The small sacs where gas exchange occurs with the bloodstream are known as?

The correct answer is alveoli because they are the tiny air sacs located at the end of the respiratory bronchioles in the lungs, where the critical process of gas exchange takes place. Alveoli are highly specialized structures that allow for the efficient transfer of oxygen into the bloodstream and the removal of carbon dioxide from it. Their walls are extremely thin and are surrounded by a network of capillaries, which facilitates this exchange of gases directly into and out of the blood.

In contrast, bronchioles and bronchi refer to the larger air passages that lead air into and out of the lungs but do not directly facilitate gas exchange. Cilia, on the other hand, are tiny hair-like structures found in the respiratory tract that help move mucus and debris out of the lungs but are not involved in the actual exchange of gases. Thus, understanding the role of alveoli emphasizes their importance in respiratory function and why they are the correct choice in this context.