The Upper Airways

Nose and Mouth.

     The upper airways are the conduits allowing air to flow from the nose, or mouth, to the lungs.

    We can breathe through our mouth, like when we have a stuffy nose, when we swim or if we are pressed to take a deep breath. But, for the most part we breath through the nose. The passage of air through the nasal cavity allows air to warm and moisten. In addition, acting as a dust filter, the nostrils are covered with hairs (hair cells, there are all along the airway).

    The airways also secrete a mucus that helps keep catch the dust. And the movement of the cilia (hairs) helps to expulse the mucus toward the nose. The movement of small particles can achieve 1 centimeter per minute.

Sneezing.

    Sneezing reflex is used to get rid of excess particles in the nasal airway. Triggered by the particles in our nose, or by other chemicals, like perfume or spices, irritation of the nasal passages initiates the reflex. Irritation is initiated by hair cells or specialized receptors located in the mucous membranes. It is then transmitted to the brain via the trigeminal nerve (fifth cranial nerve) to the medulla where the reflex is coordinated. It induces a strong inspiration (~ 2.5 liters of air) followed by a closure of the epiglottis and the vocal cords. Then, the intercostal and abdominal muscles contract to create a high pressure in the lungs (> 100 mmHg). Finally, the epiglottis and vocal cords open back allowing the air to escape at high velocity (150 - 200 km/h). Lowering of the soft palate to prevent the air to go out through mouth but escape through nasal passages.

Swallowing.

Larynx with the epiglottis open for breathing (left) or half closed for deglutition (right).

    After the nose, the air passes into the pharynx, a common passage to the air and the food. Then, unless the epiglottis closes the air will engage into the larynx (your throat), then through the trachea, through the bronchi, and reach the lungs.

    If we close the epiglottis, the air will only go into the esophagus. When food, solid or liquid, engages in the pharynx, it triggers the swallowing reflex. The larynx rises and the pharyngeal fat pad pushes on the epiglottis, closing the airway. Food can only go to the esophagus. If some food particle enters the airway, this will induce coughing or even choking.

Trachea and bronchi.

    The main section of the upper airway consists of the larynx, the trachea and the bronchi. These 'air ducts' are mostly made of cartilage, hence its skeletal aspect. The top part is called the larynx, it is where lies the vocal cords. The middle section is the trachea, which we can feel the cartilaginous rings in our throat. The air conduit give rise to many ramifications, the bronchi that distribute air throughout the lungs. In each lobe, the bronchi branch off into still smaller branches called bronchioles.

    The airways are lined with a smooth muscle layer. These muscles, by contracting or relaxing, respectively increase or decrease airway resistance. We use these muscles to change the flow rate of air according to our metabolic need, or just enough to emit sounds. Inside the airway, there are also many mucus secreting cells. This mucus helps to carry out particles and keep the lung moist. Unfortunately, when too much mucus is produced, like during infection, we must compensate for the increased resistance and, if infection persist, inflammatory processes will facilitate bronchoconstriction and further compromise breathing. In extreme case, this could be fatal.

Coughing.

    The cough reflex is similar to that of sneezing, except it is initiated by irritation of the larynx or bronchi. Irritation information is transmitted to the medulla by the vagus nerve, the tenth cranial nerve. That triggers a bronchoconstriction to increase airflow. The soft palate is not lowered, so that air is expelled primarily through the mouth and help remove mucus.

Vocalization.

    Vocalization is to produce sounds. Talking, singing, screaming and whispering are manifestations of our ability to 'vibrate' the exhaled air. These sounds result from both, the articulation of buccal and nasal cavities that helps to change resonance to the vibrations induced by the vocal cords in the larynx. Our brain is apparently developed enough to coordinate the movements necessary to produce these sounds. Sometimes, what is even more impressive, is that we can make sense of all this. From the motor centers in the cortex onto the respiratory centers in the medulla, we can influence relaxation and contraction of the diaphragm, to exhale and inhaled.

    The exhaled air, going up the larynx, vibrates after it passed the vocal cords. Two ligaments attached by muscular membrane, can vibrate and modulate airflow for a fair range of frequencies. On the left figure, we see the position of the vocal cords, photographed by laryngoscopy: top, when they are fully open during inspiration; in the middle, when they are fully closed and there is no vocalization; and bellow, during the murmur.

    By constantly modulating the different frequencies, articulations of the mouth change airway resistance, speed and sequencing of the sounds. It is the coordination of all these movements that will determine their complexity. The skull and the sinuses act as a resonators, and help at hearing ourselves. Understanding and interpretation of these sounds are performed in different hearing and speech centers, mostly in the left temporal lobe.