Maintenance of adequate blood flow to tissues is of crucial importance for proper physiological function. The properties of circulatory system and blood tissue have evolved in higher, multicellular organisms, in order to adapt to the requirements of tissue perfusion. The circulatory system consists of a pump (heart) and a very extensive network of blood vessels, which are tubes with diameters varying from several micrometers to several centimeters in vertebrates. A detailed discussion of the anatomy of the circulatory system is beyond the scope of this book; general morphological and functional features of the mammalian circulatory system are briefly described below.