Excretory System – JSS2 Intermediate Science Lesson Note (Second Term)

Excretion is the removal of waste products made by the body’s cells from the body.

Behavioural objectives

1. Define excretion and state how it differs from egestion.
2. Name the four main excretory organs and the waste each removes.
3. Name the parts of the urinary system and state the function of each.
4. State the contents of urine.
5. Explain why excretion is important and how to keep the excretory system healthy.

What is excretion?

Excretion is the process by which the body gets rid of waste made by its cells during metabolism.

• Cause: cells make waste as they release energy and build new materials.
• Danger: if waste stays in the body, it becomes poisonous (toxic).
• Everyday picture: waste in the body is like peels and tins in a kitchen. It must go into the dustbin or the kitchen becomes unhealthy.
• Waste removed: mainly liquid and gaseous waste, such as urea, carbon dioxide and excess water.

What is the difference between excretion and egestion?

• Excretion: removal of waste made inside the cells, such as urea and carbon dioxide.
• Egestion: removal of undigested food (faeces) from the body through the anus.

What are the main excretory organs?

The four main excretory organs in humans are the lungs, skin, liver and kidneys.

1. The lungs

• Waste removed: carbon dioxide and water vapour.
• How: they leave the blood and pass out when we breathe out.

2. The skin

• Waste removed: sweat, which is water, a little salt and a small amount of urea.
• Size: the skin is the largest organ of the body.
• Extra role: sweating helps to cool the body when it is hot.

3. The liver

• Role: it works like a chemical factory that cleans the blood.
• Breaks down: old red blood cells.
• Detoxifies: it changes poisons in the blood into safer substances.
• Makes urea: it changes excess amino acids into urea, which the kidneys remove.
• Bile: it makes bile, which carries waste from broken-down red blood cells into the intestines.

4. The kidneys

• Role: they are the main organs for removing liquid waste.
• Number: most people have two.
• Shape and position: bean-shaped, at the back of the abdomen.
• Function: they filter the blood like a sieve and make urine.

Summary table of excretory organs

What are the parts of the urinary system?

The urinary system is made of the kidneys, two ureters, the bladder and the urethra.

• Kidneys: filter the blood and make urine.
• Ureters: two thin tubes that carry urine from the kidneys to the bladder.
• Bladder: a stretchy bag that stores urine until we are ready to urinate.
• Urethra: the tube through which urine leaves the body.

How do the kidneys make urine?

The kidneys make urine by filtering waste and extra water out of the blood.

1. Blood carrying waste enters each kidney through a blood vessel.
2. The kidney filters the blood and removes urea, extra salts and extra water.
3. The cleaned blood leaves the kidney and returns to the body.
4. The waste and extra water form urine.
5. Urine flows down the ureters to the bladder and is stored.
6. Urine leaves the body through the urethra.

What is in urine?

Urine is mostly water, with salts and urea dissolved in it.

• Water: the largest part of urine.
• Salts: extra salts the body does not need.
• Urea: a waste chemical made in the liver.
• Dark urine: a sign of little water intake, because the waste is more concentrated.
• Clear urine: a sign of plenty of water intake, because the waste is more dilute.

Why is excretion important?

Excretion keeps harmful waste out of the body and keeps conditions inside the body steady.

• Preventing poisoning: urea and carbon dioxide are toxic. A build-up can cause a coma or death.
• Water balance (osmoregulation): with too much water, the kidneys make more urine. When dehydrated, they hold on to water.
• Salt balance: the right amount of salt in the blood helps the heart and muscles work well.
• Temperature control: sweating helps to keep the body at about 37 °C.

How can we keep the excretory system healthy?

• Drink water: take plenty of clean water every day.
• Do not hold urine: go to the toilet when you need to.
• Exercise: it makes the skin sweat.
• Watch your diet: eat less salt and avoid too many sugary drinks.
• Keep the skin clean: bathe daily so sweat does not block the skin pores.

Evaluation

1. Define the term “excretion”.
2. Name the four main excretory organs in humans.
3. Which gas is excreted by the lungs?
4. What is the name of the tube that carries urine from the kidney to the bladder?
5. State two differences between excretion and egestion.

Assignment

Draw a table with two columns, “Excretory organ” and “Waste removed”. Fill it in for the lungs, skin, liver and kidneys. Then list the parts of the urinary system in the order urine passes through them.