heart

The inner layer, or.
             endocardium, that lines the inside of the heart muscle consists of a thin layer of endothelial tissue.
             overlying a thin layer of vascularized connective tissue. The outside of the heart, the epicardium, is in.
             intimate contact with the pericardium; this serous membrane is a closed sac covering the heart muscle's.
             outside wall. Within the sac, a small amount of fluid reduces the friction between the two layers of.
             tissue. In addition to muscular and connective tissue, the heart muscle contains varying amounts of fatty.
             tissue, especially on the outside. Both anatomically and functionally, the heart is divided into a left.
             and a right half by the cardiac septum. Each half contains two separate spaces: the atrium (pl. atria),.
             or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria,.
             and the lower pumping chambers are the thick-walled ven!.
             tricles. The total thickness of the ventricular walls is about three times that of the atria; the wall of.
             the heart's left half is approximately twice as thick as that of the right half. The thickness of the.
             heart muscle varies from 2 to about 20 mm (0.1 to 0.8 in). This thickness is correlated with the maximum.
             pressure that can be attained in each chamber.
             FLOW OF BLOOD THROUGH THE HEART.
             The right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava,.
             which enter the atrium through separate openings. From the right atrium the blood passes through the.
             tricuspid valve, which consists of three flaps, or cusps, of tissue. This valve directs blood flow from.
             the right atrium to the right ventricle. The tricuspid valve remains open during diastole, or ventricular.
             filling; however, when the ventricle contracts, the valve closes, sealing the opening and preventing.
             backflow into the right atrium. Five cords attached to small muscles (papillary muscles) on the.
             ventricles' inner surface prevent the valves' flaps from being pushed backward.