Upper Respiratory Tract

The upper respiratory system encompasses the area from the nose to the larynx (Fig. 10-2, A). Air can enter the body through the mouth, but for the most part it enters the body through the two nares (nostrils) of the nose that are separated by the nasal septum. The nasal turbinates (also called nasal conchae) are three scroll-shaped bones (inferior, middle, and superior) that increase the surface area that air must pass over on its way to the lungs (Fig. 10-2, B). The vibrissae (the coarse hairs in the nose) serve to filter out large particulate matter, and the mucous membrane and cilia (small hairs) of the respiratory tract provide a further means of keeping air clean, warm, and moist as it travels to the lungs. The cilia continually move in a wavelike motion to push the sticky mucus and debris out of the respiratory tract. The air then travels up and backward, where it is filtered, warmed, and humidified by the environment in the upper portion of the nasal cavity. Damage to the cilia keeps the germ-laden mucus from leaving the body and consequently provides a hospitable environment for infection. Figure 10-3 illustrates the route of air into the body. The receptors for olfaction are located in the nasal cavity, which is connected to the paranasal sinuses, collectively named for their proximity to the nose.

The paranasal sinuses, divided into the frontal, maxillary (the largest sinus, also referred to as the antrum of Highmore), sphenoid, and ethmoid cavities, acquire their names from the bones in which they are located. The paired ethmoid sinuses are divided into anterior, middle, and posterior air cells. The function of sinus cavities in the skull is to warm and filter the air taken in and to assist in the production of sound. They are lined with a mucous membrane that drains into the nasal cavity and can be the site of painful inflammation. The ethmoid bone cradles the olfactory bulb in the cribriform plate. This sievelike bone has numerous openings through which olfactory nerves descend into the nasal cavity.

Air continues to travel from the back of the nasal cavity to the nasopharynx, a part of the throat (pharynx) behind the nasal cavity. The eustachian tubes (also called the auditory or pharyngotympanic tubes) connect the ears with the throat at this point and serve to equalize pressure between the ears and the throat. The nasopharynx is the site of the pharyngeal tonsils (adenoids), which are made of lymphatic tissue and help to protect the respiratory system from pathogens. The next structure, the oropharynx, is the part of the throat posterior to the oral cavity. It is the location of more lymphatic tissue, the palatine tonsils, so named because they are continuous with the roof of the mouth (the palate). The lingual tonsils, located on the posterior aspect of the tongue, also serve a protective function. Note that the oropharynx is part of the digestive system as well as the respiratory system; both food and air pass through it. Below the oropharynx is the part of the throat referred to as the laryngopharynx because it adjoins the opening of the larynx.

The larynx, commonly referred to as the voice box, is the main organ of sound production. It is a short tube that is composed of nine sets of supportive, protective cartilaginous structures, and two sets of vocal folds, one true and one false. The false vocal folds, also called the vestibular folds for their location at the entrance to the larynx, do not function in the production of speech. Speaking and singing are controlled by the true vocal folds (also termed vocal cords), which are composed of the glottis, two muscular folds, and the space between them (the rima glottidis). The pitch of one’s voice is determined by the degree to which the vocal cords are stretched as they vibrate. Loudness of speech is determined by the force of the exhaled air that travels out through the larynx. One of the cartilages, the epiglottis, is an oval-shaped structure that covers the trachea (windpipe) when an individual swallows to prevent food from being pulled into the windpipe instead of the esophagus. When looking at the anatomy of the neck, it is useful to note the proximity of the thyroid gland, which is anterior and inferior to the larynx. Although the thyroid gland will be described more fully in the chapter on the endocrine system, one can see that its location allows for a shared connection, the thyroid cartilage. Normally larger and more angular in the male than in the female, the thyroid cartilage consists of a pair of thin plates called laminae. These plates cover the anterior surface of the larynx, and are attached to the hyoid bone on either side with the thyrohyoid ligament. The area where the two plates join is the laryngeal prominence, commonly called the Adam’s apple. The cricoid cartilage, named for its ringlike appearance, forms the lower part of the larynx, attaching it to the trachea. The paired arytenoid cartilages, located in the back upper border of the cricoid cartilage, are attached to the vocal folds and function to close them. The corniculate cartilages are located the tip of the cricoid cartilage, while the cuneiform (wedge-shaped) cartilages are in front of the corniculate cartilage.

Lower Respiratory Tract

The lower respiratory tract begins with the trachea (or windpipe), which extends from the larynx into the chest cavity. The trachea is composed of several C-shaped rings, which prevent the airway from collapsing. The trachea lies within the space between the lungs called the mediastinum. Air travels into the lungs as the trachea bifurcates (branches) at the carina, a keel-shaped cartilage where the right and left airways, called bronchi (sing. bronchus), divide into smaller branches. The metaphor of an upside-down tree makes sense here, as one can imagine the trachea as the trunk and the bronchi and bronchioles as branches.

Each lung is composed of sections called lobes, which correspond to the secondary bronchi that supply these areas within each lung. The right lung is made up of three lobes, whereas the left has only two (see Fig. 10-1, A). The abbreviations for the lobes of the lungs are RUL (right upper lobe), RML (right middle lobe), RLL (right lower lobe), LUL (left upper lobe), and LLL (left lower lobe). Within each of these lobes, the secondary bronchi branch out to tertiary bronchi, and the areas that each supplies are referred to as segments (Fig. 10-4). Each segment is supplied with blood from a segmental artery that branches off the pulmonary arteries. The segments are named by their location (e.g., anterior, posterior, apical, basal, medial, or lateral). The lingula is the area where the superior and inferior segments appear on the left lung. Although named for its tongue-like shape, it has been postulated that it may represent the remnants of a left middle lobe. At the end of the segmental bronchi are still smaller branches called bronchioles. These bronchioles end in terminal bronchioles that branch to respiratory bronchioles. The respiratory bronchioles extend into microscopic alveolar ducts capped by air sacs called alveoli (sing. alveolus). Each alveolus is in contact with a blood capillary to provide a means of exchange of gases. At this point O₂ is diffused across cell membranes into the blood cells, and CO₂ is diffused out to be expired.

The cells that line the respiratory tract include goblet cells (that produce mucus) and ciliated basal (also termed stem) cells (that help cleanse the lining). As the bronchial tree progressively divides into smaller and smaller branches, the shapes of the cells that line it change from a thicker to a thinner appearance. Tall, simple columnar cells in the primary bronchi give way to squat, simple cuboidal cells in the terminal bronchioles. The cells in the terminal bronchioles are still equipped with ciliated cells to remove debris from the airways, but the presence of goblet cells (with their secretion of mucus) is missing. As gas exchange becomes the most important function, simple squamous (scaly, flat) cells appear in the alveoli. The epithelial lining of the alveoli is composed of type I and type II cells. Type I cells are responsible for gas exchange, while type II cells produce a substance called surfactant that keeps the lung from collapsing.

As mentioned earlier, the respiratory system is important in the maintenance of the acidity and alkalinity of the blood through regulation of the pH. Because blood pH is measured by the concentration of hydrogen ions (H⁺), one needs to understand how oxygen and carbon dioxide are involved.

When O₂ passes from the alveoli through the capillaries of the lung to begin its journey to the cells of the body, it binds to red blood cells (RBCs) and dissolves in the liquid portion of the blood, the plasma. The RBCs contain a protein called hemoglobin that increases the potential amount of oxygen that can be carried by the blood. The oxygen binds with this protein (now called oxyhemoglobin) and continues its ride to the cells where it is given off to be used in cellular respiration by the power plants of the cell, the mitochondria. Without oxygen, energy needed for cellular functions cannot be generated. This is the reason why an anemic person feels tired—oxygen is in short supply because there is a diminished number of RBCs.

The mitochondria use oxygen for the energy transformation that results in the formation of carbon dioxide. CO₂ then travels back to the lungs to be excreted, catching a ride again on the RBCs as carbonic acid (H₂CO₃), which is made up of carbon dioxide and water. If the pH is too high, the carbonic acid will split to form bicarbonate (HCO₃) and a hydrogen ion that will increase the concentration of hydrogen ions and cause the pH to fall. If the pH is too low, the bicarbonate ions will bond with unattached hydrogen ions, lowering the concentration of hydrogen ions and causing the pH to rise. Once at the capillaries, it converts back to carbon dioxide and water, where it is exhaled as warm, moist breath.

Blood pH decreases (becomes more acidic) as carbon dioxide levels increase. To compensate, we breathe faster and deeper (hyperventilation) to move the CO₂ out of our body. The carbonic acid that is carried back to the lungs can also act as a buffer, a ready donor/receiver of hydrogen ions to adjust the pH as needed. Receptors in the carotid arteries and the aorta sense the pH level and provide stimulation for its adjustment.

Ketoacidosis, a complication of diabetes mellitus, is a drop in pH caused by the excessive breakdown of fats. The resulting Kussmaul’s respirations, characterized by rapid, deep breathing, are an effort by the body to decrease the amount of CO₂ and raise the blood’s pH.

It should be noted that the lungs are assisted by the urinary system in the regulation of blood pH. The kidneys are responsible for monitoring and adjusting the concentration of bicarbonate ions, recycling them back into the bloodstream as needed and excreting excess hydrogen ions into the urine.

Each lung is also enclosed by a double-folded, serous membrane called the pleura (pl. pleurae). The side of the membrane that coats the lungs is the visceral pleura; the side that lines the inner surface of the rib cage is the parietal pleura. The two sides of the pleural membrane contain a serous (watery) fluid that facilitates the expansion and contraction of the lungs with each breath.

The muscles responsible for normal, quiet respiration are the dome-shaped diaphragm and the muscles between the ribs (intercostal muscles). On inspiration, the diaphragm is pulled down as it contracts and the intercostal muscles expand, pulling air into the lungs because of the resulting negative pressure (see Fig. 10-1, B). On expiration the diaphragm and intercostal muscles relax, pushing air out of the lungs.