Blood and blood formation (hematopoiesis)

The amount of blood in fish is around 4-7% of body weight. The blood is not evenly distributed through the body, reflecting the general and metabolic activity in the various parts of the fish's anatomy.

The blood is composed of blood serum and the blood cells. The blood serum is mostly water containing salts (sodium, chloride, calcium, magnesium ions etc.), different types of proteins, sugars and fat, dissolved gases, hormones, antibodies, immunoglobulins and vitamins. The blood cells are of two types, red and white blood cells, the red being of great majority. The main blood forming tissue is in the kidney, especially the front part, spleen and the thymus gland. The spleen is made up of the core called medulla and the surrounding cortex. Red blood cells and platelets are formed in the cortex, but white blood cells in the medulla.

Blood forming tissue has also been found in walls of blood vessels, liver, in the walls of the alimentary tract and in the yolk sac at the embryonic stage. Blood and vessels are formed relatively early at the embryonic stage. Fish do not have bone marrow nor lymph nodes as mammals, so blood formation does not take place there.

The red blood cells are egg shaped, have a large nucleus, endoplasmic reticulum and mitochondria, but these cell organs are not found in the red blood cells of mammals. Formation of haemoglobin takes place in the cells during their stay in the vascular system.

The white blood cells are of a number of types, not as many though as in mammals. Their main function is to protect the cells against foreign bodies or matter such as infective or poisonous substances and as such they are a vital part of the fish's immunity system. Lymphocytes are the most common type, forming both lymphoid type of immunity, i.e. free roaming antibodies, and also cell-based immunity or antibodies connected to or contained in the cell. Monocytes are large cells that turn into eating cells or macrophages. Granulocytes are of a few types, becoming apparent in the blood stream in periods of stress and during bacterial or parasitical infections. These cells may have macrophagocytic properties.