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 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 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 hemolymph.
More specifically, hemolymph circulates freely throughout the main body of the insect, called homocoel, and it returns to the heart via special openings called ostia.
Fish have a single circulatory system, while mammals have a double circulatory system. This refers to the mammalian double circulation of blood to the lungs for oxygenation, and separately the circulation of oxygenated blood around the body.
Fish only carry out single circulation of blood around the body, as blood gets oxygenated directly through gills which are en route to blood vessels leading to the heart.
Mammals have a double circulation system which involves:
1. Deoxygenated blood being pumped by the heart to the lungs for more oxygen
2. Oxygenated blood being pumped by the heart to the rest of the body
Having both a pulmonary circuit and a systemic circuit optimises the time taken for blood to oxygenate fully, as it’s slower to move through the lungs (giving more time for diffusion to take place), as well as a better blood flow rate to all the tissues of the body. Oxygenated blood is pumped fast and under high pressure to the rest of the body, while deoxygenated blood can take its time to reload with oxygen from the lungs.