Circulatory Systems in Various Organisms
The cnidarians are the most primitive organisms of those listed before and thus they have the simplest circulatory system. Their circulatory system does not have any circulatory vessels. The cnidarians live in an aquatic environment and use mostly diffusion to circulate materials through their bodies. Because the cnidarians live in aquatic environments, diffusion is possible because the exchange of materials between organisms and environments must take place across a moist cell membrane. Cnidarians have a rather primitive gastrovascular cavity that utilizes diffusion to spread nutrients and other materials to other parts of the body. The gastrovascular cavity acts as a digestive cavity where diffusion and distribution of needed substances can occur. It attempts to ease the process of diffusion by reducing the distance a substance must diffuse to enter or leave the cell through its large surface area in the organism. However, this gastrovascular cavity is simple compared to hearts and other pumping organs of organisms of annelids, arthropods, amphibians, and birds. Diffusion is the simplest form of circulating materials.
Earthworms, annelids, have a circulatory system. The cnidarians do not. The annelids have a closed circulatory system with circulatory vessels. This circulatory system is much more complex than that of the cnidarians. The annelid earthworms have dorsal and ventral “hearts” or pumping vessels to transport nutrients throughout its body. It has a system of circulatory vessels that take the nutrients and needed materials to where they need to go and more effectively utilize diffusion. This is a big evolutionary leap from transporting nutrients from a big central cavity alone. The circulatory vessels make diffusion more effective by acting as branches from the hearts to transport nutrients and such. In addition, the annelid is a terrestrial organism and diffusion is not as effective alone on dry land as it is in a wet aquatic environment. Thus, the earthworm prefers softer soil than to hard dry soil. I infer that the circulatory vessels and evolved physiology of the earthworm probably has something to do with its environment and the more complex nature of the organism.
The grasshopper, an arthropod, also has a circulatory system as well, but differs much from the annelid. Like the annelid, the arthropod is much more complex than the cnidarian. The arthropod however, does not have a closed circulatory system. Instead, it has an open circulatory system, and an exoskeleton. I infer that the physiology of the exoskeleton influences the evolutionary choice of an open circulatory system over a closed one. The open circulator system has blood and tissue fluids that are mixed together. This mixture is called hemolymph. The arthropods have a heart as well that pumps and circulates this hemolymph around the body to where it is needed. The fluids of the open circulatory system are not transported in vessels like the closed circulatory system. Instead, it flows about in the body cavity of the exoskeleton freely rather than through circulatory vessels. This may allow for more efficient diffusion and utilization of the circulating substances.
The amphibians also have closed circulatory systems. It has a heart with chambers. This is a more complex evolutionary trait compared to the hearts of earthworms and grasshoppers that did not have chambers. The frog has 3 chambers (2 atria and one ventricle). The oxygenated blood and the deoxygenated blood mix in the ventricle chamber of the heart. The circulatory system of the amphibian also has a pulmocutaneous route and a systemic route meaning that one route is for oxygenated blood and the other is for deoxygentated blood. This has better organization than the circulatory systems of the less complex annelids and arthropods. However, both there are some mixing in the third ventricle. Amphibians also utilize diffusion through their skin. Frogs obtain much of their oxygen through their moist skin and some through their lungs. As said before, diffusion must have a moist membrane to transport materials across from the environment to an organism or vice versa. Because the amphibian lives both on land and water, it is exposed to a semi-aquatic environment, and therefore diffusion is effective for amphibians. It also has a closed circulatory system which makes its circulatory system even more complex.
Birds are the most complex of the listed above. Birds have the ability to fly and in doing so they expend much energy keeping themselves in flight. Thus, evolution has granted them a more complex circulatory system to more efficiently deliver oxygenated blood to their muscles, which are used for flight and such. The aves group has four chambers with a complete separation of oxygenated blood and deoxygenated blood. This is more organized and efficient than the circulatory system of frogs as the oxygenated blood and deoxygenated blood do not mix. Birds also have double circulation with a pulmonary and systemic route of blood circulation.
