Final answer:
Oxygen enters the bloodstream at the alveoli where it diffuses across the membranes into the capillaries, with the correct answer being D) Alveolar sacs.
Step-by-step explanation:
The process through which inhaled oxygen gets into the bloodstream occurs in the lungs, specifically in the alveoli, which are tiny sacs at the end of the air passages. The alveoli are lined with capillaries, and gas exchange occurs by the simple diffusion of gases across these thin membranes.
The oxygen moves from the alveoli into the bloodstream because of the higher concentration of oxygen in the alveoli compared to the blood in the capillaries, creating a concentration gradient favoring this diffusion process. In response to the student's question, the correct option reflecting where oxygen moves to the hemoglobin by passing through a semipermeable membrane is D) Alveolar sacs.