1. Compare the roles of respiratory, circulatory and excretory systems
Respiratory System
The respiratory system enables gaseous exchange to occur between an organism and its external environment. Organisms take in oxygen which is essential to keep alive and remove carbon dioxide as it can becomes toxic if present in large quantities.
The organs responsible differ between animals as mammals have lungs and fish have gills.

2. Circulatory (transport) System A transport system ensures that all cells in an organism are supplied with necessary nutrients and gases as well as removing waste. The circulatory system transports gases eg oxygen and carbon dioxide, nutrients, waste products, hormones and antibodies. It is responsible for maintaining a constant internal environment ie pH and osmotic pressure. The circulatory system also removes any toxins or pathogens and is responsible for the distribution of heat.

3. Excretory System
The excretory system is responsible for removing metabolic wastes from the blood and to expel them from the organism.
Two main waste products are:
Nitrogenous wastes – excreted as part of urine
Carbon dioxide – expelled from the lungs
Excretory system is closely linked to water balance in an organism. The more toxic the waste, the greater amount of water is needed to dilute it for excretion.
Some organisms also use the excretory system to eliminate excess salts, regulate pH of body fluids and blood pressure.