly water with some salt and a little waste. Sweating also cools you down.
  • Urine is made by your kidneys. It carries away many wastes, extra salts and extra water.

Solid waste (feces) leaves through the digestive system. It is mostly food the body could not digest.

A clear glass of water and a metal reusable water bottle on a sunny kitchen counter.
Drinking enough water helps your kidneys flush wastes out of your body in urine, and replaces the water you lose when you sweat. AI-generated image
Wastes from working cells,carried by the bloodlungscarbon dioxide(breathed out)skinsweat: water, saltsand a little wastekidneysurine: wastes, extrasalts and water
Three ways wastes leave: the lungs breathe out carbon dioxide, the skin sweats, and the kidneys make urine.

The urinary organs: kidneys, ureters, bladder, urethra

  1. Kidneys. You have two, each about the size of a fist and shaped like a bean. They sit toward your back, one on each side of your spine, just above your waist. Blood flows into them all the time. Each kidney has about a million tiny filters that remove wastes and extra water from the blood. The cleaned blood goes back into the body. The wastes and water become urine.
  2. Ureters. Two thin tubes carry urine down from the kidneys.
  3. Bladder. A stretchy, muscular bag stores the urine. As it fills, it stretches, and nerves tell your brain, "Time to find a bathroom!"
  4. Urethra. A tube carries urine from the bladder out of the body.

The liver helps too. It changes some harmful wastes into a less harmful one called urea, which the kidneys then remove in urine.

kidneyureterblood vesselskidneybladderurethra
The urinary organs of the excretory system. Front view: the person is facing you, so their left side is on your right. The kidneys sit toward the back, on each side of the spine, just above the waist.
OrganWaste it removes
kidneys (with ureters, bladder, urethra)urea, other wastes, extra salts and extra water, as urine
lungscarbon dioxide
skinwater and salts in sweat (and a little waste)
liver (helper)changes harmful wastes into urea for the kidneys to remove
Organs that help remove wastes.

Keeping your excretory system healthy

  • Drink water through the day, more when it is hot or you are active. Pale yellow urine is usually a sign you are drinking enough.
  • Go to the bathroom when you need to; don't hold it for a long time.
  • Eat a variety of foods without too much salt.

A person can live a healthy life with just one working kidney. That is why some people can donate a kidney to someone whose kidneys are not working.

Worked example

Let's follow a waste out of the body

A muscle cell in your leg makes waste. How does it leave your body in urine?

  1. The waste passes out of the cell into a capillary (circulatory system).
  2. The blood carries it through veins back to the heart, and the heart pumps it out again through arteries.
  3. Some blood goes to the kidneys. Tiny filters take the waste and some water out of the blood.
  4. The waste and water become urine, which flows down a ureter to the bladder.
  5. When the bladder is full, urine leaves through the urethra.

Cell → blood → kidney → ureter → bladder → urethra. The circulatory and excretory systems work together!

Just for fun

Your kidneys are super-filters. In an adult they filter about 180 litres of liquid out of the blood every day, enough to fill a bathtub, then put almost all of it back! Only about 1 to 2 litres leaves as urine. Talk about careful recycling.

Hands-on activity

Engineering challenge: build a kidney-model filter

Your kidneys filter blood. Can you design a filter that cleans muddy water?

You need: a cup of water mixed with a spoonful of soil or sand, two clear plastic cups, a funnel (or the top of a plastic bottle), and filter materials such as coffee filters, paper towel, cotton balls, a clean cloth, gravel or sand. A grown-up.

  1. Define the problem: design a filter that makes muddy water as clear as possible.
  2. Plan: choose materials and draw your design with labels. Predict how clear the water will be.
  3. Build your filter in the funnel over an empty cup.
  4. Test: pour the muddy water through slowly. Rate the clearness from 1 (still muddy) to 5 (very clear).
  5. Improve: change one thing (add a layer, change the order) and test again. Record what worked best.
  6. Communicate: explain to your family how your filter is like a kidney and how it is different.

Grown-up note: the filtered water is not safe to drink, even if it looks clear. Supervise if a grown-up cuts a plastic bottle. Wash hands after handling soil.

Your turn

1

Why does your body need an excretory system?

2

Name the four urinary organs in the order urine travels.

3

What is the main job of the kidneys?

4

Which organ removes carbon dioxide?

5

Where are your kidneys?

6

What does the bladder do?

More practice: warm-up

1

How many kidneys do most people have?

2

What liquid waste do the kidneys make?

3

What comes out of your skin when you are hot?

4

Which tube carries urine out of the body?

5

What shape are the kidneys?

More practice: on your own

1

Explain how the circulatory system and the excretory system work together.

2

Why might your urine be darker on a hot day when you didn't drink much?

3

Your filter made the water look clear. Is it safe to drink? Explain.

4

How does the liver help the excretory system?

5

Why can a person donate a kidney and stay healthy?

Challenge

1

Name two ways a real kidney is different from your water filter.

2

Sweating helps remove a little waste, but what is its main job? Explain.

3

Mia tested two filter designs but changed both the materials and the amount of muddy water. What should she do differently?

Check your understanding

Quiz

1.

Which organs filter wastes out of the blood?

2.

Which order does urine travel?

3.

Which organ removes carbon dioxide from the body?

4.

The bladder's job is to...

5.

Where are the kidneys?

6.

In the engineering design process, after you test your design you should...

For parents and teachers

Big idea: the excretory system removes wastes. The kidneys filter blood and make urine (kidneys → ureters → bladder → urethra). The lungs and skin also remove wastes.

How to help at home:

  • Encourage your child to notice thirst and drink water, especially on hot days or after sports.
  • During the filter challenge, praise the redesign step: engineers learn from what doesn't work.

Common difficulties:

  • Thinking the kidneys are in the front of the belly. They are toward the back.
  • Mixing up ureter and urethra: there are two ureters (kidneys to bladder) and one urethra (bladder to outside).
  • Use correct names (urine, bladder, urethra) calmly; they are the curriculum's science words.

Safety: filtered water is never safe to drink; wash hands after handling soil.