The heart’s duties are much broader than simply pumping blood continuously throughout life. The heart must also be able to respond to changes in the body’s demand for oxygen. The heart works very differently during sleep, for example, than in the middle of a 5-kilometer run. Moreover, the heart and the rest of the circulatory system can respond almost instantaneously to constantly shifting situations—when a person stands up or lies down, for example, or when a person is faced with a potentially dangerous situation.

Blood pressure, the pressure exerted on the walls of blood vessels by the flowing blood, also varies during different phases of the cardiac cycle. Blood pressure in the arteries is higher during systole, when the ventricles are contracting, and lower during diastole, as the blood ejected during systole moves into the body’s capillaries. Blood pressure is measured in millimeters (mm) of mercury using a sphygmomanometer, an instrument that consists of a pressure recording device and an inflatable cuff that is usually placed around the upper arm. Normal blood pressure in an adult is about 120 mm of mercury during systole, and about 80 mm of mercury during diastole. Blood pressure is usually noted as a ratio of systolic pressure to diastolic pressure—for example, 120/80. A person’s blood pressure may increase for a short time during moments of stress or strong emotions. However, a prolonged or constant elevation of blood pressure, a condition known as hypertension, can increase a person’s risk for heart attack, stroke, heart and kidney failure, and other health problems.

Impulses generated by the SA node spread rapidly throughout the atria, so that all the muscle cells of the atria contract virtually in unison. Electrical impulses cannot be conducted through the partition between the atria and ventricles, which is primarily made of fibrous connective tissue rather than muscle cells. The impulses from the SA node are carried across this connective tissue partition by a small bridge of muscle called the atrioventricular conduction system. The first part of this system is a group of cells at the lower margin of the right atrium, known as the atrioventricular (AV) node. Cells in the AV node conduct impulses relatively slowly, introducing a delay of about two-tenths of a second before an impulse reaches the ventricles. This delay allows time for the blood in the atria to empty into the ventricles before the ventricles begin contracting.

After making its way through the AV node, an impulse passes along a group of muscle fibers called the bundle of His, which span the connective tissue wall separating the atria from the ventricles. Once on the other side of that wall, the impulse spreads rapidly among the muscle cells that make up the ventricles. The impulse travels to all parts of the ventricles with the help of a network of fast-conducting fibers called Purkinje fibers. These fibers are necessary because the ventricular walls are so thick and massive. If the impulse had to spread directly from one muscle cell to another, different parts of the ventricles would not contract together, and the heart would not pump blood efficiently. Although this complicated circuit has many steps, an electrical impulse spreads from the SA node throughout the heart in less than one second.

The journey of an electrical impulse around the heart can be traced by a machine called an electrocardiograph (see Electrocardiography). This instrument consists of a recording device attached to electrodes that are placed at various points on a person’s skin. The recording device measures different phases of the heartbeat and traces these patterns as peaks and valleys in a graphic image known as an electrocardiogram (ECG, sometimes known as EKG). Changes or abnormalities in the heartbeat or in the heart’s rate of contraction register on the ECG, helping doctors diagnose heart problems or identify damage from a heart attack.

Although the SA node generates the heartbeat, nerves and certain chemicals in the bloodstream may influence the heart rate. Impulses from nerves cause the heart to speed up or slow down almost instantaneously (see Nervous System). The nerves that regulate heart rate are part of the autonomic nervous system, which directs activities of the body that are not under conscious control. The autonomic nervous system is made up of two types of nerves, sympathetic and parasympathetic fibers. These fibers come from the spinal cord or brain and deliver impulses to the SA node and other parts of the heart. Sympathetic nerve fibers increase the heart rate. These fibers are activated in times of stress, and they play a role in the so-called fight or flight response that prepares humans and other animals to escape danger. In addition to fear or physical danger, exercising or experiencing a strong emotion can also activate sympathetic fibers and cause an increase in heart rate. In contrast, parasympathetic nerve fibers slow the heart rate. The heart receives impulses from both sympathetic and parasympathetic fibers constantly. In fact, in the absence of nerve impulses the SA node would fire about 100 times each minute—parasympathetic fibers are responsible for slowing the heart to the normal rate of about 70 beats per minute.

Chemicals known as hormones carried in the bloodstream also influence the heart rate. Hormones generally take effect more slowly than impulses from nerves. They work by attaching to receptors, proteins on the surface of heart muscle cells, to change the way the muscle cells contract. Epinephrine (also called adrenaline) is a hormone made by the adrenal glands, which are located on top of the kidneys. Released during times of stress, epinephrine increases the heart rate much as sympathetic nerve fibers do. Thyroid hormone, which regulates the body’s overall metabolism, also increases the heart rate. The levels in the bloodstream of other chemicals—especially calcium, potassium, and sodium—also affect heart rate and rhythm.

An increase in either heart rate or stroke volume—or both—will increase cardiac output. During exercise, sympathetic nerve fibers increase heart rate. At the same time, stroke volume increases, primarily because venous blood returns to the heart more quickly and the heart contracts more vigorously. Many of the factors that increase heart rate also increase stroke volume. For example, impulses from sympathetic nerve fibers cause the heart to contract more vigorously as well as increasing the heart rate. The simultaneous increase in heart rate and stroke volume enables a larger and more efficient increase in cardiac output than if, say, heart rate alone increased during exercise. In a healthy adult during vigorous exercise, cardiac output can increase six-fold, to 18 liters per minute per square meter of body surface.

                        Conceived by Ajit Kumar , contact me at mr_ajit_kumar@indiatimes.com