Unit 4: The circulatory and excretory systems                       1   2   3   4

4. The excretory system

In addition to the CO₂ produced during cellular respiration, the metabolic activity of our cells produce other many waste substances that we have to expulse out of our bodies.

First these waste substances pass from cells to blood and then, blood carries them to the excretory organs which eliminate them.

Excretion is the process of removing waste substances produced by vital activity from blood.  

The excretory system is formed essentially by the urinary system and the sweat glands, although other organs and organs systems share this function with them. These other organs are:

- Lungs (Respiratory system): They eliminate the CO₂ produced in cellular respiration.

- Liver (Digestive system): It expulses bile pigments (rests of erythrocytes that cannot be recycled) and toxins ingested (e.g. medicines, alcohol, etc).

- Large intestine (Digestive system): It makes and expulses faeces which are non-digested remains of food mixed with excretion products of the liver.

Anyway, the most toxic waste substances produced by our bodies come from the destruction of proteins and nucleic acids. They are respectively urea and uric acid and they have to be eliminated by specific organs: kidneys and sweat glands.