The upper airway represents the beginning of the respiratory system, serving as the gateway for all inhaled air. This pathway processes and conducts air from the external environment into the lungs. Its primary role is to ensure a continuous and clean supply of oxygen, which is fundamental for the body’s metabolic functions. If the upper airway operation is compromised, the entire process of respiration is impacted.
Defining the Anatomical Components
The upper airway is a continuous passage starting at the nose and mouth and extending down the throat. It includes the nasal cavity and the paranasal sinuses, which are air-filled spaces within the skull bones. Inside the nasal cavity, scroll-shaped bones called conchae or turbinates project inward, creating a large surface area for air to pass through.
Air then moves into the pharynx, or throat, a muscular tube divided into three segments. The nasopharynx is the uppermost section, located behind the nasal cavity, functioning solely as an air passage. Below it is the oropharynx, which connects to the mouth and is shared by both the respiratory and digestive systems.
The lowest part is the laryngopharynx, which directs air into the larynx and food into the esophagus. The final structure is the larynx, or voice box, a framework of nine cartilages. The epiglottis, a leaf-shaped cartilage within the larynx, acts like a hinged trapdoor to separate the respiratory and digestive tracts during swallowing.
Essential Functions of Air Processing
A primary function of the upper airway is preparing inhaled air before it reaches the lungs. This process includes three actions: warming, humidification, and filtration. Warming is necessary because cold air can damage the lower respiratory tract, so the airway raises the temperature of inspired air to approximately 34°C by the time it reaches the nasopharynx.
Humidification involves adding moisture, ensuring the air is nearly 100% saturated with water vapor as it moves deeper into the body. The turbulent airflow created by the conchae allows for maximal contact between the air and the moist mucosal lining. This saturation prevents the lower airways from drying out and maintains the fluidity of mucus.
Filtration acts as the body’s first line of defense, removing particulate matter like dust, debris, and pathogens. Nasal hairs trap larger particles, while smaller contaminants are caught in the sticky mucus lining. Tiny, hair-like projections called cilia sweep this contaminated mucus toward the pharynx to be swallowed or expelled. The larynx also plays a secondary role in sound production, using the vocal folds to vibrate air for speech.
Understanding Airway Obstruction
Obstruction occurs when the upper airway narrows or becomes blocked, significantly reducing the body’s ability to exchange gases. This ranges from a chronic, partial blockage to an acute, life-threatening event. A common chronic cause in adults is obstructive sleep apnea (OSA), where muscle relaxation during sleep causes the soft tissues of the pharynx to collapse inward.
Acute obstruction can arise from issues including the aspiration of a foreign body, which is common in young children. Infections like croup or epiglottitis cause severe inflammation and swelling, rapidly narrowing the air passage. In an unconscious patient, loss of muscle tone can allow the tongue to fall back against the posterior pharyngeal wall, creating a mechanical blockage.
A partial obstruction often results in turbulent airflow, producing abnormal breathing sounds. These include stridor, a high-pitched sound heard during inhalation, or stertor, a coarser noise. Untreated obstruction can lead to hypoxia, a lack of oxygen in the blood, which may result in respiratory distress and cardiac arrest.
The Boundary Between Upper and Lower Airway
The division between the upper and lower respiratory tracts is anatomically precise. The boundary is considered the area just below the vocal folds, or vocal cords, within the larynx. The section of the larynx above the vocal cords is part of the upper airway, while the area immediately beneath them, known as the subglottis, marks the beginning of the lower airway.
The lower airway continues down into the trachea and the bronchi, which lead into the lungs. This distinction is important in clinical medicine because diseases often affect one tract differently than the other. For example, upper airway infections frequently involve the nose and pharynx, while lower airway conditions affect the bronchi and lungs, requiring different treatment approaches.