Excretion in Humans
ICSE Class 7 · Biology · Chapter 5
What this chapter covers
- What excretion is and why the body needs it
- Excretory products and waste products
- Organs of excretion and their products
- The urinary system
- How the kidneys filter the blood
- Skin and lungs as excretory organs
- Common disorders of the urinary system
Key points
- Excretion is the removal of wastes made by the body's cells during metabolism, and of extra water and salts taken in beyond the body's needs.
- The main excretory products are urea, carbon dioxide, extra water and extra salts.
- Excretion also keeps the amount of water and salts in the blood steady.
- Undigested food is not an excretory product; its removal as faeces is called egestion.
- The kidneys remove urea, water and salts as urine; the skin removes water, salts and a little urea as sweat; the lungs remove carbon dioxide and water vapour.
- The urinary system is made of two kidneys, two ureters, one urinary bladder and one urethra.
- Urine travels: kidney → ureter → urinary bladder → urethra → outside.
- Each kidney contains about a million nephrons, the tiny units that filter the blood.
- The kidneys filter the blood and then take back useful substances and most of the water; what remains is urine.
- A urinary tract infection is caused by germs in the urinary tract; it brings burning and a frequent urge to pass urine.
- A kidney stone is a hard lump of salts formed in the kidney; drinking too little water raises the risk.
- Drinking enough water, not holding urine and eating less salt keep the urinary system healthy.
Questions answered
What is excretion, and why is it necessary for the body?
Excretion is the removal of the wastes made by the body's cells during metabolism, such as urea and carbon dioxide, and of the extra water and salts taken in beyond the body's needs. It is necessary because wastes such as urea and carbon dioxide are poisonous if they build up, and because the body must keep the amount of water and salts in the blood steady.
Name the three main excretory organs and one excretory product of each.
Kidneys remove urea (in urine); the skin removes salts and water (in sweat); the lungs remove carbon dioxide.
Why is the passing out of faeces not called excretion?
Faeces consist mostly of undigested food that never entered the blood or the body's cells. Its removal is egestion. Excretion removes substances that have been in the blood: wastes made by the cells during metabolism, and extra water and salts.
Write the path that urine follows from where it is made to where it leaves the body.
Kidney → ureter → urinary bladder → urethra → outside.
State one function each of the ureter, the urinary bladder and the urethra.
The ureter carries urine from the kidney to the bladder; the urinary bladder stores urine; the urethra carries urine from the bladder to the outside.
Where in the body are the kidneys found?
High in the abdomen against the back wall, one on each side of the backbone, with their upper parts sheltered by the lowest ribs.
What are nephrons, and roughly how many does each kidney contain?
Nephrons are the tiny filtering units of the kidney that remove wastes from the blood and form urine. Each kidney contains about a million of them.
The kidneys filter about 180 L of liquid a day but make only about 1.5 L of urine. What happens to the rest?
About 178.5 L, containing useful substances such as glucose, salts and most of the water, is taken back into the blood.