Figure 24. The heart.
d. Blood Supply. Blood from the upper part of the body enters the right atrium
by way of a large vein, the superior vena cava, and from the lower part of the body by
the inferior vena cava. When the right atrium becomes filled, it contracts, and blood is
forced through the open tricuspid valve into the right ventricle.
e. Heart Regulation.
(1) The sinoatrial node. The sinoatrial (SA) node is a small, specialized
tissue located in the posterior of the right atrium between the opening of the superior
vena cava and the coronary sinus. The SA node is the pacemaker of the heart. Under
normal conditions, it produces the impulses that determine the heartbeat.