Respiratory Tract

The Larynx

The larynx is a cartilaginous structure that serves as a passageway for air between the pharynx and the trachea. The larynx can be pictured as a triangular box whose apex, the Adam’s apple, is located at the anterior of the neck. The larynx is called the voice box because it houses the vocal cords. The vocal cords are mucosal folds supported by elastic ligaments, and the slit between the vocal cords is an opening called the glottis (Fig. 14.3). When air is expelled past the vocal cords through the glottis, the vocal cords vibrate, producing sound. At the time of puberty, the growth of the larynx and the vocal cords is much more rapid and accentuated in the male than in the female, causing the male to have a more prominent Adam’s apple and a deeper voice. The voice “breaks” in the young male due to his inability to control the longer vocal cords. These changes cause the lower pitch of the voice in males.
The high or low pitch of the voice is regulated when speaking and singing by changing the tension on the vocal cords. The greater the tension, as when the glottis becomes more narrow, the higher the pitch. When the vocal cords relax, the glottis is wider, and the pitch is lower (Fig. 14.3b). The loudness, or intensity, of the voice depends upon the amplitude of the vibrations-that is, the degree to which the vocal cords vibrate.

When food is swallowed, the larynx moves upward against the epiglottis, a flap of tissue that prevents food from passing through the glottis into the larynx. You can detect this movement by placing your hand gently on your larynx and swallowing.

The Trachea

The trachea, commonly called the windpipe, is a tube connecting the larynx to the primary bronchi. The trachea lies ventral to the esophagus and is held open by C-shaped cartilaginous rings. The open part of the C-shaped rings faces the esophagus, and this allows the esophagus to expand when swallowing. The mucosa that lines the trachea has a layer of pseudostratified ciliated columnar epithelium. (Pseudostratified means that while the epithelium appears to be layered, actually each cell touches the basement membrane.) The cilia that project from the epithelium keep the lungs clean by sweeping mucus, produced by goblet cells, and debris toward the pharynx:

Figure 14.3 Placement of the vocal cords. a. This frontal section of the larynx shows the location of the vocal cords. b. Viewed from above, the vocal cords can be seen to stretch from anterior to posterior across the larynx. When air is forced past the vocal cords, they vibrate, producing sound. The vocal cords are taut when we produce a high-pitched sound (top), and they relax as the pitch deepens (bottom).

Smoking is known to destroy these cilia, and consequently the soot in cigarette smoke collects in the lungs. If the trachea is blocked because of illness or the accidental swallowing of a foreign object, it is possible to insert a breathing tube by way of an incision made in the trachea. This tube acts as an artificial air intake and exhaust duct. The operation is called a tracheostomy.

The Bronchial Tree

The trachea divides into right and left primary bronchi (sing., bronchus), which lead into the right and left lungs (see Fig. 14.1). The bronchi branch into a great number of secondary bronchi that eventually lead to bronchioles. The bronchi resemble the trachea in structure, but as the bronchial tubes divide and subdivide, their walls become thinner, and the small rings of cartilage are no longer present. During an asthma attack, the smooth muscle of the bronchioles contracts, causing bronchiolar constriction and characteristic wheezing. Each bronchiole leads to an elongated space enclosed by a multitude of air pockets, or sacs, called alveoli (sing., alveolus). The components of the bronchial tree beyond the primary bronchi, including the alveoli, compose the lungs.

The Lungs

The lungs are paired, cone-shaped organs that occupy the thoracic cavity except for the mediastinum, a central area that contains the primary bronchi, the heart, and other organs. The right lung has three lobes, and the left lung has two lobes, allowing room for the heart whose apex points left. A lobe is further divided into lobules, and each lobule has a bronchiole serving many alveoli. The apex is the superior narrow portion of a lung, and the base is the inferior broad portion that curves to fit the dome-shaped diaphragm, the muscle that separates the thoracic cavity from the abdominal cavity. The lateral surfaces of the lungs follow the contours of the ribs in the thoracic cavity. Each lung is enclosed by a double layer of serous membrane called the pleura. The visceral pleura adheres to the surface of the lung, while the parietal pleura lines the thoracic cavity. The pleura produces a lubricating serous fluid that allows its two layers to slide against one another. Surface tension is the tendency for water molecules to cling to each other (due to hydrogen bonding between the molecules) and to form a droplet. Surface tension holds the two pleural layers together when the lungs recoil during expiration.

The Alveoli

With each inhalation, air passes by way of the bronchial tree to the alveoli. An alveolar sac is made up of simple squamous epithelium surrounded by blood capillaries. Gas exchange occurs between the air in the alveoli and the blood in the capillaries (Fig. 14.4). Oxygen diffuses across the alveolar and capillary walls to enter the bloodstream, while carbon dioxide diffuses from the blood across these walls to enter the alveoli. The alveoli must stay open to receive the inhaled air if gas exchange is to occur. Gas exchange takes place across moist cellular membranes, and yet the surface tension of water lining the alveoli is capable of causing them to close up. The alveoli are lined with a surfactant, a film of lipoprotein that lowers the surface tension and prevents them from closing. The lungs collapse in some newborn babies, especially premature infants, who lack this film. The condition, called infant respiratory distress syndrome, is now treatable by surfactant replacement therapy.

Figure 14.4 Gas exchange in the lungs. The lungs consist of portions of the bronchial tree leading to the alveoli, each of which is surrounded by an extensive capillary network. Notice that the pulmonary artery and arteriole carry O₂-poor blood (colored blue), and the pulmonary vein and venule carry O₂-rich blood (colored red).

Respiratory Membrane

Gas exchange occurs very rapidly because of the characteristics of the so-called respiratory membrane (Fig. 14.5). The respiratory membrane consists of the juxtaposed alveolar epithelium and the capillary endothelium. At times, their basement membranes are fused, meaning that very little tissue fluid separates the two portions of the respiratory membrane and they are indeed one membrane. This membrane is extremely thin-only 0.2-0.6 mm thick. The total surface area of the respiratory membrane is the same as the area of the alveoli, namely 50-70 m². The blood that enters the many pulmonary capillaries spreads thin. The red blood cells within the capillaries are pressed up against the narrow capillary walls, and little plasma is present. This too facilitates the speed of gas exchange during external respiration.

Figure 14.5 The respiratory membrane consists of the alveolar wall and the capillary wall.