Introduction

Diffusion can serve only the smallest of organisms, such as those that are unicellular, in terms of transporting things around the body.

Additional systems must be in place to serve this purpose in larger organisms, and they do so in varied ways.

Earthworms

Earthworms have a system similar to mammals as they have vascularisation (blood vessels), and transport gases dissolved in the blood which is pumped by 5 “hearts”.

Unlike other organisms, they… well, they have 5 hearts!

Insects

Insects do not carry respiratory gases in the blood, and have an open circulatory system. Their heart is towards the back of the body (dorsal) and has a tube shape.

An open circulation system means that there is no “blood” flowing enclosed into vessels. Instead, the heart pumps a fluid which is a mixture of blood contents and interstitial fluid (tissue fluid between cells) throughout the head and body, which then returns to the heart as it relaxes. This fluid is called…