Respiratory System Histology

What Is the Histology of the Respiratory System?

The histology of the respiratory system is its microscopic structure: the layers, cells and fibers that make up every airway wall from the nasal cavity to the alveoli. Studying these tissues explains why the upper airways can clean and warm air, while the deepest parts of the lungs can pass oxygen into the blood.

Histologists divide the airways into two zones. The conducting zone, from the nose to the terminal bronchioles, is built for moving and cleaning air, so its walls are thick and supported. The respiratory zone, from the respiratory bronchioles to the alveoli, is built for diffusion, so its walls are as thin as living tissue can be. The guide to the conducting zone vs respiratory zone shows where one ends and the other begins.

Explained simply

Picture the airways as a pipe system that turns into a sponge. The big pipes near the top have stiff rings, a wet sticky lining and tiny hairs that sweep dirt away. Deeper down the pipes get narrower and softer, and at the very end they become bubbles with walls so thin that oxygen can slip straight through them into the blood.

What Tissues Make Up the Respiratory System?

All four basic tissue types are found in the respiratory system: epithelium lines the airways, connective tissue (including cartilage) supports them, smooth muscle adjusts their width, and nerve tissue controls glands, muscle and reflexes. Most airway walls arrange these tissues in four layers.

1. Mucosa: the inner lining, made of respiratory epithelium resting on a basement membrane and a loose connective tissue layer called the lamina propria, rich in elastic fibers and immune cells.
2. Submucosa: denser connective tissue holding seromucous glands, which add watery and mucous secretions to the lining.
3. Cartilage and smooth muscle layer: hyaline cartilage rings or plates that keep the airway open, plus smooth muscle that can narrow it.
4. Adventitia: the outer connective tissue that anchors the airway to nearby organs.

The key cell of the lining is the ciliated columnar cell, which carries hundreds of motile cilia. Goblet cells between them release mucus, and the two together form the mucociliary escalator that carries trapped dust up and out of the airways. Basal cells near the basement membrane act as stem cells that replace worn-out lining cells.

What Does the Upper Respiratory Tract Look Like Under a Microscope?

The upper respiratory tract is lined by three kinds of epithelium, each matched to the wear it faces: stratified squamous where friction is high, respiratory epithelium where air must be cleaned, and olfactory epithelium where odors are sensed. The upper respiratory tract page covers the anatomy of each part.

• Nasal vestibule: the entrance of the nose is lined by skin, keratinized stratified squamous epithelium with coarse hairs, sweat glands and sebaceous glands.
• Respiratory region of the nasal cavity: pseudostratified ciliated columnar epithelium over a lamina propria packed with thin-walled veins, which warm the air passing over the nasal conchae.
• Olfactory region: a small patch in the roof of the nasal cavity with olfactory receptor neurons, supporting cells and Bowman glands, described on the sense of smell page.
• Pharynx: the nasopharynx carries respiratory epithelium, while the oropharynx and laryngopharynx, which also carry food, are lined by nonkeratinized stratified squamous epithelium.
• Larynx: mostly respiratory epithelium, but the true vocal cords and the upper part of the epiglottis are covered by stratified squamous epithelium that resists the constant rubbing of speech and swallowing.

Trachea Histology: What Is the Function of Each Layer?

Trachea histology shows a mucosa of respiratory epithelium, a submucosa with seromucous glands, 16 to 20 C-shaped rings of hyaline cartilage, and an outer adventitia, and each layer has a clear function. The epithelium cleans the air, the glands keep it moist, and the cartilage stops the trachea from collapsing when pressure in the chest changes.

The open ends of each cartilage ring face backward toward the esophagus and are bridged by the trachealis, a band of smooth muscle. When it contracts, as in a cough, the trachea narrows and air rushes out faster. Between the rings, fibroelastic ligaments let the trachea bend and stretch when the neck moves or the lungs fill.

Under the microscope, the tracheal epithelium looks layered because its nuclei sit at different heights, but every cell actually touches the basement membrane, which is why it is called pseudostratified. The basement membrane itself is unusually thick here, and the lamina propria contains a band of elastic fibers that helps the airway recoil.

Bronchi and Bronchioles Histology: How Do the Walls Change?

As the airways branch from the bronchi into the bronchioles, the cartilage breaks up and then disappears, the glands and goblet cells fade out, and smooth muscle becomes the main tissue controlling airway width. This gradual change is the heart of bronchi and trachea histology.

Bronchi

The main bronchi look much like the trachea. Inside the lungs, the C-shaped rings are replaced by irregular plates of hyaline cartilage, and a continuous layer of spirally arranged smooth muscle appears between the mucosa and the cartilage. The epithelium is still respiratory epithelium, but it becomes shorter with each division, and small clusters of lymphoid tissue sit in the wall.

Bronchioles

The bronchioles have no cartilage and no glands, so they rely on smooth muscle and on the elastic pull of the surrounding lung to stay open. Their lining shrinks from ciliated columnar cells to simple cuboidal cells, and goblet cells give way to club cells, dome-shaped cells that release protective proteins and help repair the lining. In the terminal and respiratory bronchioles, the cuboidal wall is interrupted by the first alveoli, marking the start of gas exchange. The full branching pattern is shown on the bronchial tree page.

Lung Histology: What Is the Function of the Alveolar Wall?

Lung histology is dominated by the alveolar wall, a sheet of simple squamous epithelium wrapped around capillaries, whose function is to let oxygen and carbon dioxide diffuse between air and blood. A labeled section of lung looks like lace: thin walls around empty air spaces, with small airways and blood vessels scattered between them.

Three cell types line the alveoli. Type I pneumocytes are extremely flat and cover most of the alveolar surface; type II pneumocytes are plump cuboidal cells that make pulmonary surfactant and can divide to replace type I cells; and alveolar macrophages wander over the surface eating dust and microbes.

Between neighboring alveoli lies the interalveolar septum, which holds capillaries, elastic and reticular fibers and a few fibroblasts. Where a capillary presses against a type I cell, their basement membranes fuse, creating the respiratory membrane, which is about 0.5 micrometer thick in most places. Elastic fibers in the septa store energy as the lungs stretch and release it to push air out during quiet exhalation. The outer surface of each lung is covered by the visceral pleura, a layer of flat mesothelial cells over connective tissue.

Common Conditions That Change Respiratory Tissue

Many lung and airway diseases are defined by what they do to these tissues, which is why histology is the basis of pathology. Our respiratory diseases section explains each condition in more depth.

• Squamous metaplasia: long-term irritation, especially from tobacco smoke, can replace ciliated epithelium with tougher stratified squamous cells that cannot clear mucus.
• Chronic bronchitis: the submucosal glands enlarge and goblet cells multiply, producing excess mucus.
• Asthma: the airway wall thickens, the basement membrane zone thickens, smooth muscle grows and mucus can plug small airways.
• Emphysema: alveolar walls are destroyed, leaving fewer, larger air spaces.
• Pulmonary fibrosis: scar tissue thickens the interalveolar septa and the respiratory membrane.