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 [Page 635]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 ischémie 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.

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