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The heart is about the size of a clenched fist. It weighs between 300g and 450g and is shaped like an upside-down pear. The heart is located between the two lungs in the centre of the chest, and leans its bottom a little towards the left.
The heart is a double pump made up of four chambers. The right side of the heart consists of the right atrium and right ventricle. The right atrium takes in deoxygenated blood and squeezes it into the right ventricle. The blood is then pumped by artery to the lungs. The left side of the heart consists of the left atrium and left ventricle. Oxygenated blood enters the heart via the left atrium and is squeezed into the left ventricle. The blood is then pumped by arteries around the body.
The heart rate is regulated by a special cluster of cells located in the right atrium, which is called the sinus node. The sinus node prompt the atria to contract first, and then an electrical impulse is sent to a second node (the atrioventricular node), which is found between the atria and their ventricles. After a brief delay, the ventricles contract.
Like any other organ or tissue, the heart needs oxygen. The coronary arteries that supply the heart are connected directly to the aorta, ensuring a rich supply of oxygenated blood. Deoxygenated blood from heart muscle is ‘dumped’ by coronary veins straight into the right atrium.
The arteries eventually divide down into the smallest blood vessel, the capillary. Capillaries are so small that blood cells can only move through them one at a time.
Oxygenated blood from the heart is pumped along arteries, which are muscular. Arteries divide like tree branches until they are slender. The largest artery is the aorta, which connects to the heart and picks up oxygenated blood from the left ventricle.
Veins have one-way valves instead of muscles to stop blood from running back the wrong way. Generally, veins carry deoxygenated blood from the body to the heart, where it can be sent to the lungs.
The heart pumps blood around the body through the blood vessels. Blood pressure is the amount of force exerted on the artery walls by the pumping blood. Blood pressure doesn’t stay the same all the time - it changes to meet your body’s needs. For example, hard physical exercise will raise your blood pressure.