Transcription
The primary function of the respiratory system is to exchange oxygen and carbon dioxide between the body and the environment. The actual gas exchange process occurs in the respiratory division within the lungs. The rest of the respiratory system—the nose, pharynx, larynx, trachea, bronchi, and bronchioles—functions primarily as passageways for air to flow into and out of the lungs, and constitutes the conducting division.
The nasal cavity is lined with a ciliated mucous membrane. Sticky mucus traps inhaled particles, while the beating of cilia propels the mucus, laden with debris, toward the throat to be swallowed. Inhaled bacteria are destroyed by lysozyme in the mucus. Additional protection against potential pathogens is provided by lymphocytes and antibodies.
There are three shelves of tissue that arise from the wall of the nasal cavity, called the nasal conchae, or turbinates. These structures increase the surface area for contact with inhaled air, allowing the nose to rapidly warm, humidify, and clean it. The roof of the nasal cavity contains olfactory neurons in the lining, and these are responsible for the sense of smell.
From the nose, inhaled air turns 90 degrees downward as it reaches the pharynx. This turn is another trap for large dust particles, which, due to their inertia, strike the posterior wall of the throat and stick to the mucous membrane. The pharynx houses several tonsils. These immunologically competent tissues of the immune system are well-positioned to respond to inhaled pathogens. In addition to inhaled air, which is on its way to the lungs, the pharynx also conducts food and drink from the mouth to the esophagus.
Because the inhalation of food or drink into the lungs can be life-threatening, mechanisms are in place to prevent this from happening. The larynx is most important in this regard. The opening of the larynx is guarded by a flap of tissue called the epiglottis. During swallowing, the larynx is pulled upward and the epiglottis folds over, directing food and drink into the esophagus. Importantly, the vocal folds also close to protect the airway.
From the larynx, air passes into the trachea, or windpipe, which then divides into two main bronchi, supplying the lungs. Within the lungs, the primary bronchi branch into smaller and smaller bronchi and bronchioles, forming the bronchial tree with its millions of alveolar ducts, or airways. The airways contain a layer of smooth muscle in their walls that allows them to constrict or dilate. In response to the body’s increased demand for air, such as during exercise, the bronchioles dilate to increase airflow. On the other hand, in the presence of airborne pollutants, the airways constrict to reduce their entry into the lungs.
The larynx, trachea, and bronchi are lined with ciliated pseudostratified columnar epithelium, which produces mucus and acts as a mucociliary escalator: mucus traps inhaled particles, while the beating of cilia moves the mucus upward toward the throat, where it is swallowed. The final component of the conducting division, the terminal bronchioles, branch into several respiratory bronchioles that mark the beginning of the respiratory division.
Respiratory bronchioles terminate in microscopic, sac-like chambers called alveoli, each surrounded by capillaries. This is where gas exchange occurs. The alveolar wall is composed primarily of Type I squamous cells, which are thin and allow for rapid diffusion of gases. Inhaled oxygen moves from the alveoli into the blood in the capillaries, while carbon dioxide moves from the blood into the alveoli to be expelled from the body.
There are also a small number of cuboidal Type II cells that secrete surfactant, the function of which is to reduce the surface tension at the air-liquid interface and prevent the alveoli from collapsing at the end of each exhalation. The alveoli also contain a large number of macrophages, which are ready to engulf any inhaled particles that have managed to bypass the previous barriers of the lungs. The debris-laden macrophages are then carried up the mucociliary escalator to the throat to be swallowed and digested.