Gases enter and leave the blood during respiration primarily by (c) Diffusion and active circulation. Diffusion allows for the passive movement of gases like oxygen and carbon dioxide across the thin membranes of the alveoli in the lungs and the capillaries surrounding them. Meanwhile, active circulation, driven by the pumping action of the heart, facilitates the transport of oxygen-rich blood from the lungs to the body tissues and the return of carbon dioxide-rich blood from the tissues back to the lungs for exchange, ensuring a continuous supply of oxygen and removal of carbon dioxide.