There are two phases of the heartbe

There are two phases of the heartbeat. These phases are called diastole (relaxation) and systole (contraction). Diastole occurs when the ventricle walls relax and blood flows into the heart from the venae cavae and the pulmonary veins. The tricuspid and mitral valves are open in diastole, as blood passes from the right and left atria into the ventricles. The pulmonary and aortic valves are closed during diastole
Systole occurs next, as the walls of the right and left ventricles contract to pump blood into the pulmonary artery and aorta. Both the tricuspid and the mitral valves are closed during systole, thus preventing the flow of blood back into the atria (see Fig 11-7)
This diastole-systole cardiac cycle occurs between 70 and 80 times per minute (100000 times a day). The heart pumps about 3 ounces of blood with each contraction. This means that about 5 quarts of blood are pumped by the heart in 1 minute (75 gallons an hour and about 2000 gallons a day)
The beat of the heart as felt through the walls of the arteries is called the pulse. What is felt is a shock wave that travels along the fibers of the arteries as the heart contracts in systole. The pulse is usually felt just inside the wrist below the thumb at the radial artery (the radius is one of the two bones in the lower arm). Other sites of pulsation include the side of the neck (carotid artery), the elbow (brachial artery), and the anterior side of the hip (femoral artery)
Closure of the heart valves is associated with audible sounds, such as "lub,dub,lub,dub" that can be heard when listening to a normal heart with a stethoscope. The "lub" is associated with closure of the tricuspid and mitral valves at the beginning of systole and the "dub" with the closure of the aortic and pulmonary valves at the end of systole. The "lub" sound is called the first heart sound and the "dub" is the second heart sound because the normal cycle of the heartbeat starts with the beginning of systole. An abnormal heart sound is known as a murmur

Conduction system of the heart
What keeps the heart at its perfect rhythm? Although the heart does have nerves that can affect its rate, they are not primarily responsible for its beat. It is known that the heart starts beating in the embryo before the heart is supplied with nerves and it will continue to beat in experimental animals even when the nerve supply is cut
Primary responsibility for initiating the heartbeat rests with a small region of specialized muscle tissue in the posterior portion of the right atrium, where an electrical impulse originates. This region of the right atrium is called the sinoatrial node (SA node). The SA node is also called the pacemaker of the heart. The current of electricity generated by the pacemaker causes the walls of the atria to contract and force blood into the ventricles (ending diastole)
Almost like ripples in a pond of water when a stone is thrown, the wave of electricity passes from the pacemaker to another region of the myocardium. This region is at the posterior portion of the interatrial septum and is called the atrioventricular node (AV node). The AV node immediately sends the excitation wave to a bundle of specialized muscle fibers called the bundle of His (pronounced hiss). Within the interventricular septum, the bundle of His divides into right and left bundle branches, which carry the impulse to the right and left ventricles, causing them to contract. Thus, systole occurs and blood is pumped away from the heart. A short rest period follows, and then the pacemaker begins the wave of excitation across the heart again. Figure 11-8 shows the location of the SA node, the AV node, and the bundle of His
The record used to detect these electrical changes in heart muscle as the heart beats is called an electrocardiogram (EKG or ECG, from the greek root kardia). The normal EKG show five waves, or deflections, that represent the electrical changes as a wave of excitation spreads through the heart. The deflections are called P, QRS, and T waves. The P wave represents electrical activity of the SA node impulse formation and the change in the electrical activity in the wall of the atria (atrial depolarization). The QRS wave is ventricular depolarization as electricity passes through the bundle of His and the ventricular wall. This is the largest wave because the ventricle contains the most muscle. The T wave represents ventricular repolarization, which is when the ventricular wall relaxes and recovers from contraction. Figure 11-9 illustrates P, QRS, and T waves in a normal EKG. The EKG is used to diagnose a heart attack (myocardial infarction), which causes abnormal deflections.
Normal heart rhythm (originating in the SA node and traveling through the heart) is called sinus rhythm. Sympathetic nerves speed up the heart rate during conditions of emotional stress or vigorous exercise. Parasympathetic nerves slow the heart rate when the need for extra pumping is past