There is high pressure created in heart chambers during contraction, and lower pressure in blood vessels. Lowest pressure is at the vena cava just before blood enters the right atrium. Blood flow is faster through smaller vessels because flow velocity = flow rate/cross- sectional area. Maintains force over longer period of time. Depolarization of cell from an adjacent cell activates voltage gated. Ca++ channels for ca++ intake. outside the cell, not by depolarization of t-tubule. This allows the atria to empty blood into the ventricles. Impulse travels down to the atrioventricular bundle, and branches down to the base of the heart where they divide into the purkinje fibers. These purkinje fibers contract: refractory period much longer than in skeletal muscle fibers. This is to avoid: the hcn channels that slowly activate control pacemaker potential and, the tetanus rhythm. More na+ flows in than k+ flows out slowly causing depolarization. Hyperpolarization of autorythmic cells activates hcn gated channels.