Summary
Highlights
Alveolar ducts are elongated passages with alveoli opening directly into them. Alveolar sacs are spaces surrounded by multiple alveoli. Alveoli are the primary sites of gas exchange, characterized by a very thin septum. They are lined by two main types of cells: Type I alveolar cells (simple squamous, thin for gas exchange) and Type II alveolar cells (cuboidal, produce surfactant to prevent alveolar collapse and act as progenitor cells for Type I).
Alveoli also contain alveolar macrophages, or 'dust cells,' which phagocytize dust particles, carbon, bacteria, and red blood cells (in cases of heart failure, known as 'heart failure cells'). These macrophages contribute to the mottled appearance of lungs in smokers or city dwellers. The lung surface is covered by the visceral pleura, consisting of simple squamous mesothelial cells on connective tissue.
The bronchial tree is divided into a conducting portion, which transports air, and a respiratory portion, where gas exchange occurs. The conducting portion includes the trachea, main bronchi, lobar bronchi, segmental bronchi, and bronchioles. The respiratory portion consists of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
The trachea's wall has four layers: mucosa, submucosa, cartilage, and adventitia. The mucosa has pseudostratified columnar ciliated epithelium with goblet cells and lies on a lamina propria. The submucosa contains seromucous glands. The cartilage is C-shaped hyaline cartilage rings, providing structural support and preventing collapse, with the posterior part lacking cartilage and containing the trachealis muscle.
The respiratory epithelium is pseudostratified columnar ciliated epithelium, characterized by ciliated cells for sweeping mucus, goblet cells for mucus production, basal cells (stem cells), and brush cells (sensory). Cilia are vulnerable to damage from toxins and infections, leading to reduced mucus clearance and possibly squamous metaplasia. Small granule cells (neuroendocrine) regulate smooth muscle tone.
The submucosa of the trachea contains seromucous glands with mucous and serous acini, often capped by serous demilunes. These glands secrete mucus through ducts opening onto the mucosal surface. The C-shaped cartilage rings ensure the trachea remains patent, while the trachealis muscle posteriorly allows for caliber control.
Bronchi share similar histology with the trachea but have important distinctions. Instead of C-shaped rings, they have discontinuous cartilage plates distributed around the wall. The smooth muscle layer is circumferential and complete, unlike the trachealis muscle in the trachea. Bronchi are found within the lung, surrounded by alveoli.
Bronchioles, with a diameter less than 1 mm, lack cartilage and submucosal glands. Their epithelium gradually changes from pseudostratified columnar ciliated to simple columnar and eventually cuboidal, with fewer goblet cells. They have a prominent circumferential layer of smooth muscle, which contracts in conditions like asthma, causing wheezing. Respiratory bronchioles connect to alveoli, allowing some gas exchange.