The respiratory system enables the body to take in oxygen and release carbon dioxide. This complex system relies on various specialized cells, each performing distinct functions to ensure air is properly prepared and gases are exchanged efficiently, from the initial intake of air to its final interaction with the bloodstream.
Cells of the Conducting Airways
The conducting airways, extending from the nasal cavity through the trachea and bronchi down to the terminal bronchioles, prepare inhaled air for gas exchange. Ciliated epithelial cells are abundant in these airways, equipped with cilia. These cilia beat rhythmically, sweeping mucus and trapped particles upwards towards the throat, where they can be swallowed or expelled, a process known as mucociliary clearance.
Interspersed among ciliated cells are goblet cells, which produce mucus. This viscous, carbohydrate-rich substance traps dust, bacteria, viruses, and other airborne particles. Mucus also provides hydration and lubrication, helping to maintain tissue health.
Basal cells act as stem cells for the airway epithelium, capable of differentiating into other epithelial cell types, including ciliated and goblet cells, to repair damaged tissue. They also contribute to oxidant defense and transepithelial water movement.
In the smaller airways, particularly the bronchioles, club cells (formerly Clara cells) become more prevalent as goblet cell numbers decrease. These cells secrete a surfactant-like substance that prevents the bronchioles from collapsing during exhalation. Club cells also detoxify harmful inhaled substances using cytochrome P450 enzymes and can act as stem cells, regenerating both ciliated and non-ciliated epithelial cells after injury.
Cells of the Alveoli
The alveoli are tiny air sacs at the ends of the bronchioles, serving as the primary sites for gas exchange within the lungs. There are approximately 500 million alveoli in the human body, providing a vast surface area for efficient oxygen and carbon dioxide transfer.
Type I pneumocytes are thin, flat cells that cover about 95% of the alveolar surface. Their extremely thin structure, approximately 0.1 to 0.2 micrometers, minimizes the distance gases must travel for diffusion. These cells form a part of the blood-air barrier by sharing a basement membrane with the pulmonary capillary endothelium.
Type II pneumocytes are cuboidal cells, less abundant than Type I cells, covering about 5% of the alveolar surface. They produce and secrete pulmonary surfactant, a lipoprotein complex that reduces surface tension within the alveoli, preventing their collapse during exhalation. These cells also act as progenitor cells, capable of dividing and differentiating into Type I pneumocytes to repair the alveolar epithelium after injury.
Adjacent to the alveoli, endothelial cells line the capillaries, forming the other component of the blood-air barrier. These cells are also remarkably thin, facilitating the rapid diffusion of oxygen into the blood and carbon dioxide out of it. The close proximity and shared basement membrane between Type I pneumocytes and capillary endothelial cells create a minimal diffusion distance, optimizing gas exchange.
Immune Cells of the Respiratory System
The respiratory system is constantly exposed to inhaled pathogens and foreign particles, necessitating a robust immune defense. Alveolar macrophages are the primary phagocytes in the lower respiratory tract. Their main function is to engulf and digest dust, bacteria, viruses, and cellular debris, serving as the first line of defense against invaders that bypass the conducting airway’s mechanical defenses.
When faced with a significant threat, alveolar macrophages can release pro-inflammatory cytokines and chemokines, such as interleukins and tumor necrosis factor-alpha. This action recruits other immune cells, including neutrophils, lymphocytes, and mast cells, to the site of infection or injury. Neutrophils are phagocytic cells that help control bacterial and fungal infections.
Lymphocytes, including T lymphocytes and B lymphocytes, contribute to adaptive immunity by recognizing and targeting specific pathogens. Mast cells, found in various tissues including the respiratory epithelium, can rapidly respond to external triggers by releasing mediators like histamine, initiating an inflammatory response.