Heart beat - Cardiac cycle - Heart sounds and regulation of heart beat

Heartbeat 
Blood is forced out of the ventricles with each heartbeat. The stroke volume is the volume of blood pumped by a ventricle with each beat. The cardiac output is the volume of blood pumped by one ventricle per minute. In a resting adult, the cardiac output is usually about 5 liters-approximately the amount of blood in the body. The Medical Focus reading on this page describes congestive heart failure, a condition that occurs when the cardiac output is insufficient to meet the body's needs.
Cardiac Cycle 
From the description of the path of blood through the heart, it might seem that the right and left side of the heart beat independently of one another, but actually, they con­tract together. First, the two atria contract simultaneously; then, the two ventricles contract at the same time. The word systole (sis'to-le) refers to contraction of heart mus­cle, and the word diastole (di-as'to-Ie) refers to relaxation of heart muscle; therefore, atrial systole is followed by ven­tricular systole.
If the heart contracts, or beats, about 70 times a minute, then each heartbeat lasts about 0.85 second. Each heartbeat, or cardiac cycle, consists of the follow­ing elements:
Time                Atria                  Ventricles
0.15 see.        Systole                  Diastole
0.30 see.        Diastole                Systole
0.40 see.        Diastole                Diastole  
 
 This shows that, while the atria contract, the ventricles re­lax, and vice versa, and that all chambers rest at the same time for 0.40 second. The short systole of the atria is ap­propriate since the atria send blood only into the ventri­cles. The muscular ventricles require a longer systole because they pump blood throughout the whole body. When the word systole is used alone, it usually refers to the left ventricular systole.
The heartbeat is divided into three phases: (1) the atria contract during atrial systole, (2) the ventricles contract during ventricular systole, and (3) both the atria and ventricles relax during diastole.
Heart Sounds 
A heartbeat produces the familiar "lub-dub" sounds. The sounds are due to vibrations caused by pressure changes that occur when the chambers contract and the valves close. The "lub" sound is heard when the ventricles contract and the atrioventricular valves close. This sound lasts longer and has a lower pitch than the "dub" sound, which is heard when the semilunar valves close and the ventricles relax. Heart murmurs, or a slight slush sound after the "lub," are often due to ineffective valves that allow blood to pass back into the atria after the atrioventricular valves have closed.
Rheumatic fever resulting from a streptococcal infection is one cause of a faulty valve, particularly the mitral valve. Mitral stenosis is a narrowing of the opening of the bicus­pid valve. If operative procedures are unable to open and/or restructure the valve, it may be replaced by an artificial valve.
The heart sounds are due to the closing of the heart valves.
Cardiac Conduction System
The heart contains specialized cardiac muscle fibers with both muscular and nervous characteristics: They can conduct cardiac impulses throughout the myocardium. These fibers are a part of the cardiac conduction system.
The heartbeat is intrinsic, meaning that the heart will beat independently of outside nervous stimulation. This ability is due to a specialized mass of cardiac fibers called the sinoatrial (SA) node. The SA node is located in the poste­rior wall of the right atrium, just below the opening for the superior vena cava. The SA node is known as the pacemaker because it initiates the heartbeat and automati­cally sends out an excitation impulse every 0.85 second. The atria then contract, and the impulse is sent on to a second node, called the atrioventricular (AV) node. This node is lo­cated in the wall of the right atrium near the septum just su­perior to the ventricles. The AV node conducts the impulse to a group of large fibers called the atrioventricular (AV) bundle (bundle of His). The A V bundle, which is in the up­per part of the interventricular septum, divides into right and left bundle branches. These branches give rise to Purkinje fibers (pur-kin'je), which cause the ventricles to contract. Contraction begins at the apex and moves toward the base, where the blood vessels are located.
With the contraction of any muscle, including the myocardium, electrolyte changes occur that can be de­tected by electrical recording devices. A record of the changes that occur during a cardiac cycle is called an
electrocardiogram (ECG).
The conduction system of the heart includes the SA node. the AV node. the AV bundle. the bundle branches. and the Purkinje fibers. The SA node causes the atria to contract. The AV node and the rest of the conduction system cause the ventricles to contract.
Regulation of the Heartbeat 
While, as discussed in the preceding section, the heartbeat is intrinsic, the rate is regulated by the nervous system. A cardiac control center in the medulla oblongata of the brain can alter the heartbeat rate by way of the autonomic nervous system. Parasympathetic motor im­pulses conducted by the vagus nerve cause the heartbeat to slow down, and sympathetic motor impulses conducted by sympathetic motor fibers cause the heartbeat to increase.
The cardiac control center receives sensory input from receptors within the circulatory system. For example, pres­soreceptors (baroreceptors) are present in the aorta just after it leaves the heart and in the carotid arteries, which take blood from the aorta to the brain. Ifblood pressure falls, as it might if we stand up quickly, the pressoreceptors signal the cardiac control center. Thereafter, sympathetic motor impulses to the heart cause the heartbeat rate to increase. Once blood pressure begins to rise above normal, nerve impulses from the cardiac control center cause the heart­beat rate to decrease. Such reflexes also help control cardiac output and, therefore, blood pressure, as discussed later.
The cardiac control center is under the influence of the cerebrum and the hypothalamus. Therefore, when we feel anxious, the sympathetic motor nerves are activated, and the adrenal medulla releases the hormones norepineph­rine and epinephrine. The end result is an increase in heart­beat rate. On the other hand, activities such as yoga and meditation lead to activation of the vagus nerve, which slows the heartbeat rate.
Other factors also influence the heartbeat rate. A cold temperature slows the heartbeat rate, which is why the body temperatures of a person undergoing open-heart sur­gery is lowered. The correct plasma concentration of elec­trolytes, such as potassium (K+) and calcium (Ca2+), is important to a regular heartbeat.
The heartbeat rate is regulated largely by the autonomic nervous system.