In larger organisms, there is usually the need for some type of circulation of fluid to transport various molecules. There is no separation of blood and lymph. Internal organs are bathed in the hemolymph rather than being infiltrated with capillaries as is in a closed system. Closed circulatory systems are common in animals with well developed coeloms. Origin of the heart contraction .
Blood is a word used for the liquid contained within a closed circulatory system.
The plasma contains mostly water, but proteins, glucose, amino acids, salts, enzymes, hormones, metabolic waste molecules, and gases are dissolved in the water. In vertebrates, the cellular component includes several types of elements. In mammals the mature erythrocyte does not contain a nucleus. In mammals and birds the bone marrow is the primary site of production, in other vertebrates the kidneys and spleen are the primary sites. In invertebrates, if a respiratory pigment is present it is sometimes dissolved in the hemolymph rather than contained within cells.