Emergency Medical Technician (EMT) Intermediate Practice Exam

How do oxygen and carbon dioxide pass across the alveocapillary membrane?

1. Osmosis

2. Diffusion

3. Active transport

4. Facilitated diffusion

The correct answer is: Diffusion

Oxygen and carbon dioxide pass across the alveocapillary membrane primarily through diffusion. This process occurs because the gases move from an area of higher concentration to an area of lower concentration, following their respective concentration gradients. In the lungs, oxygen present in the alveoli is at a higher concentration than in the deoxygenated blood entering the pulmonary capillaries, prompting oxygen to move into the bloodstream. Conversely, carbon dioxide, which is at a higher concentration in the blood than in the alveoli, diffuses into the alveolar spaces for exhalation. This mechanism is critical to the respiratory process, allowing for efficient gas exchange necessary for maintaining proper levels of oxygen and carbon dioxide in the body. Unlike osmosis, which specifically refers to the movement of water through a semi-permeable membrane, and active transport, which requires energy to move substances against their gradient, diffusion operates passively and naturally. Facilitated diffusion, while a means of transporting some substances across membranes, typically involves specificity through carrier proteins, which is not the case for the gases involved in this context.