Many animals use a circulatory system 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.
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The open circulatory system is common among small animals such as arthropods. Instead of blood, the fluid that is circulated is called hemolymph, and it is pumped by the heart into a body cavity called a hemocoel, where it sloshes around and bathes the internal organs in nutrients and gases. There is very low blood pressure, so this is only a suitable system for animals with low metabolisms that do not need quick energy or immune defenses, or blood to reach to far extremities.
Larger animals and vertebrates have closed circulatory systems, including humans. The main functions of the circulatory system are gas exchange, hormone and nutrient distribution, and waste elimination. The two major processes of the closed system are pulmonary circulation and systemic circulation. Deoxygenated blood is passed through the lungs to receive oxygen from inhaled air. Next, systemic circulation distributes the newly oxygenated blood 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 and from all extremities of the body at a rapid rate.
Open Circulatory System
The open circulatory system is the simpler of the two systems. This system is common among arthropods. The heart pumps blood - or as it is commonly known for open circulatory systems, hemolymph - into an open cavity called a hemocoel. The hemolymph mixes with interstitial fluid and sloshes around the hemocoel, bathing the internal organs and delivering nutrients and in some cases, gases such as oxygen. In some animals, the heart is simply an aorta or other blood vessel, and the hemolymph is pulsed throughout the body by muscle contractions.
There are no arteries or major veins to pump the hemolymph, so blood pressure is very low. Organisms with an open circulatory system typically have a relatively high volume of hemolymph and low blood pressure. Examples of animals with open circulatory systems 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 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, and waste elimination.
The two major processes of the system are pulmonary circulation and systemic circulation. In the former process, deoxygenated blood is passed through the lungs for gas exchange, in order to receive oxygen from inhaled air. Next, systemic circulation distributes the newly oxygenated blood throughout the body. The blood picks up carbon dioxide, a metabolism waste product, from cells, and brings it back to the lungs again.
In a closed circulatory system, blood is directed through arteries to veins and to smaller blood vessels 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 and from 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 processes such as locomotion, digestion and respiration, it has need for less oxygen. Since oxygenated blood takes more time to reach the extremities of the body, the open system is only feasible in small animals.
Advantages of the Closed System
The closed system operates with a much higher blood pressure. It is more efficient in that it uses less blood for even higher and faster levels of distribution. Since oxygenated blood may reach the extremities of the body faster than with an open system, organisms with a closed system may have higher metabolisms, allowing them to move, digest and eliminate wastes more rapidly. Due to the efficient distribution of antibodies, immune responses are stronger, helping the body to fight off infection more effectively.