Type II pneumocyte

A pulmonary alveolus (plural: alveoli, from Latin alveolus, 'little cavity') is a hollow cup-shaped cavity found in the lung parenchyma, and is the basic unit of ventilation. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the capillaries that surround the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed. A pulmonary alveolus (plural: alveoli, from Latin alveolus, 'little cavity') is a hollow cup-shaped cavity found in the lung parenchyma, and is the basic unit of ventilation. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the capillaries that surround the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed. The alveoli are located in the alveolar sacs of the lungs in the pulmonary lobules of the respiratory zone, representing the smallest functional units in the respiratory tract. They are also present in the respiratory bronchioles as scattered outpockets, extending from their lumens. The respiratory bronchioles lead into alveolar ducts which are deeply lined with alveoli. Each respiratory bronchiole gives rise to between two and eleven alveolar ducts. Each duct opens into five or six alveolar sacs into which clusters of alveoli open. New alveoli continue to form until the age of eight years. A typical pair of human lungs contain about 300 million alveoli, producing 70m2 of surface area. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. The diameter of an alveolus is between 200 and 500 µm. The alveoli consist of an epithelial layer and an extracellular matrix surrounded by capillaries. In the alveolar walls there are interconnecting air passages between the alveoli known as the pores of Kohn. The alveoli contain some collagen fibers and elastic fibers. The elastic fibres allow the alveoli to stretch when they fill with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air. There are three major types of alveolar cell. Two types are pneumocytes known as type I and type II cells are found in the alveolar wall, and a large phagocytic cell known as an alveolar macrophage moves about in the lumens of the alveoli and in the connective tissue between them. Type I cells are squamous, thin and flat and form the structure of the alveoli. Type II cells release pulmonary surfactant to lower surface tension. Type II cells can also differentiate to replace damaged type I cells. Type I cells are thin and flat epithelial lining cells, that form the structure of the alveoli. They are squamous (giving more surface area to each cell) and their long cytoplasmic extensions line more than 95% of the alveolar surface. Type I cells are involved in the process of gas exchange between the alveoli and blood. These cells are extremely thin sometimes only 25 nm – the electron microscope was needed to prove that all alveoli are lined with epithelial. This thin lining enables a fast diffusion of gas exchange between the air in the alveoli and the blood in the surrounding capillaries. The nucleus of a type I cell occupies a large area of free cytoplasm and its organelles are clustered around it reducing the thickness of the cell. This also keeps the thickness of the blood-air barrier reduced to a minimum. The cytoplasm in the thin portion contains pinocytotic vesicles which may play a role in the removal of small particulate contaminants from the outer surface. In addition to desmosomes, all type I alveolar cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space.