The alveoli are a part of the respiratory system that exchange gas and blood. The alveoli's elastic fibers allow for them to expand with air (when you breath in) and contract when you breath out. The alveoli are only found in mammals, while other structures are used for gas exchange in other vertebrates. The alveoli has the form of a hollow cavity. There are around 150 million alveoli in each lung; these alveoli are wrapped in a mesh of capillaries which in total cover about 70% of it's area. A full grown adults's alveoli are around 0.2-0.3 mm in diameter and expand more when you inhale. There are 3 different types of alveolar cells. There is the squamous alveolar cells that form the alveolar wall. Then, there are the great alveolar cells which release a pulmonary surfactant which is used to reduce surface tension. Lastly, microphages are used to destroy foreign bacteria which have entered the respiratory system.
Capillaries are the bodies smallest blood cells measuring one cell thick. These capillaries are used to exchange blood, water, oxygen, and waste. Arteries branch into arterioles which branch into capillaries. There are three types, continuous capillaries allow the transport of water and ions to diffuse. These are located mostly in the skeletal muscles, lungs, gonads and skin while ones with few vesicles can be found in the central nervous system. Fenestrated capillaries are larger and allow the diffusion of proteins as well. They are found mostly in endocrine glands, the pancreas, the intestines, and the kidney. Sinusoidal capillaries are the largest of the three and are special fenestrated capillaries that allow for the transport of white and red blood cells. They can be found in the liver, spleen, bone marrow, lymph nodes, as well as the adrenal gland.