In most animals, the heart is a muscular organ that pumps blood through the circulatory system's blood arteries. The pumping blood transports oxygen and nutrients to the body, as well as metabolic waste like carbon dioxide, to the lungs. The heart is roughly the size of a fist.
Did you know that the human heart beats 35 million times per year and pumps 1 million barrels of blood in its lifetime?
The heart is made up of 3 layers of muscle tissue pericardium, myocardium and endocardium and it has four main chambers. The right atrium, left atrium, right ventricle and left ventricle. It is connected to arteries and veins. Arteries carry blood away from the hearts and veins carry blood to the heart.
This diagram shows the circulatory system in humans. The colour blue represents deoxygenated blood (Blood that doesn't have oxygen). The colour red represents oxygenated blood (Blood that has oxygen).
The right atrium receives deoxygenated blood from the rest of the body. This blood is passed onto the right ventricle. From the right ventricle the blood goes to the lungs so it can become oxygenated.
Then the oxygenated blood is received in the left atrium from the lungs which is passed onto the rest of the body. The left side of the heart has a thicker wall because it needs to be strong enough to pump the oxygenated blood to the rest of the body.
Because blood circulates twice through the heart each circuit, it is referred to as double circulatory system.
Watch this 3-minute video about the circulatory system in more detail
The heart also contains valves these prevent the backflow of blood. The right side has the tricuspid valve (3 flaps) and the left side has the bicuspid valve (2 flaps). Both sides have semi-lunar valves. The beating sound of the heart is caused by the valves opening and closing.
The blue whale has the largest heart weighing a whopping 1500 pounds (Roughly 630kg)!
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