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Heartbeat

Heartbeat

The heart is the muscular organ responsible for circulating blood through the body. The aver­age animal heart will beat several billion times throughout life. The human heart, for example, will beat approximately 2.8 billion times over 75 years at 72 beats per minute. The rate at which the human heart beats is controlled by many different factors. The central nervous system controls heart rate with the medulla oblongata. Activation of the parasympathetic nervous system, using the vagus nerve, causes a decrease in heart rate, while activa­tion of the sympathetic nervous system produces an increase in the rate at which the heart beats. The endocrine system controls the heartbeat with hor­mones, such as epinephrine (adrenaline), which increases heart rate.

Generally, the size of an animal will correlate with how many times its heart will beat in a min­ute. Smaller animals tend to have a higher resting heart rate, such as the mouse with a heart rate around 500 beats per minute. Larger animals, like the whale or elephant, tend to have a lower resting heart rate, roughly around 20 and 35 beats per minute, respectively. During times of hibernation, the heartbeat in some animals can drop to rates drastically lower than while not hibernating. During summer months, a black bear has a heart rate between 40 and 50 beats per minute. While hibernating, this can slow to as few as 8 beats per minute.

The embryonic heart in humans begins to beat around 3 weeks after conception, at which time it beats at around 75 beats per minute, a rate near the mother’s. It then increases linearly to over 170 beats per minute, peaking 7 weeks after concep­tion. The heart rate then decreases to around 145 beats per minute by the 13th week, where it remains until birth. Heart rate remains high throughout childhood, usually not becoming 70 beats per minute until after adolescence.

Young children are often born with heart mur­murs. Although these can indicate a defective heart valve, most heart murmurs are from a more benign cause, a patent foramen ovale (PFO). This is an incomplete closure of the wall between the two atria that closes over time as part of normal neonatal development. If a PFO fails to close, as it does in between 20% to 25% of persons, and persists through adulthood, it can increase risk of stroke. This is because minute blood clots in the deoxygenated blood from the peripheral tissues can bypass the lungs, where they are normally filtered out by the microvasculature of the lungs, and pass through the hole between the two atria. These small aggregates can then find their way to the brain and block blood flow. The result can be either a stroke or a transient ischemic attack (TIA), also know as a mini-stroke.

Blood carries oxygen from the lungs to the peripheral tissues and transports nutrients, hor­mones, and white blood cells. It also aids in waste removal. By beating, the heart is able to pump blood continuously. The rhythmic contractions of the atria and ventricles occur in a synchronized sequence that ensures efficient blood flow. A sin­gle heartbeat begins as the result of a spontane­ous, rapid depolarization of the pacemaker cells located in the sinoatrial node on the right atrium of the heart. This generates a stimulus for contrac­tion. Pacemaker cells in humans depolarize 70-80 times per minute, resulting in a heart rate of 70-80 beats per minute.

Modern technology has developed artificial pacemakers for patients who suffer from heart problems in which either the natural pacemaker cells in the sinoatrial node do not signal at a high enough rate, or they fail to transmit a stimulus for contraction altogether. The small electronic device is surgically inserted into the chest and connected to the heart with electrodes. The electrodes send electrical impulses to the heart, stimulating each heartbeat. More advanced artificial pacemakers can even control the rate of the electrical impulses, increasing heart rate during physical exertion and decreasing it during times of rest and sleep.

In the late 1960s and early 1970s, around the same time that artificial pacemakers were being successfully implanted and becoming more reli­able, the first artificial hearts were being created. Designed by Domingo Liotta, the first artificial heart was successfully implanted in 1969 by sur­geon Denton Cooley, although it was used for only a short period of time until a donor heart became available. The Jarvik-7, designed by Robert Jarvik, was used in about 90 patients as a permanent heart replacement before the practice became banned because of the low survival rate of its recipients. Although most patients lived less than a year with the Jarvik-7 as a permanent replace­ment, it was still used as a temporary device for patients waiting for donor hearts to become avail­able for transplant. In 2004, CardioWest’s tempo­rary Total Artificial Heart (TAH-t) was approved by the Food and Drug Administration, the first implantable artificial heart to receive FDA approval. Developed from the Jarvik-7, it is used only to extend life in patients awaiting heart trans­plant surgery. One patient lived almost one full year with the TAH-t before receiving a donor heart. Over 75% of patients that receive the TAH-t survive through and after the following human donor transplant surgery, most over 5 years.

As the heart ages over time, it can suffer from an array of diseases that ultimately lead to death. Some diseases can be fatal in a very short time; others can take several years until the heart fails. Coronary heart disease is the result of the buildup of plaque in the arteries that sup­ply oxygen to the heart muscle. This occurs over an extended period, sometimes decades. When the plaque then ruptures, the blood forms clots on the plaque and blocks the passage of blood to the heart tissue. Without oxygen it takes only minutes for the heart tissue cells to begin to die and the heartbeat to stop. This is called a myocardial infarction, or heart attack. The heart can also fail as the result of infection by bacteria or a virus. Even poor diet, hyperten­sion, high cholesterol, and structural defects can affect heartbeat and shorten the lifespan of the heart. Although it is possible to survive for prolonged periods of time with some heart dis­eases, it generally takes only minutes for oxygen- deprived tissues in the heart to die and the heartbeat to stop.

Michael F. Gengo

See also Decay, Organic; Diseases, Degenerative; Dying and Death; Healing; Medicine, History of

Further Readings

Martini, F., & Bartholomew, E. (2007). Essentials of anatomy and physiology (4th ed.). San Francisco: Pearson/Benjamin Cummings.

McMillian, B. (2006). Human body: A visual guide. Buffalo, NY: Firefly Books.

Tyson, P. (2000). Secrets of hibernation. Retrieved July 12, 2008, from www.pbs.org/wgbh/nova/satoy ama/hibernation.html

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