The trachea is a long, flexible tube with 20 tracheal cartilages, which are C-shaped rings of hyaline cartilage that keep the lumen open. The trachealis muscle, a bundle of smooth muscle and fibroelastic tissue, fills the spaces between the cartilage rings. These hold the trachea lumen open while allowing for flexibility during inspiration and expiration.

The larynx begins the lower respiratory system and connects to the trachea. It allows speech by collaborating with the throat, tongue, and lips. Vocal folds, or real vocal cords, and vestibular folds, or false vocal cords, are found in the larynx and create sounds and resonance, respectively. Both are lined by stratified squamous epithelium and, in some parts, respiratory epithelium, with serous-mucous glands in the underlying loose connective tissue. A thick layer of skeletal muscle (vocalis muscle) covers the vocal folds, allowing them to move and make sounds.

The alveolar epithelium is made up of two types of cells:

Type I pneumocytes are big flattened cells that cover 95% of the alveolar surface area and provide a very thin gas diffusion barrier.
pneumocytes of type II (making up 5 percent of the total alveolar area, but 60 percent of cells). Surfactant is a substance secreted by these cells that reduces the surface tension between the thin alveolar walls, preventing alveoli from collapsing when you exhale.