(i) The heart collects blood through both atria and then distributes it through ventricles. 

(ii) The action of heart includes contractions and relaxations of atria and ventricles respectively called as systole and diastole.

(iii) The right atrium receives.de-oxygenated blood from different parts of body and from the heart itself through the openings-superior vena cava, inferior vena cava and coronary sinus respectively. 

(iv) The left atrium receives oxygenated blood from the lungs through the openings of the pulmonary veins. 

(v) The deoxygenated and oxygenated bloods are forced into their respective ventricles. 

(vi) The contraction of atria is initiated and activated by SA node which spreads the wave of contraction through AV node which are latter transmitted to all parts of ventricles via bundle of His and Purkinje's fibres. 

(vii) During the contraction of ventricles the backward flow of blood into the atria is prevented by bicuspid and tricuspid valves. 

(viii) Thus, blood from the left ventricle and right ventricle is forced into the aorta and pulmonary trunk respectively. 

(ix) In this way the oxygenated blood is supplied to the various parts of the body including the heart itself.