Coordinating the activity of the four chambers to maintain proper blood flow through the heart and its vessels present some challenges. First, when the ventricle contract, the blood must be directed out through the arteries, the large vessels that carry blood away from the heart, and not back up into the atria. Then, once blood has the arteries, it must be prevented from flowing back as the blood relaxes. Four simple one-way valves solve these problems. Pressure in one direction opens them readily, while reverse pressure forces them tightly closed. Atrioventricular valves separate the atria from the ventricles; a tricuspid valve separates the right ventricle and right atrium, and a bicuspid valve lies between the left atrium and the left ventricle. Two
semilunar valves allow blood to enter the pulmonary artery and the aorta when the ventricles contract but prevent it from returning as the ventricles relax.

A second challenge is to create smooth, coordinated contractions of the muscle cells that make up each chamber. When they are excited, muscle cells produce electrical signals that cause the muscle cells to contract. In the heart, the individual heart muscle cells communicate directly with one another through gap junctions in their adjacent membranes. These connecting pores allow electrical signals that cause contractions to pass freely and rapidly between heart cells. The contraction of the heart is initialed and coordinated by a pacemaker, a cluster of specialized muscle cells that produce spontaneous electrical signals at a
regular rate. Although the nervous system can alter the rate of these signals, they are initialed by the pacemaker muscle cells.
The heart primary pacemaker is the sinoatrial node, located in the wall of the right atrium. Signals from the SA node spreadrapidly through both the right and left atria, causing the atria to contract in smooth synchrony.
A final challenge is to coordinate contraction of all four chambers. The atria must contract first and empty their contents into the ventricles in order to be able to refill while the ventricle contract. Thus there must be a delay between the contractions of the atria and those of the ventricles. From the SA node, an electrical impulse creates a wave of contraction that sweep through the atria until it reaches a barrier of unexcitable tissues separating the atria from the ventricles. Here the excitation is channeledthrough a second small mass of specialized muscle cells, the atrioventricular (AV) node, located in the floor of the right atrium.
The impulse is delayed at the AV node, postponing the ventricular contraction for about 0.1 second after contraction of the atria. This delayed give the atria time to complete the transfer of blood into the ventricle before the ventricular begin. From the AV node, the signal to contract spread to the base of the two ventricles along tracts of excitable fibers. The impulse then travel rapidly from these fibers through the communicating muscle fibers, causing the ventricles to contract in unison. When the pacemaker fails to coordinate muscle contractions, uncoordinated, irregular contraction impairs the heart's function, a condition known as fibrillation. Fibrillation of the ventricle is soon fatal because blood is not pump out of the heart to the brain and other organs but is merely sloshed around. Left on its own, the SA node pacemaker would maintain a steady rhythm of about 100
beats per minute. However, the heart rate is significantly altered by the influence of nervous impulses and hormones. In the
resting person, activity of parasympathetic nervous system slows the heart rate to around 70 beats per minutes. When exerciseor stress creates a greater demand of blood flow to the muscles, the parasympathetic influence is reduced and the sympathetic nervous system accelerates the heart rate. Likewise, the hormone epinephrine increases heart rate as it immobilizes the entire body for response to threatening or unfamiliar events. When astronauts were landing on the moon, their heart rates were over 170 beats per minute, even though they were sitting still!