Which of the following processes causes most of the carbon dioxide from the blood to move into the alveoli?
A.
Diffusion down a concentration gradient
B.
Conversion of carbon monoxide
C.
Passive transport using carrier proteins
D.
Active transport using energy
The Correct Answer and Explanation is:
The correct answer is A. Diffusion down a concentration gradient.
Explanation:
The movement of carbon dioxide (CO2) from the blood into the alveoli of the lungs is primarily facilitated by diffusion. This process occurs as a result of the concentration gradient between the blood and the alveoli. Here’s how it works:
- Concentration Gradient: In the pulmonary capillaries, the concentration of carbon dioxide is higher in the blood than in the alveoli. This difference in concentration creates a gradient that drives the diffusion process. Gases naturally move from areas of higher concentration to areas of lower concentration in an attempt to reach equilibrium.
- Diffusion Mechanism: CO2 molecules passively diffuse across the alveolar-capillary membrane. This membrane is thin and selectively permeable, allowing for the efficient exchange of gases. Since the concentration of CO2 in the alveoli is lower than in the blood, CO2 diffuses out of the blood and into the alveoli, where it can then be exhaled.
- Role of Oxygen: Simultaneously, oxygen (O2) is moving in the opposite direction—from the alveoli into the blood—also by diffusion. This simultaneous exchange of gases is essential for respiratory function, allowing for the oxygenation of blood while facilitating the removal of carbon dioxide, a waste product of metabolism.
- Other Options: The other options provided do not accurately describe the primary mechanism of CO2 transport. B. Conversion of carbon monoxide is irrelevant in this context as CO2 and CO are different gases with different physiological roles. C. Passive transport using carrier proteins typically applies to molecules that require assistance to cross membranes, which is not the case for CO2 diffusion. D. Active transport using energy is not applicable here because the process of gas exchange in the lungs does not require energy; it is driven by the natural tendency of molecules to move down their concentration gradients.
In conclusion, diffusion down a concentration gradient is the key process that enables the transfer of carbon dioxide from the blood into the alveoli for exhalation.