Alveoli – Tutorial
Please read Unit 9 – Introduction to Respiratory System Tissues prior to completing the activities in this chapter.
Introduction to Alveoli
Alveoli are microscopic air sacs in the lungs at the end of the respiratory tree where gas exchange occurs between air and the bloodstream. Respiratory bronchioles at the terminal end of the respiratory tree connect with groups of alveoli along alveolar ducts which deliver air to individual alveoli. Each lung contains hundreds of millions of alveoli with an average diameter of approximately 200 micrometers, however the size of alveoli increases slightly during inhalation (alveoli stretch) and decreases slightly during exhalation (alveoli recoil).
The walls of alveoli are extremely thin and comprised primarily of simple squamous epithelium. The thin simple squamous cells that form the majority of an alveolar wall are known as type I pneumocytes (type I alveolar cells). The interstitial spaces between alveoli are filled with an extensive network of capillaries (alveolar capillaries). Gas exchange occurs across the thin fused membranes formed by the simple squamous linings of alveoli and the simple squamous walls of alveolar capillaries.
Scattered among the simple squamous cells of alveolar walls are larger cuboidal-like cells known as type II pneumocytes (type II alveolar cells) that secrete surfactant. Surfactant is an oily substance that coats the inner surface of an alveolus and breaks the surface tension of water which helps prevent alveoli from collapsing. Also found inside the lumen of an alveolus are large phagocytic cells known as alveolar macrophages. Alveolar macrophages patrol the lumen of an alveolus and phagocytize any foreign particles that were not cleared from inspired air along upper portions of the respiratory tract.
Tutorial: Use the image slider below to learn more about the structure and characteristics of alveoli.
Microscopy: Use the image slider below to learn how to use a microscope to study alveoli on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure and function of alveoli.
