Respiratory System Anatomy and Cellular Functions
The Conducting Portion of the Respiratory System
The conducting portion consists of the nasal passages, nasal pharynx, larynx, trachea, and bronchioles.
The Larynx: Phonation and Protection
The larynx anatomically links the pharynx to the trachea. Its primary functions include phonation, swallowing, and breathing. It prevents different substances from entering the interior and features a cartilaginous structure with stratified squamous epithelium and flat folds. Internally, it is lined with pseudostratified ciliated columnar epithelium, which extends from the larynx through the trachea and into the bronchi.
The Trachea: Structure and Cell Types
The trachea extends from the larynx to the carina. It is composed of 16-20 semilunar cartilaginous rings held together by connective tissue and fibroelastic smooth muscle. The pseudostratified epithelium contains mucous cells and ciliated columnar cells, along with several other specialized cell types:
- Ciliated cells
- Goblet cells: Responsible for producing mucus.
- Brush cells: These serve a chemoreceptor function.
- Neuroendocrine cells: These produce serotonin and polypeptide hormones.
The lamina propria consists of loose connective tissue with elastic fibers and contains some tubular alveolar glands.
Differences Between the Trachea and Bronchi
Bronchi: These contain irregular hyaline cartilage. Smooth muscle is present around the entire circumference of the mucosa and cartilage, and the structure features longitudinal folds.
Trachea: This features anterior C-shaped (circular) cartilage with smooth muscle located specifically in the posterior region.
Bronchioles and Clara Cells
The bronchioles consist of 10-32 ramifications. They contain ciliated epithelial cells and Clara cells (non-ciliated). Their function is producing pulmonary surfactant, which decreases surface tension in the lungs to prevent collapse. Bronchioles are divided into three structural types: preterminal, terminal, and respiratory, terminating in the alveolar sacs to deliver inspired air to the lobules. The respiratory bronchioles serve the alveolar ducts and alveolar sacs.
Alveoli and the Diffusion Barrier
The alveoli (approximately 3x109) represent the functional structure of the respiratory apparatus. Their structure is formed by two types of cells:
- Pneumocyte I: Facilitates the diffusion of gas exchange.
- Pneumocyte II: A secretory cell believed to give rise to both Type I and Type II cells through mitosis.
The Diffusion Barrier: The alveoli are coated with a network of blood vessels. The barrier consists of the following layers:
- Liquid layer of surfactant
- Alveolar epithelium
- Basal lamina
- Interstitial space
- Capillary basal lamina
- Capillary endothelium
The entire system is innervated by the respiratory autonomic nervous system.