Through which structure does oxygen transfer to the blood?

Oxygen transfer to the blood primarily occurs in the alveoli, which are tiny air sacs located in the lungs. The alveoli have thin walls and are surrounded by a network of capillaries, allowing for efficient gas exchange. When air is inhaled, oxygen diffuses across the alveolar membrane into the blood vessels within the capillaries, where it binds to hemoglobin in red blood cells. This process is crucial for delivering oxygen to the body's tissues and organs, enabling cellular respiration and overall metabolic functions.

The bronchioles are air passages that lead to the alveoli but do not participate in the gas exchange process themselves. The lungs are the overall organ responsible for respiration, containing the alveoli, but oxygen transfer specifically occurs at the alveolar level. Veins, on the other hand, carry deoxygenated blood back to the heart, and while they play a role in the circulatory system, they are not involved in the initial transfer of oxygen from the atmosphere to the bloodstream. Thus, the alveoli are the structures responsible for the crucial function of oxygen transfer to the blood.