Diffusion in gas exchange occurs as molecules move from what to what?

Diffusion in gas exchange occurs as molecules move from what to what?

• A. Higher concentration to lower concentration
• B. Lower concentration to higher concentration
• C. Equal concentrations
• D. Inside cells only

Answer: (A)

Diffusion moves molecules down a gradient: from where their concentration (or partial pressure) is higher to where it is lower. In gas exchange, oxygen goes from the air in the alveoli, where O2 partial pressure is high, into the blood where it’s lower. Conversely, carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air, which has a lower partial pressure. So the general rule is movement from higher to lower concentration (or partial pressure).

Diffusion moves molecules down a gradient: from where their concentration (or partial pressure) is higher to where it is lower. In gas exchange, oxygen goes from the air in the alveoli, where O2 partial pressure is high, into the blood where it’s lower. Conversely, carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air, which has a lower partial pressure. So the general rule is movement from higher to lower concentration (or partial pressure).