Models aim to gain a better understanding of an aspect or feature of the world by making it easier to visualize, define, quantify and the like by referencing it to something more commonly accepted by all. There are different types of models and each can be used depending on the aim.
If you want people to better quantify a certain aspect, then mathematical models can be used. If you want people to better visualize a certain aspect of the world, then graphical models be best. These are just some examples of models.
Respiratory and circulatory systems work hand in hand to expel wastes. respiratory system removes CO2 and the circulatory uses the blood as a medium to move excess salts and water from source to sink.