Human circulatory system is also called blood vascular system. Heart is also called myogenic because it is not operated by CNS ,it control itself by nodes.
Heart is situated in the thoracic cavity slightly towards the left in between the lungs this is why left lungs is little smaller than the right one .It has size the of a clenched fist.
Structure
Heart is protected by double walled membrane called as pericardium which is filled by pericardial fluid which prevents friction between two walls during beating of the heart. Heart has four chambers the upper two chambers are called as auricles and other two are called as ventricles. The auricles are divided by tissue called as auricle septum and The ventricles are divided by tissue called as ventricular septum. Auricles and ventricles are divided by atrio-ventricular valves also called as cuspid valves. Between right auricle and right ventricle there are three cuspid valves also called as tricuspid valve. Between left auricle and left ventricle there are two cuspid valves also called as bicuspid valve .Aorta which is the largest artery in our body and which carries oxygenated blood from left ventricle to whole body also, have valves at its opening in left ventricle called as semilunar valves .Pulmonary artery ,which carries deoxygenated blood from right venticles to the lungs also have valves at its opening in right ventricle called as semilunar valves .Blood is brought to the heart by venacava.
Functioning
Blood is brought to the heart from upper part of the body by superior venacava and from lower part of the body by inferior venacava. And is poured in the right auricle which is deoxygenated ,then blood is passed to right ventricles which is then pushed into pulmonary artery ,which carries deoxygenated blood to lungs for oxygenation,then from there the blood is carried to the left auricle by the pulmonary viens. Then the blood is poured into left ventricle,the the bood is pushed into aorta ,and the blood is distributed to whole body.
Note: Ventricles walls are thicker than auricles espeacially left ventricle because it has to pump the blood to whole body regions which needs pressure since right ventricle carry blood only to lungs which needs quiet low pressure. While pumping blood from auricles into ventricles ,semilunar valves are closed. while pumping blood from ventricles into aorta or pulmonary artery ,cuspid valves are closed, to prevent back flow of blood.
Image source
https://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Diagram_of_the_human_heart_%28cropped%29.svg/1200px-Diagram_of_the_human_heart_%28cropped%29.svg.png
