=To trace one complete circuit of the blood= (Fig. 54), let us begin with the blood in the _capillaries_ of the outer tissues, such as the skin or muscles. The blood goes through small veins which unite into _two large veins_, through which it enters the receiving chamber, or _right auricle_, goes through the _tricuspid valve_ into the expelling chamber, or _right ventricle_, then through a _semilunar valve_ into the _pulmonary artery_ leading to the _lungs_. Becoming purified while passing through the _capillaries of the lungs_, the blood goes _through the pulmonary veins_ to the _left auricle_ (Fig. 54), then through the _bicuspid_ or mitral valve, to the _left ventricle_, whence it is forced through a _semilunar valve_ into the largest artery of the body, called the _great aorta_ (Fig. 54). Thence it goes to the _smaller arteries_, and then to the _capillaries_ of the tissues in general, thus completing the circuit. [Illustration: FIG. 55.--THE LEFT SIDE OF HEART (plan), showing the left ventricle at the moment when relaxing and receiving the blood from the auricle; and the same at the beginning of contraction to send blood into aorta. Notice action of the valve.] =Structure of Veins and Arteries.=--Seen under the microscope the arteries and veins show that they are made of _three kinds of tissues_ arranged in _three coats_ (Fig. 56): a tissue resembling epithelial tissue (Chap. 1), as a lining to lessen friction; an outer connective tissue (Chap. 1), to give elasticity; and a middle coat of muscular tissue to enable the vessels to change in size. Let us see why blood vessels must have these three properties. =Why the Blood Vessels must be Elastic.=--The aorta and its branches are always full of blood. When the left ventricle with its strong, muscular walls contracts, the blood in the aorta and small blood tubes _cannot move forward fast enough to make room for the new supply so suddenly sent out of the ventricle_. Where can this blood go? If a cup is full, it cannot become more full; not so with an artery. The elastic connective tissue allows it to expand as a rubber hose does under pressure. The first part of the aorta having expanded to receive the incoming blood, _the stretched walls contract_ because of the elasticity of the outer connective tissue coat _and force blood into the portion of the aorta just ahead_, forcing it to expand in turn. Thus _a wave of expansion_ travels along the arteries. This wave is called the _pulse_.