The alveoli and capillaries both have thin walls to maximize gas exchange; alveoli can be found at the ends of the respiratory bronchioles.
The lungs are composed of a series of branching structures called bronchi, each one is smaller than the one before it. The bronchi branch into the bronchioles. The terminal end of each bronchiole becomes a respiratory bronchiole. The respiratory bronchiole then connects to alveolar sacs, which are made up of alveoli. An alveolus (plural, alveoli) is a cavity in the lungs where gas exchange takes place. These air sacs have many tiny capillaries. A capillary is any of a vast network of tiny vessels that enable the exchange of nutrients, gases, and wastes between tissues and blood. There are approximately 300 million alveoli in the lungs. These structures provide the surface area used for gas exchange.
Lower Respiratory System Overview
The surfaces of the alveoli are covered with capillaries, which together form the alveolar-capillary membrane system. These vessels are so thin that gases can pass freely across their surfaces by way of simple diffusion. The lungs of an adult human have over 300 million bronchi and a surface area for oxygen exchange of approximately 800 sq ft. Oxygen passes from the alveoli to the blood and carbon dioxide leaves the blood and passes into the alveoli to be expelled during expiration. The rate of diffusion between the alveoli and the capillaries is dependent upon the thickness of the tissues and the surface area through which the gases can pass.
Alveoli are surrounded by a net of elastic fibers that hold the structures together. There are also pores that connect alveoli lying next to each other. These pores allow for the equalization of air pressure in the lungs and also provide another mechanism for air to travel through damaged alveoli.
There are three main types of cells found within the alveoli, each playing a role in gas exchange:
Type I alveolar cells—a single layer of squamous epithelium surrounded by a very weak basement membrane. These cells form a layer that is thinner than a piece of paper.
Type II alveolar cells—cuboidal shaped epithelial cells found scattered within the Type I cells. Type II cells release a fluid called a surfactant. Surfactant is a fluid with lipoprotein that coats the surfaces of the alveoli and keeps them from collapsing during expiration; it also reduces tension of the alveolar surfaces. Type II alveolar cells also release several types of proteins that protect the alveoli from being invaded by pathogens.
Alveolar macrophages—cells that move freely along the alveolar surfaces, taking in dust particles, bacterial invaders, and other particles that may damage the lungs. The surfaces of alveoli are surprisingly sterile considering how many infectious organisms get brought into the lungs.