2. Respiratory Tract

The respiratory tract (or system) is a complex arrangement of organs and tissues. Its main functions are taking in oxygen and getting rid of carbon dioxide.

The respiratory tract can be separated into two parts (see Figs. 1 and 2):

(a) The proximal-conducting, nonrespiratory airways that include the nose, pharynx, larynx, trachea, bronchi, and nonalveolar bronchioles.

(b) The distal respiratory region (i.e., respiratory bronchioles, alveolar ducts, and alveoli).

Figure 1. Anatomical features of the respiratory tract.

Figure 2. Upper respiratory tract with the upper portion of the trachea and esophagus included.
Abbreviations: ST = superior turbinate; MT = middle turbinate; IT = inferior turbinate.

Gas exchange between air and blood occurs in the respiratory region. The geometry of the respiratory tract differs among different individuals. This contributes to differences in the amount of airborne material each person would deposit in his/her respiratory tract for a given exposure scenario.

Key structural features of the respiratory tract (nose, pharynx, larynx, trachea, bronchi and bronchioles, gas exchange airways, and lymphatic system) are summarized below.

2.1 Nose. The nose is the first potential target for inhaled substances except when one breathes through the mouth.

The nose is important in odor detection. The interior parts of the nose warm, moisten, and filter air. This helps to protect the lungs from harm. Some inhaled material can be trapped in the nasal cavity. The trapped material can, however, be transported from there to the throat, where it is swallowed. Nose blowing can also remove deposited material from the nose.

The nasal septum divides the nasal airway into two passages. Each passage extends from the nares (nostrils) to the nasopharynx.

Inhaled air flows through the nares into the vestibule (the section just inside the nares before the main chamber of the nose). The nasal vestibule is surrounded primarily by cartilage, unlike the more distal main nasal chamber that is surrounded by bone.

After inhaled air goes through the nasal vestibule, the air then passes through the narrowest part of the upper airway, the nasal valve, into the main chamber. The lumen of the main chamber is lined with mucous membranes that are covered by a layer of mucus and are associated with numerous nerves and blood vessels.

The mucous layer, which contains entrapped material, is moved by underlying cilia to the oropharynx where it is swallowed. Again, this is one way in which trapped substances are transported from the nasal region.

Turbinates (bony structures) project into the airway lumen from the lateral walls. The nasal turbinates are of three types: superior, middle, and inferior. They increase the inner surface area of the nose, which is important in filtering, humidifying, and warming the inhaled air.

2.2 Pharynx. Inhaled substances that do not deposit in the nose or mouth could deposit in the pharynx.

The pharynx connects the nasal and/or oral airway with the laryngeal airway during breathing. The pharynx is shaped somewhat like a funnel. It can be anatomically divided into nasal, oral, and laryngeal regions.

The pharynx lies just behind the nasal and oral cavities and extends partway down the neck. The wall of the pharynx consists of striated muscle lined with a mucous membrane. The pharynx allows passage of air and food and provides a resonating chamber for speech sounds.

The middle portion of the pharynx is called the oropharynx; the lowest portion is called the laryngopharynx, which extends downward and becomes continuous with the esophagus and larynx. Both the oropharynx and laryngopharynx are digestive and respiratory pathways.

2.3 Larynx. Some inhaled substances may deposit in the larynx.

The larynx is a short cavity that has a slit-like narrowing in its central portion. The narrowing is caused by two pairs of folds in the larynx walls. The lower folds are called vocal chords and the upper folds the false vocal chords.

The larynx connects the pharynx with the trachea and has three functions:

2.4 Trachea. Inhaled substances that pass through the larynx with the inhaled air could deposit in the trachea.

The trachea is continuous with the larynx in the neck and extends into the thoracic cavity where it branches to form the left and right main bronchi. The structure of the trachea airway is maintained during breathing because of many rings made of cartilage found within its wall.

2.5 Bronchi and Bronchioles. Inhaled substances that pass through the trachea with the inhaled air could deposit in the bronchi or bronchioles.

The main bronchi branch into the lobar bronchi, then into segmental and subsegmental bronchi, and the conducting airways and eventually end at the smallest of the conducting airways, the terminal bronchioles.

2.6 Gas Exchange Airways. Inhaled substances that do not deposit in other parts of the respiratory tract could deposit in gas-exchange airways.

The conducting airways terminate in gas-exchange airways, which are made up of respiratory bronchioles and alveolar ducts.

2.7 The Lymphatic System. The lymphatic system of the lungs plays an important supportive role in maintaining liquids, respiratory defenses, and in removal of inhaled material deposited in the lungs. The large flow of lymph from the lung tissue spaces toward the blood helps in removing excess fluid.