North Carolina EMT State Practice Exam 2025 - Free EMT Practice Questions and Study Guide

Oxygen diffuses into the blood primarily in the alveolar sacs, which are the tiny air-filled structures located within the lungs. When you breathe in, air enters the alveoli, where it comes into contact with the thin walls of the capillaries that surround them. This is where gas exchange occurs: oxygen from the air diffuses across the alveolar membrane into the blood in the capillaries, while carbon dioxide in the blood diffuses into the alveoli to be exhaled. The design and function of the alveolar sacs are specifically adapted to maximize this diffusion process due to their vast surface area and thin walls, allowing for efficient oxygen uptake and carbon dioxide removal.

In contrast, the mainstem bronchi, trachea, and larynx play crucial roles in conducting air to the lungs but are not sites of gas exchange. The mainstem bronchi direct air into each lung, the trachea serves as the windpipe leading to the bronchi, and the larynx is involved in sound production and protects the airway during swallowing. None of these structures facilitate the actual diffusion of oxygen into the bloodstream.