This chapter discusses various aspects of the cardiovascular system, consisting of the heart and blood vessels. Blood from capillaries is collected by small venules that join to form veins and the veins return blood to the heart. Outside the tunica intima there is the tunica media or middle layer that may consist predominantly of elastic tissue or of smooth muscle. Longitudinally arranged muscle is present in the media of arteries that undergo repeated stretching or bending. Examples of such arteries are the coronary, carotid, axillary, and palmar arteries. The initial segment of each lateral branch is surrounded by a few smooth muscle cells that constitute the precapillary sphincter. The walls of venules have considerable permeability and exchanges between blood and surrounding tissues can take place through them. Sinusoids are found typically in organs that are made up of cords or plates of cells such as liver, adrenal cortex, hypophysis cerebri, and parathyroid glands.