The circulatory system is the physiological system within organisms that is responsible for providing necessary nutrients and gases to different parts of an organism's body. Different organisms may have different types of circulatory systems. The two main types of circulatory systems are the open circulatory system and the closed circulatory system. Vertebrates and other complex organisms tend to have a closed circulatory system, while many small invertebrates have an open circulatory system.
Distribution of nutrients and oxygen is achieved through a closed circulatory system in humans and in other vertebrates. Closed circulatory systems are systems in which the blood or circulatory fluid is contained in a network of blood vessels that carry the nutrient- and oxygen-rich blood where it needs to go within the body. There are two primary parts to the human circulatory system. Pulmonary circulation occurs when the heart pumps blood to the lungs in order to oxygenate the blood. In systemic circulation, the heart pumps the oxygenated blood through the blood vessels to the rest of the body, which needs oxygen to carry out cellular metabolism.
In open circulatory systems, by contrast, a fluid called hemolymph is present in a body cavity. To receive oxygen and nutrients, organs are bathed directly in this fluid, which is essentially a mix of blood, proteins, water, hormones, sugars, fats, and other biological substances. Hemolymph cannot be directed to specific parts of the body in open circulatory systems, as there is no system of blood vessels in place to direct the movement of the fluid. Muscular movements, as when the organism is moving through its environment, are generally able to shift the hemolymph around within the organism's body.
There are some simple organisms, such as flatworms, that entirely lack circulatory systems of any form. The nutrients obtained through digestion diffuse directly to the cells that need them. Some more complex creatures lacking circulatory systems have highly branched digestive systems so that the nutrients obtained can still be diffused to cells throughout the body, even to parts that are greatly removed from the digestive system.
Circulatory systems can serve purposes other than oxygen and nutrient transportation, particularly in complex organisms. Blood flow in vertebrates, for example, can help control body temperature. This is especially evident in cold environments when blood warmed by the body's internal heat is pumped to the extremities. The lack of blood transportation greatly inhibits this function in organisms with an open circulatory system.