Alveoli

Alveoli

The alveolar ducts, feed the alveolar sacs where pores connect one alveolus to the adjacent alveolus. 

Some exchange occurs in the respiratory bronchiole. However, the alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. of the alveolar ducts and sacs are where most gas exchange transpires.  Here the walls of the alveoli, devoid of smooth muscle, are surrounded by a web of elastic fibers. 

Cells

The simple squamous epithelia are called type I pneumocytesThin cells that form the alveolar walls, allowing for gas exchange. or alveolar cellsThe basic structural and functional units of life.. They facilitate gas exchange between the air of the alveoli and the blood in the capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. surrounding the alveoli. The respiratory membrane is composed of these type I pneumocytes fused to the endothelial cells of the surrounding blood capillaries. 

Scattered among the type I pneumocytes are the surfactant-making type II pneumocytesCells that produce surfactant to reduce alveolar surface tension. also called great alveolar cells. These cells appear as simple cuboidal cells with a dome-shaped apical surface projecting into the lumenThe inside space of a hollow organ or structure. of the alveolus. A certain level of alveolar surface tensionThe force exerted by the liquid lining the alveoli, which tends to collapse them; reduced by surfact is needed to help the lungs decrease in volume passively during expirationThe process of exhaling, which can be passive (relaxation of respiratory muscles) or active (involvi. However, surfactantA substance secreted by Type II pneumocytes that reduces alveolar surface tension. is needed to break up that surface tension. By doing this, surfactant makes it easier to inhale with less effort. It increases what is known as complianceThe ease with which the lungs expand and contract during breathing.. Compliance is the ability to produce pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. changes with very small changes in volume.  

Alveolar macrophages are monocytes that have permanently moved into lung tissue. They roam the lung tissues. They also engulf any foreign material for immune surveillance.