Along the length of the alveoli ducts there are small pouches known as pulmonary alveoli. Pulmonary alveoli gather in clusters and are encased in sacs referred to as alveolar sacs. The respiratory section of the lungs is comprised of the alveoli sacs, the alveolar ducts, and the pulmonary alveoli.


The pulmonary alveoli is considered the functioning necessity of the lungs, as this is where the gaseous exchanges take place. At only .25 to .50 mm in diameter, these tiny little expansions provide the necessary oxygen and waste gas exchanges which allows the entire human body to function properly.

There are approximately 350 million pulmonary alveoli per lung, creating a vast network of functional units throughout the lungs. This actually creates about 760 square feet of surface area of pulmonary alveoli and gas diffusion.

The point of diffusion is very thin, helping to speed up the process. The air-blood barrier is constructed of only a single celled layer that creates the wall of the pulmonary alveolus, the pulmonary alveoli cell, the basement membrane, and on blood capillary cell, which totals a thickness of about 2 micrometers.


Image: Alveoli

The averages of thickness vary slightly, as there are two types of alveolar cells, which vary in thickness.

Type I alveolar cells are the ones which are responsible for the diffusion while type II alveolar cells are responsible for the secretion of a fluid that minimizes the likelihood of the collapse of the pulmonary alveoli. This secretion is known as surfactant.


Pulmonary alveoli resemble the basic structure of a honeycomb, with their polyhedral shape and their basic clustered structures. They are clustered inside the alveolar sacs at the end of the alveolar ducts.

Combined, these units create the majority of the mass of the lungs. The clusters are about .5 mm apart from each other.

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