The operating principle is a process called diffusion, which causes a substance to move from a region of high concentration to a region of lower concentration. Think of a squirt of perfume diffusing from a corner in a room until you can smell it everywhere in the room. Blood returning from the body to the lungs has a higher percentage of carbon dioxide (CO2) than the air inhaled into the lungs does. Conversely the concentration of oxygen (O2) in the inhaled air is greater than the concentration of O2 in the returning blood. Haemoglobin, which can (loosely) bond to both O2 and CO2, facilitates the exchange of gasses from respective regions of high concentration to the regions of lower concentration. Specifically, CO2 moves from the returning blood (higher concentration) to the air in the lungs (lower concentration) and oxygen moves in the other direction, thus oxygenating the blood.
At the time when the blood reaches the lung alveoli
at this stage the partial pressure of the oxygen is higher in the lung alveoli and hence it get diffuse in the blood at the same time the co2 level in the blood is more and less in alveoli hence it get passively diffuse in the lung and exhaled out
low-oxygen blood is pumped from the right ventricle of the heart into the pulmonary arteries and through the lungs, where red blood cells pick up oxygen (usually trading carbon dioxide for oxygen) by way of capillaries in contact with the alveoli. (Oxygen travels through the air we breathe into the alveoli in our lungs.) Some (but very little) oxygen can also be picked up by the plasma at the alveoli. The oxygen rich blood travels from the lungs back to the heart via the pulmonary veins, into the left atrium then into the left ventricle and is pumped into the aorta where then the oxygen rich blood is circulated systemically.
By doing it
Blood becomes oxygenated in the lungs.
Blood is pumped into the lungs and the blood is oxygenated when oxygen is taken into the lungs
Blood is de-oxygenated when it it pumped into the lungs, and after going through the lungs, is now oxygenated.
lungs
Arteries and veins carry freshly-oxygenated blood away from the lungs.
The body picks up oxygen through the lungs.
Pulmonary vein carries oxygenated blood to the heart from the lungs
Blood leaving the lungs is oxygenated
oxygenated blood
Pulmonary veins, these are the only viens that carry oxygenated blood
The left side of the heart pumps oxygenated blood.
The pulmonary vein carries oxygenated blood away from the lungs, to the left atrium of the heart.