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The heart: All you need to know

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  The human heart is a finely-tuned instrument that serves the whole body. It is a muscular organ around the size of a closed fist, and it sits in the chest, slightly to the left of center. The heart beats around  100,000 times a day , pumping approximately 8 pints of blood throughout the body 24/7. This delivers oxygen- and nutrient-rich blood to tissues and organs and carries away waste. The heart sends deoxygenated blood to the lungs, where the blood loads up with oxygen and unloads carbon dioxide, a waste product of metabolism. Together, the heart, blood, and blood vessels — arteries, capillaries, and veins — make up the circulatory system. In this article, we explore the structure of the heart, how it pumps blood around the body, and the electrical system that controls it. Anatomy of the heart Below is an interactive 3D model of the heart. Explore the model using your mouse pad or touchscreen to learn more. The heart consists of  four chambers : The atria:  These are the two upper
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fertility rate

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  fertility rate , average number of children born to women during their reproductive years. For the   population   in a given area to remain stable, an overall total   fertility   rate of 2.1 is needed, assuming no immigration or emigration occurs. It is important to distinguish  birth rates —which are defined as the number of live births per 1,000 women in the total population—from fertility rates. The single most important factor in population growth is the  total fertility rate (TFR). If, on average, women give birth to 2.1 children and these children survive to the age of 15, any given woman will have replaced herself and her partner upon death. A TFR of 2.1 is known as the replacement rate. Generally speaking, when the TFR is greater than 2.1, the population in a given area will increase, and when it is less than 2.1, the population in a given area will eventually decrease, though it may take some time because factors such as age structure, emigration, or immigration must be cons
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The heart’s electrical system

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  The heart’s electrical system To pump blood throughout the body, the muscles of the heart must work together to squeeze the blood in the right direction, at the right time, and with the right force. Electrical impulses coordinate this activity. The electrical signal begins at the sino-atrial node, sometimes called the sinus, or SA, node. This is the heart’s pacemaker, and it sits at the top of the right atrium. The signal causes the atria to contract, pushing blood down into the ventricles. The electrical impulse then travels to an area of cells at the bottom of the right atrium, between the atria and ventricles, called the atrioventricular, or AV, node. These cells act as a gatekeeper. They coordinate the signal so that the atria and ventricles do not contract at the same time. There needs to be a slight delay. From here, the signal travels along fibers, called Purkinje fibers, within the ventricle walls. The fibers pass the impulse to the heart muscle, causing the ventricles to con
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