Oxygen is vital for life. This oxygen is diffused into the body cells via a blood circulatory system. This circulation not only helps carry oxygen to cells, but also carries nutrients and waste to-and-fro from the body tissues. There are two types of circulatory systems in animals. These include a closed circulatory system and an open circulatory system. Most vertebrates have a closed circulation, while an open circulation is found in arthropods and a few mollusks.
Closed Circulatory System
A closed circulatory system is found in vertebrates as well as some of the higher invertebrates. The blood in the body flows through a system of arteries, veins and capillaries. Each organ in the body is surrounded by capillaries that provides nourishment, oxygen as well as takes away waste products. There are two major functions of a closed circulatory system. The pulmonary circulation helps carry deoxygenated blood from the organs to the lungs and the systemic circulation helps in carrying the oxygenated blood from the lungs to the various parts of the body. The blood moves through the veins and arteries due to a difference in pressure. This system suits larger animals perfectly as there is more area to be covered. As this is a closed system, the organs are not bathed in blood directly. The volume of the blood is lower than the blood in animals with an open circulatory system. The blood is pumped into the blood vessels with the help of the heart. This helps maintain the high pressure that is necessary for the blood to reach each and every extremity of the body.
Open Circulatory System
An open circulatory system is one where there is no true heart or capillaries. Instead of the heart, blood vessels pump the blood with force. The blood vessels are connected with the open sinuses. The blood is not actually blood, but a combination of blood and interstitial fluid called the 'hemolymph'. This hemolymph is forced from the blood vessels into the large sinuses. This causes the blood vessels to literally bathe the organs in blood. The interconnected system of sinuses is provided with nutrients, fluids and oxygen directly through the hemolymph. In most cases, these animals have a low blood pressure with the exceptions being arthropods, noncephalopods, mollusks, etc. The metabolic rates of these animals are very low due to the limits of diffusion in their body. However, as it requires less energy for distribution, it is suited for smaller animals.
Comparison between Open Vs. Closed Circulatory System
|Open Circulatory System||Closed Circulatory System|
|The blood leaves the blood vessels and flows through the body cavity in an open circulatory system. This causes the organs in the body to be bathed in blood.||Blood in a closed circulatory system does not flow freely, but is contained within the blood vessels. Thus, the organs are not bathed in blood, but the blood supply is controlled by valves.|
|The blood flow in the body is very slow as the blood pressure is low. The animal is supposed to move its body muscles to help the blood flow throughout the sinuses.||The blood flow in a closed circulatory system is very fast due to high blood pressure. This pressure is provided by the pumping action of the heart.|
|This system suits small animals with a low metabolism and less active respiration, digestion and locomotion.||This system is suitable for larger animals who have a faster metabolism and need to eliminate waste from the body quickly.|
Most arthropods and mollusks have an open circulatory system. The exceptions in this case include cephalopods, nautilus, etc. Higher invertebrates like annelids, squid and octopi as well as vertebrates, have a closed circulatory system. Each system has its advantages and disadvantages, but they suit the animals in which they are found. The above information will help give you an idea about the types of blood circulatory systems found in different animals.