Transport: blood and circulation

The blood system transports nutrients, oxygen, carbon dioxide and nitrogenous wastes; other functions include defence. Fish have a closed, single circulation in which blood is pumped by a contractile heart via a ventral aorta to the gills, then via the dorsal aorta to vessels supplying the tissues and organs, with a venous return to the heart. Large venous sinuses occur in elasmobranchs. Air-breathing fish have modifications of the circulation. Complex networks of narrow blood vessels can occur as red patches, retia, maximizing transfer of nutrients, oxygen or heat. Most fish have nucleated red blood cells (erythrocytes) with haemoglobin. The types of white blood cells (leucocytes) are similar to those of other vertebrates but there are thrombocytes rather than platelets. Nutrient transport is in the plasma, the fluid component of the blood, which may also carry antifreeze agents and molecules (e.g. urea in elasmobranchs) which counteract deleterious osmotic effects