Blood is a suspension of differentiated cells in a specialized extracellular fluid, the plasma. The most abundant cells are the erythrocytes, or red blood cells, and the haemoglobin which they contain transports oxygen from the gills to the tissues. The protein moiety of the haemoglobin has also a crucial role in the transport of carbon dioxide, for its cations buffer the protons formed from the dissociation of carbonic acid; there is a complex and changing interrelation between the CO2 in the plasma and the red cell as it passes from the tissues to the gills. The several kinds of leucocytes protect and repair tissues and are transported to sites where their services are needed. The neutrophils are phagocytic, and ingest bacteria. Monocytes move around the body in the blood and differentiate into tissue macrophages. Lymphocytes are carried in the blood to sites of injury and pathogen invasion, and there differentiate into plasma cells which produce specific antibodies. Other lymhocytes react directly to foreign invading cells. Plasma also contains specific proteins which bind and transport particular ions such as copper, iron and iodine. The anions of strong acids, such as chloride and phosphate, in solution in the plasma, are buffered by the presence of bicarbonate ion, and thus are prevented from exerting a large change of pH. This list does not exhaust the transportive functions of the blood; to do that other protective agents, nutrients, hormones, trace elements and vitamins would need to be included.

The crucial role of red blood cells and their contained haemoglobin can be judged by considering the problem of transporting oxygen in a 1 kg trout.