How do you make a 3D model of the human respiratory system?

You can use reference (2d) images of human muscles in the viewport of a 3d program (max, maya, cinema4d, etc) and star modeling based on that reference. or you could just use your imagination

well does it have to work? or can it just be for visual use only? clay, tubes, straws,

Try using tissue paper on a flat surface like a piece of cardboard. Label important things like the towns around the river and what kind of wildlife is there.

Out of clay , plastic

cos day breathe in H2O innit and there trachea is more complex and blood vessels spread out on the gills to create a bigger surface area. :)