In systemic circulation, O2 enriched blood enters the aorta or aorta from the left ventricle of the heart. From the aorta, blood flows through arteries and capillaries to various organs, tissues and cells. CO2, nutrients, hormones etc. reach the cells through the blood.
Then blood containing CO2 enters the capillary network from various organs, tissues and cells. Blood comes from the capillary network into the veins. Blood from the veins reaches the veins. Veins from the upper part of the body supply blood to the superior vena cava and veins from the lower part to the inferior vena cava. Finally the blood containing CO2 enters the right atrium of the heart through superior vena cava and inferior vena cava. In this way, it takes 25-30 seconds for the blood to flow from the heart and return to the heart.