Alveoli
The small thin-walled balloon-like structures that help in the exchange of gases are called alveoli. These tiny air sacs are found in the lungs and play a crucial role in the process of respiration.

Anatomy of Alveoli
The human lungs are made up of millions of alveoli, which are surrounded by a network of blood vessels called capillaries. Each alveolus is a hollow, thin-walled structure lined with a layer of epithelial cells. The walls of the alveoli are extremely thin, allowing for efficient gas exchange.

Gas Exchange in Alveoli
When we breathe in, oxygen from the air enters the alveoli through the bronchioles and diffuses across the thin alveolar walls into the surrounding capillaries. At the same time, carbon dioxide, a waste product of cellular respiration, moves from the capillaries into the alveoli to be exhaled.

Surface Area and Efficiency
The alveoli provide a large surface area for gas exchange due to their immense number and microscopic size. This increased surface area allows for a greater amount of oxygen to diffuse into the bloodstream and a larger amount of carbon dioxide to be removed from the body.

Importance of Thin Walls
The thin walls of the alveoli are essential for efficient gas exchange. The thinness of the walls reduces the distance over which gases need to diffuse, allowing for a quicker exchange. It also ensures that the exchange occurs easily through passive diffusion, without requiring much energy.

Conclusion
In summary, alveoli are small thin-walled balloon-like structures present in the lungs that facilitate the exchange of gases. Their microscopic size, large number, and thin walls optimize the efficiency of gas exchange, allowing for the uptake of oxygen and removal of carbon dioxide during respiration.

D) Alveoli