Circulatory System

Blood platelets, or thrombocytes, have a short lifespan of approximately 5 to 9 days. After that, they are removed by the spleen. Their primary role is in blood clotting and repair of damaged blood vessels.

Fishes have a two-chambered heart that receives only deoxygenated blood from the body and pumps it to the gills for oxygenation. In contrast, birds (Aves) and mammals have four-chambered hearts that separate oxygenated and deoxygenated blood.

In reptiles, sinus venosus is present in primitive forms but conus arteriosus is either absent or merged into the ventricle. In amphibians, both structures are present. In birds and mammals, neither sinus venosus nor conus arteriosus is separately present.

Haemoglobin binds oxygen more efficiently under high oxygen pressure , low carbon dioxide pressure , and alkaline conditions (high pH). These conditions are typically found in the lungs.