What is the complete cycle of blood from hearth to lungs?

Pulmonary Circulation

The pulmonary circulation is the portion of the circulatory system which carries deoxygenated blood from the heart, to the lungs, and returns oxygenated blood back to the heart. Oxygen-depleted (deoxygenated) blood leaves the right ventricle of the heart and traverses the main pulmonary artery. At the level of the hilum of each lung, the main pulmonary artery gives off lobar branches (right main pulmonary into right and left pulmonary arteries, left main pulmonary artery directly into left pulmonary artery), which lead to segmental vessels and further into arterioles to supply blood to a pulmonary capillary plexus that surrounds millions of microscopic air sacs known as alveoli.

Within the capillary beds in the thin-walled alveoli, carbon dioxide passes from the surrounding capillaries into the alveoli in the lungs during a process called external respiration, while molecular oxygen moves from the alveoli through the pulmonary capillaries into the bloodstream, a process called internal respiration or gas exchange.

Now oxygenated blood returns to the heart by way of pulmonary venules that converge into four pulmonary veins (two from each lung), which discharge into the left atrium in order to complete the pulmonary circulation.

Systemic Circulation

Systemic circulation on the other hand is defined as the movement of oxygenated blood from the left ventricle of the heart to supply oxygen & nutrients to all the remaining vital tissues & organs (organs other than the heart or the lungs) and then the return of de-oxygenated blood with certain metabolic waste products (like carbon dioxide & lactic acid) from the same set of organs & vital tissues of the periphery back to the right atrium of the heart; to once again repeat the pulmonary & finally systemic circulation all over again.