A pulmonary alveolus (plural: alveoli, from Latin alveolus, "little cavity") is a hollow 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.[1] Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.[2] 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 absorbed.
Structure
The alveoli are located in the respiratory zone of the lungs, at the ends of the alveolar ducts and alveolar sac, representing the smallest units in the respiratory tract. They provide total surface area of about 75m2.[5]
Bronchial anatomy
A typical pair of human lungs contain about 480 million alveoli,[6] producing 70m2 of surface area.[7][clarification needed] Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 µm, with an increase in diameter during inhalation.[6]
Microanatomy
The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn. The alveoli contain some collagen fibers and elastic fibers. The elastic fibres allow the alveoli to stretch as they are filled 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 cell in the alveolar wall: two types of alveolar cell (also called pneumocytes) and a large phagocyte known as an alveolar macrophage.
Type I cells
Type I cells are thin and flat and form the structure of the alveoli. Type I alveolar cells are squamous (giving more surface area to each cell) and cover approximately 90–95% 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 covered with an epithelial lining. These cells need to be so thin to be readily permeable for enabling an easy gas exchange between the alveoli and the blood.
Organelles of type I alveolar cells such as the endoplasmic reticulum, Golgi apparatus and mitochondria are clustered around the nucleus. The nuclei occupy large areas of free cytoplasm. This reduces the thickness of the cell, thereby reducing the thickness of the blood-air barrier. 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.
Type I pneumocytes are unable to replicate and are susceptible to toxic insults. In the event of damage, type II cells can proliferate and differentiate into type I cells to compensate.