What structure in the lungs help to increase absorption of oxygen?

The structure in the lungs that help to increase absorption of oxygen are thin-walled sacs called alveoli. They are tiny, bulb-shaped structures that form the end of the smallest air sacs in the lungs, called bronchioles. Alveoli are surrounded by a network of tiny blood vessels called capillaries. Oxygen from the inhaled air diffuses across the thin walls of the alveoli and into the capillaries, where it is carried by hemoglobin in the red blood cells to the rest of the body. The alveoli provide a large surface area for the exchange of gases, which allows for efficient absorption of oxygen and release of carbon dioxide.