The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs except in crocodilians and birds. (c) Reptiles also have two circulatory routes however, blood is only oxygenated through the lungs. The blood is pumped from a three-chambered heart with two atria and a single ventricle. (b) Amphibians have two circulatory routes: one for oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. (a) Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-chambered heart through the gills and then the rest of the body. Figure 21.4 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. Arthropods like this bee and most mollusks have open circulatory systems.įor more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body therefore, more complex circulatory systems evolved. Hemolymph returns to the blood vessel through openings called ostia. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outcompeted by the arrival of birds 150 million years ago. An open circulatory system does not use as much energy as a closed system to operate or to maintain however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen. This movement allows for gas and nutrient exchange. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As opposed to a closed system, arthropods-including insects, crustaceans, and most mollusks-have an open circulatory system, as illustrated in Figure 21.2 b. In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure 21.2 a.

In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity. The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart.

Heart blood flow diagram with lungs