Tibor Balint                                             

Master of Space Studies - International Space University - Strasbourg - France

The respiratory system

This section gives a brief overview of the respiratory system and its functions. It also shows where the upper airway fits into the whole respiratory system, which is given as two boxes entitled "external nares and nasal cavity'' and ''pharynx and larynx''.  The respiratory system, its functions and the connection between them are demonstrated in Figure 1.

Figure 1: The respiratory system
 
 

Anatomical stucture of the upper airway

The upper airway with which we are concerned here extends from the nares and lips to the larynx.  It is only a few centimeters long and a few square centimeters in cross section.  It consists of a complex muscle structure, soft tissue and bones in its walls.  It has many functions; it is a conduit for air, food and drinks.  It also warms, humidifies and filters the air to the lungs.  Cough reflex protects the lungs from inhaling foreign objects and clears secretions.  Additional roles are gastrointestinal function and speech.

The major organs of the respiratory system are (Ref: The Editors of the Market House Book Ltd., "The Bantam Medical Dictionary", Second Revised Edition, Bantam Books, 1996} (Ref: Henry Gray, "Gray's Anatomy", A revised American from the Fifteenth English Edition, Gramercy Books, New York, 1977)
 


The incoming air enters the nose (and/or the mouth) and passes through the nasal cavity (and/or the oral cavity). The nasal cavity is the space inside the nose that lies between the floor of the cranium (or skull) and the roof of the mouth.  It is divided into two halves by the septum (a dividing wall). Each half communicates with the outside via the nostrils and with the nasopharynx through the posterior (or internal) nares.  (Internal nares are the openings leading from the nasal cavity into the pharynx.)  The nasal and oral cavities are divided by the palate, also called the roof of the mouth.  It consists of two portions: the hard palate and the soft palate.  The hard palate, at the front of the mouth, is formed by processes of the maxillae (or upper jaw) and palatine bones and is covered by mucous membrane.  The soft palate, further back, is a movable fold of mucous membrane that tapers at the back of the mouth to form a fleshy hanging flap of tissue, called the uvula.

From the nasal and oral cavities the incoming air then continues through the pharynx (or throat).  The pharynx is a 12-14 cm (about 5 inches) long tube, extending from the nasal cavity to the larynx (and esophagus). The pharynx is divided into four portions (see Figure 2), named for the structures behind which they lie, such as (1) {\it nasopharynx}, (2){\it velopharynx}, (3){\it oropharynx} and (4) {\it laryngopharynx}.
 

  1.  The nasopharynx has four openings and contains tonsils within its walls.  Two openings are the internal nares, which are passageways into the nasal cavity and separated by the nasal septum.  Two openings are passageways into the auditory - or Eustachian tubes - leading to the ears.
  2. The velopharynx lies behind the soft palate between the nasopharynx and oropharynx.
  3. Oropharynx is the part of the pharynx that lies between the soft palate and the hyoid bone.  The hyoid bone is situated near the upper portion of the epiglottis and is a small isolated U-shaped bone in the neck.  It supports the tongue and it is held in position by muscles and ligaments between it and the styloid process of the temporal bone.  The epiglottis is a thin leaf-shaped flap of cartilage, covered with mucous membrane, situated immediately behind the the roof of the tongue.  It covers the entrance to the larynx during swallowing.
  4. Laryngopharynx is the part of the pharynx that lies below the hyoid bone.
Vocal sound is produced in the larynx, within a pair of vocal cords.  It also serves as an air passage conveying air from the pharynx to the lungs.  It is situated in the front of the neck, above the trachea (see Figure 2).  It is made up of a framework of nine cartilages bound together by ligaments and muscles and lined with mucous membrane.

Parts of the upper airway are illustrated in Figure 2.
 

Figure 2: A midline segittal view of the upper airway
 
 

Airway resistance

There are several factors affecting upper airway resistance, such as:

Alae nasy section

The anterior nose is a major but variable site of resistance (Strohl, K.P., O'Cain C.F., Slutsky, A.S., "Alae nasi activation and nasal resistance in healthy subjects", Journal of Applied Physisology, Vol.52, 1982, pp.1432-1437) and this is controlled by the {\it alae nasi} (the two lateral flared portions of the external nose) muscles.
 

Nasal cavity section

Further back in the nasal cavity, the two principal determinants of airway caliber are the skeletal structure of the nose and the thickness and vascular congestion of the nasal mucosa.  Blood supply to the nasal mucosa is influenced by autonomical vascular tone.  At any given time, one nostril has high resistance while the other has low resistance. This pattern alternates between the nostrils with a cycle of approximately 90 minutes.
 

Pharyngeal section

Pharyngeal resistance is determined by the structure of the facial bones, particularly the maxilla and mandible, and by pharyngeal lymphoid tissue, fat deposition in the pharyngeal walls and the activities of the upper airway muscles.

Obesity is associated with decreased upper airway patency, principally related to increased fat deposition in the lateral walls of the pharynx.

Gender is another important influence.  Women tend to have smaller upper airways than men and lower pharyngeal resistance than men (Brown, I.G., Zamel, N, Hoffstein, V., "Pharyngeal cross sectional area in normal men and women", Journal of Applied Physiology, Vol.61, 1986, pp.890-895).

Advancing age is associated with reduction in upper airway size, with a redistribution of body fat from peripheral to truncal.  This results in increased pharyngeal resistance in men and increase in airway compliance "Brown, I.G., Zamel, N, Hoffstein, V., "Pharyngeal cross sectional area in normal men and women", Journal of Applied Physiology, Vol.61, 1986, pp.890-895).

The pharynx is the part of the upper airway, which is most vulnerable to collapse.  Its surrounding compliant walls lack bony support and depend on the balance between the dilating and constricting muscle activities for maintenance of potency.
 

Velopharyngeal section and soft palate

The caliber of airway around the mobile soft palate depends on the position of the soft palate, which is controlled by the palatal muscles.  These palatal muscles play an important role in determining the breathing route - oral or nasal -, although oral breathing also requires the mouth to be open.  In general, quiet breathing at rest is via the nose.  When the breathing flow rate increases above 35-45 L/min -- for example during exercise -- ventilation is both nasal and oral.

The upper airway resistance is markedly reduced through oral breathing, however other functions, such as heating, humidifying and filtering of the inspired air is less efficient.
 

Laryngeal section

The larynx} also offers some resistance that is greater in quiet expiration when the vocal cords adduct than in inspiration when they abduct.  Thus the laryngeal muscles are the major determinants of laryngeal resistance.

The larynx provides 25-30% of total airway resistance (Tully, A., Brancatisano, A., Loring, S.H., Engel, L.A., "Influence of porterior cricoarytenoid muscle activity on pressure-flow relationship of the larynx", Journal of Applied Physiology, Vol.70, 1991, pp.2252-2258), with the glottic aperture between the vocal cords being the major site of laryngeal resistance.
 

Inspiration and expiration

The upper airway offers lesser resistance to airflow during inspiration than expiration (White, D.P., Lombard, R.M., Cadieux, R.J., Zwillich, C.W., "Pharyngeal resistance in normal humans: Influence of gender, age and obesity", Journal of Applied Phusiology, Vol.58, 1985, pp.365-371).

During inspiration the negative pressure tends to collapse the airway.  This collapse can be counteracted by coordinated constriction of the upper airway muscles.

During expiration the pharyngeal constrictor muscles contract, consequently breaking airflow, thus increasing airflow resistance.  In normal adults this pharyngeal resistance is almost negligible (Spann, R.W., Hyatt, R.E., "Factors affecting upper airways resistance in conscious man", Journal of Applied Physiology, Vol.31, 1971, pp.708-712).
 

Body position

The position of the head and neck also affects upper airway resistance.  During either quiet breathing or panting, hypertension or hyperflexion of the head, resistance increases in the mouth and the larynx.

The upper airway resistance is less while standing, compared to when lying horizontally, due to the gravity shifting the lower jaw (or mandible) and hypopharyngeal sructures backward and to increase blood flow to the nasal mucous membrane (or mucousa).

The upper airway is also less collapsible when sitting up at 30° compared with lying down.


last updated: January 10, 2000