Structures ensuring that fluids pass through them in only one direction are called
The contraction of the atrial walls forces the blood from the atria through the AV
valves and into the ventricles. When the ventricles contract, the openings between the
atria and the ventricles are sealed by the cusps of the AV valves.
When the ventricles relax, the openings between the great arteries and the
ventricles are sealed by the cuplike cusps of the semilunar valves. (para 10-27)
The action of the heart is accelerated by the sympathetic portion of the nervous
system. It is slowed down by the parasympathetic portion.
The sinoatrial (SA) node is often called the pacemaker of the heart because it
initiates each cycle of the contractions of the heart chamber. Impulses begin in the SA
node, pass to the AV node, and then descend through the septal bundles to stimulate
the myocardium of the ventricular walls to contract. (para 10-28a)
Nutrient blood is supplied to the walls of the heart by the right and left coronary
The openings leading into the coronary arteries are located in the base of the
ascending aorta, just behind the cusps of the semilunar valve. When this valve is open,
its cusps cover the openings of the coronary arteries. When the valve is closed, the
backpressure within the aorta fills the coronary arteries with blood. The coronary
arteries then distribute the blood to all of the tissues of the relaxed heart.
Many of the branches of the coronary arteries are of the end artery type. If such
a branch is closed for any length of time, the tissue in the area supplied will die.
The blood from the tissues of the heart is collected by the cardiac veins. These
veins empty into the coronary sinus, a vessel which in turn empties into the right atrium.
The pumping action of the heart continues without stopping until death. During
each cycle, the heart changes in shape and size and tends to rotate. The number of
cycles per minute is called the heart rate. To reduce friction, the heart is enclosed
Intimately covering the surface of the heart is the visceral pericardium, also
called the epicardium. The outer serous membrane is the parietal pericardium. The
pericardial fluid provides lubrication and reduces the amount of work done by the heart.