Many organisms require a circulatory system in order to distribute nutrients and materials throughout the body in an efficient matter. There are two types of circulatory systems: open and closed. Each system has its advantages and disadvantages. Although the closed system is more advanced and allows for quicker distribution, many invertebrates and other animals are better suited to the simpler open system.
Functions of the Circulatory System
The circulatory system consists primarily of blood, the heart and a network of blood vessels. The main functions of the circulatory system are gas exchange, hormone and nutrient distribution, as well as waste elimination. The heart pumps the blood throughout the body, which is transported by the blood vessels to the necessary tissues and organs. Gas exchange involves spreading oxygen throughout the body and removing carbon dioxide waste. Oxygen must be delivered to all functioning body cells in order for them to metabolize, or carry out their functions and activities. Blood also transports helpful nutrients and antibodies so that the body's immune system may be healthy and responsive.
Open Circulatory System
The open circulatory system is the simpler of the two systems. Here, the heart pumps blood into open cavities, where blood vessels carry the blood throughout the body at a low pressure. First, the open system bathes all organs and tissues throughout the body with blood and, second, there are no arteries or major veins to increase blood pressure and direct distribution. Organisms with an open circulatory system typically have lots of blood and low blood pressure. Examples of organisms having an open circulatory system include insects, spiders, prawns and most mollusks.
Closed Circulatory System
Larger and more active animals, including all vertebrates, have a closed circulatory system. This more complex system includes two major processes: pulmonary and systemic circulation. In the former process, deoxygenated blood is passed through the lungs in order to receive oxygen. Afterward, systemic circulation distributes the newly oxygenated blood throughout the body. In a closed circulatory system, blood is directed through arteries to veins throughout the body. As opposed to bathing all tissues and organs with blood, the blood remains in vessels and is transported at high pressures to all extremities of the body at a rapid rate.
Advantages of the Open System
The open circulatory system requires less energy for distribution. This system is more suited to animals that have a slower metabolism and a smaller body. Due to the absence of arteries, blood pressure remains low, and oxygen takes longer to reach the body cells. If an organism has a low metabolism, meaning it is generally less active in such processes as locomotion, digestion and respiration, it has less of a need for oxygen. Also, since oxygenated blood takes more time to reach the extremities of the body, the open system is only feasible in smaller animals.
Advantages of the Closed System
The closed system operates with a much higher blood pressure. In addition, it is more efficient in that it uses much less blood for even higher and faster levels of distribution. Since oxygenated blood may reach the extremities of the body much faster than with an open system, organisms with a closed system may metabolize much faster, allowing them to move, digest and eliminate wastes much more quickly. In addition, due to the efficient distribution of antibodies, immune responses are much stronger, helping the body to fight off infection more powerfully.