What transport mechanism allows oxygen to move from the lungs to the blood?

• A. Active transport
• B. Facilitated diffusion
• C. Passive diffusion
• D. Osmosis

Answer: (C)

Oxygen moves from the lungs to the blood primarily through passive diffusion. This process relies on the gradient of partial pressures of oxygen between the air in the alveoli of the lungs and the blood in the surrounding capillaries.

In the lungs, the partial pressure of oxygen is significantly higher than that in the blood vessels. Due to this concentration gradient, oxygen molecules spontaneously move from an area of higher concentration (the alveoli) to an area of lower concentration (the blood) without the need for energy input. This movement continues until an equilibrium is reached, where the partial pressures of oxygen in the alveoli and the blood become matched, although it will never be completely equal during normal respiration.

Active transport is not involved in this process because it requires energy to move substances against a concentration gradient, which is not applicable for oxygen transfer under normal conditions. Facilitated diffusion involves the use of protein channels to assist the movement of molecules across cell membranes but is not the primary mechanism for oxygen absorption in this context. Osmosis specifically refers to the movement of water through a selectively permeable membrane, which is unrelated to the transport of oxygen.