The heart is a double sided muscular pump nestled between the two lungs in the centre of the chest, behind the breastbone. The muscular organ keeps the blood circulating around the body. The mammalian circulatory system is known as a double circulatory system because, on every circuit of the body, blood passes through the heart twice.
The mammalian heart consists of four chambers: the right and left atria (singular atrium) and the right and left ventricles. The heart is divided into two halves by the septum, and blood in the right and left sides does not mix. The upper chambers or atria receive blood from the veins. The one-way flow of blood through the heart is controlled by valves at two points. The atrioventricular valves (mitral and tricuspid values) are like flags anchored by fibrous cords between the atria and the ventricles. The semilunar valves (aortic and pulmonary valve) are cup shaped and are found at the opening of the arteries. The human heart normally beats between 60 and 80 times a minute at rest. This can increase to well over 100 times a minute during exercise. Blood that has circulated through your body that has had most of its oxygen and nutrients removed, enters the heart from the body via the systemic canal veins into the right atrium. When the Atrium contracts, it pumps the blood through the tricuspid valve to the right ventricle. Contraction of the ventricle closes the tricuspid valve so blood cannot flow backwards, and the blood is pushed out via the