Cells make wastes as they work, the way a kitchen makes scraps. The blood from The Heart and Blood carries those wastes away. Several organs then remove them from the body.
Key idea
The kidneys filter wastes and extra water and salt from the blood and make urine, which is stored in the bladder. The lungs remove carbon dioxide. The skin releases some water and salt in sweat.
Why it works
When cells break down proteins, they make nitrogen wastes such as urea, which would be harmful if they built up. Each kidney contains about a million tiny filters. Blood is pushed through them, and then most of the water, sugar and salt is taken back into the blood. What is left (the wastes and extra water) becomes urine. In a day the kidneys filter about 150 quarts of blood but make only about 1 to 2 quarts of urine.
Worked example: Which organ removes it?
Name the organ that removes each waste: (a) carbon dioxide from respiration (b) urea (c) extra salt on a hot day.
- (a) Carbon dioxide diffuses out in the alveoli and is breathed out: the lungs.
- (b) Urea is filtered from the blood: the kidneys.
- (c) Salt leaves in urine (kidneys) and some in sweat (skin).
Answer: (a) lungs (b) kidneys (c) kidneys and skin.
Worked example: How much is kept?
The kidneys filter about 150 quarts of blood a day and make about 1.5 quarts of urine. About what share of the filtered fluid is kept?
- Kept: 150 − 1.5 = 148.5 quarts.
- Share: 148.5 ÷ 150 = 0.99.
Answer: About 99%.
Common mistake
Students think solid waste from digestion is the main waste the body makes. Most of it is food the body never absorbed, plus gut bacteria: it passed through the digestive tube without entering your cells. The wastes that cells make are carried in the blood and removed by the kidneys, lungs and skin.