What are the features of the circulatory system in birds?

All birds have a closed circulatory system. The four-chambered heart is well developed and reaches 1% of body weight. The heart is called four-chambered because it has four “parts.” These are two atria, two ventricles. The blood passes only through the vessels and is divided into venous and arterial. Arterial blood does not mix with venous blood and passes through two circles of blood circulation – large and small circles.