1. Section 2: Structure and functions in living organisms i) Excretion – humans
Learning objectives:
Recall that the lungs, kidneys and skin are organs of excretion (pg. 112).
Understand how the kidney carries out its roles of excretion and of osmoregulation (pg. 113)

2. Homeostasis
Your body keeps the environment inside your body almost the same all the time. In the tissue fluid surrounding your cells, the temperature and amount of water are kept almost constant. So is the concentration of glucose. Keeping this internal environment constant is called homeostasis.

3. Excretion
The metabolic reactions that take place in your cells constantly produce waste products.
Eg: Carbon dioxide and urea.
Some of these waste products are toxic and can damage your cells.
Therefore, your body removes these toxic substances.
This removal of toxic waste products is called excretion.

4. Egestion Is the removal of indigestible food from the body.
This is removed as faeces.

5. Organs of excretion
There are three important excretory organs in humans
that you need to know about. 
Skin
Skin excretes sweat, containing water, ions (salts) and urea (excretory product).
Lungs
Lungs excrete carbon dioxide and water (waste from metabolic process of respiration).
Kidneys
Kidneys excrete urine, containing urea (excretory waste product), mineral ions, water and other “foreign” chemicals from the blood.

6. The importance of Waste removal
As long as homeostasis is maintained, body cells function efficiently and the body stays healthy. If homeostasis is not maintained a disease may occur, if the imbalance is great enough it may result in death.
Nitrogenous waste products and carbon dioxide are poisons to the body and without their effective removal by the kidneys and lungs respectively, the body would very quickly begin to feel their effects. As soon as nitrogenous waste and carbon dioxide levels rise, the body’s homeostasis mechanisms ensure that the levels are returned to normal. If these systems fail to work then the levels of harmful waste products will rise and have detrimental effects on the individual.