Blood

All vertebrates circulate blood within blood vessels. Because blood is enclosed within blood vessels, the circulatory systems of vertebrates are called closed circulatory systems. Some animals without vertebrae, called invertebrates, have circulatory systems that do not contain blood vessels. In these open circulatory systems, the fluid analogous to blood is called hemolymph (Greek, hemo, blood + lympha, water). Examples of animals that circulate hemolymph include insects and aquatic arthropods such as lobsters and crawfish. Like blood, hemolymph transports oxygen and carbon dioxide and has a limited clotting ability. Unlike blood, hemolymph is colorless. Other invertebrates have no true circulatory system. In these animals, it is not possible to distinguish blood or hemolymph from the watery fluid that bathes the tissues. This fluid contains a few defensive cells, proteins, and salts. However, oxygen and carbon dioxide are not transported in this fluid.