In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation. In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation). The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The blood then continues through the rest of the body before arriving back at the atrium this is called systemic circulation. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated this is called gill circulation. FishĪs illustrated in Figure 2a, fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. Figures 2 and 3 illustrates the basic circulatory systems of some vertebrates: fish, amphibians, reptiles, and mammals. Closed circulatory systems are a characteristic of vertebrates however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy. The larger more complex crustaceans, including lobsters, have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as squids, have evolved a closed circulatory system and are able to move rapidly to catch prey. In an open system, an elongated beating heart pushes the hemolymph through the body and muscle contractions help to move fluids. ![]() Most arthropods and many mollusks have open circulatory systems. Instead, gases, nutrients, and wastes are exchanged by diffusion.įor more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body therefore, more complex circulatory systems evolved. Figure 1. Simple animals consisting of a single cell layer such as the (a) sponge or only a few cell layers such as the (b) jellyfish do not have a circulatory system.
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