The circulatory system is critical to animal life and functions to transport fluids and molecules throughout an animal’s body. However, the complexity and specific components which comprise this system vary greatly among different species. Furthermore, some unicellular species and even some multicellular animals lack a circulatory system and use diffusion to transport molecules.
Although some metazoans (multicellular animals) rely on diffusion for transporting substances, most animals have a circulatory system. The transportation of fluids and molecules is the major role of this system, along with heat exchange and osmotic/water/ionic balance. The types of substances which the circulatory system transports include carbon dioxide (CO2), oxygen (O2), immune cells, signalling molecules, nutrients and waste products.
The components of the circulatory system vary between invertebrates and vertebrates, with invertebrate systems being comprised of fewer circulatory cells. In vertebrates, humans have the most complex systems and fish have the least specialized systems. Blood cells which are common among most vertebrates, but vary in terms of shape and/or size include erythrocytes, lymphocytes (such as T cells), granulocytes (such as neutrophils) and thrombocytes (platelets). Veins, arties and capillaries are other components of the circulatory system.
Another important difference between animal circulatory systems depends on whether the system is open or closed. Open circulatory systems are simpler and have the blood pumped by the heart into the body cavity, once it exits the artery. The body cavity acts as part of the system. All tissues within the body cavity (hemocoel) are surrounded by blood. An open circulatory exists in many invertebrates like arthropods and molluscs.
Conversely, a closed circulatory system has blood maintained within vessels (arties, arterioles, capillaries, venules and veins) and pumped by the heart through these vessels, which vary in terms of their size and thickness. Most vertebrates, including all jawed vertebrates have a closed system. However, the diversity among these circulatory systems and among the heart organ itself, is incredibly large.
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