Respiratory and Circulatory System
Their blood is transparent because it contains no hemoglobin. Red blood cells are usually absent and if present are rare and defunct. Oxygen is dissolved in the plasma and transported throughout the body without the hemoglobin protein. The fish can live without hemoglobin because of their low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment. However, the oxygen carrying capacity of their blood is less than 10% that of their relatives with hemoglobin.
To compensate for the loss of hemoglobin, they have larger blood vessels (including capillaries), greater blood volumes (four times that of other fish), bigger hearts and greater cardiac outputs (fivefold greater) compared to other fish. Their hearts lack coronary arteries and the ventricle muscles are very spongy, enabling them to absorb oxygen directly from the blood they pump. Their hearts, large blood vessels and low viscosity (RBC free) blood are specialized to carry out very high flow rates at low pressures. This helps to reduce the problems caused by the loss of hemoglobin. In the past, it has been widely supposed that their scaleless skin helps absorb oxygen. However, current analysis has shown that the amount of oxygen absorbed by the skin is much less than that absorbed through the gills. The little extra oxygen absorbed by the skin may play a part in supplementing the oxygen supply to the heart which receives venous blood from the skin and body before pumping it to the gills.
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