Trachea

Organization of the Respiratory System

The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, and pharynx. The lowerrespiratory tract includes the larynx, trachea, and lungs. The lungs contain the bronchi, bronchioles, and alveoli. FIGURE 21-1 shows the structures of the respiratory system.

Trachea

The trachea or windpipe is a cylindrical tube about 2 cm in diameter. It is extremely mobile and flexible, with a length of approximately 10–12 cm or 4 inches (FIGURE 21- 5). The trachea extends downward from the larynx, anterior to the esophagus into the medias-tinum of the thoracic cavity, where it splits into right and left bronchi. The trachea has layers known as the mucosa, submucosa, and adventitia as well as a hya-line cartilage layer. The ciliated mucosa, like nearby structures,­ contains goblet cells in a pseudostratified epithelium. It moves trapped particles up into the pharynx where they can be swallowed along with mucus. The submucosa is a connective tissue layer that contains seromucous glands, which help to pro-duce sheets of mucus. The adventitia is the outermost layer of connective tissue, encasing the hyaline carti-lage rings of the trachea.

Inside the trachea are about 20 pieces of hyaline cartilage, each shaped like the letter “c.” Their open ends are toward the spine. These rings of cartilage pre-vent the trachea from collapsing. The soft tissue near the back of each ring allows the esophagus to expand as food moves toward the stomach. The open poste-rior parts of these rings of cartilage are connected by smooth muscles in the trachealis as well as by soft con-nective tissues. When the trachealis muscle contracts, the diameter of the trachea increases and expired air is caused to rush upward with great force from the lungs. This helps to expel mucus when coughing, at speeds of approximately 100 miles per hour. A cartilage struc-ture called the carina projects posteriorly from the final tracheal cartilage. This is the point where the tra-chea branches into the two primary bronchi.

Conducting Zone Structures

The structures of the conducting zone include all respi-ratory passageways except for those that make up the respiratory zone. The conducting zone structures are relatively rigid and the organs within function to clean, humidify, and warm the incoming air. There-fore, when this air reaches the lungs, it contains less dust, bacteria, and other irritants than when it entered the nose and has become warm and damp.

The trachea divides into the right main bronchi and the left main bronchi, also known as primary bronchi. Both are found at the level of T₇when a personis standing. The bronchi run obliquely in the medias-tinum. They then move downward into the hilum or medial depression of the lungs. The left main bronchus is thinner, longer, and less vertical than the right main bronchus. It is more common for a foreign object to be inhaled and stuck in the right main bronchus because of its size, shape, and position.

Branched airways leading from the trachea to the alveoli make up the bronchial tree. These branches begin with the right and left primary bronchi near the level of the fifth thoracic vertebra. Each primary bronchus divides into a secondary bronchus, then into tertiary bronchi, and even finer tubes. The right bronchus or bronchus dexter is wider, shorter, and more vertical than the left bronchus or bronchussinister. The right bronchus branches off to theupper lobe of the right lung. This branch is called the eparterial branch because it arises above the right pulmonary artery. It then passes below the artery at the hyparterial branch and divides into two branches for the middle and lower lobes. The left bronchus has no eparterial branch because there is no third lobe in the left lung. Bronchi have less cartilage than the tra-chea and bronchioles have no cartilage. Smaller and smaller respiratory tubes have smooth muscle instead of cartilage.

Bronchioles are smaller tubes that continue todivide and include terminal bronchioles. The alveoli provide a large surface of epithelial cells that allow easy exchange of gases. Oxygen diffuses from the alveoli into the capillaries and CO₂ diffuses from the blood into the alveoli. From each tertiary bron-chus, about 6,500 terminal bronchioles arise. The lumen of each terminal bronchiole is only between 0.3 and 0.5 millimeters in width. The bronchiole walls have no cartilage and are mostly made up of smooth muscle. When the bronchioles change in diameter, they control resistance to airflow and distribution of air inside the lungs.

Diameter of the bronchioles is controlled by the autonomic nervous system, accomplished by regulating smooth muscle activity. Bronchodilation is caused by sympathetic activation, and airway diameters enlarge. Bronchoconstriction is caused by parasympatheticactivation, and airway diameters are reduced. Bron-choconstriction also occurs in anaphylaxis and other allergic reactions, a response to histamine released by basophils and activated mast cells.

Both bronchodilation and bronchoconstriction adjust resistance to airflow, and direct airflow to or from certain areas of the respiratory exchange sur-faces. When there is tension of the smooth muscles, the bronchiole mucosa usually forms a series of folds, limiting airflow. In conditions such as asthma, excessive stimulation can nearly totally prevent airflow along the terminal bronchioles.

Respiratory Zone Structures

The respiratory zone is where actual gas exchange occurs. It is made up of microscopic structures, which include the respiratory bronchioles, alveolarducts, and alveoli. The alveoli are thin-walled airsacs. The respiratory zone starts at the point of the ter-minal bronchioles feeding into the respiratory bron-chioles inside the lungs. Scattered alveoli protrudefrom the respiratory bronchioles, which also lead into twisting and turning alveolar ducts. The walls of the alveolar ducts are completely made up of diffuse rings of connective tissue fibers, alveoli, and smooth muscle cells. The ducts lead to alveolar sacs or alveolar sac-cules, which are terminal clusters of alveoli.

The alveoli are not the same as the alveolar sacs. The alveoli are the actual sites of gas exchange. The alveolar sacs resemble bunches of grapes, with the alveoli being the individual grape-like structures. There are approximately 300 million alveoli in the lungs, all of them filled with gas. They make up most of the body’s lung volume, creating a huge surface area for gas exchange. The alveoli are what give the lungs their open and spongy appearance. Each alveolus has an extensive capillary network. The capillaries are sur-rounded by a network of elastic fibers that help main-tain positions of the alveoli and bronchioles. When the fibers recoil during exhalation, the size of the alve-oli is reduced, which helps to push air out of the lungs.

In each alveolus, the walls are mostly made up of one layer of simple squamous epithelial cells, also called type I alveolar cells. These are surrounded by a thin respiratory basement membrane. The alveolar walls are 15 times thinner than one sheet of paper. There are also scattered, cuboidal type II alveolar cells, which are also calledseptal cells. These secretesurfactant, an oily substance containing phospholip-ids and proteins, which coats the alveolar surfaces that are exposed to gas. Surfactant forms a superfi-cial coating over a thin H₂O layer. The type II cells also secrete many antimicrobial proteins, needed for innate immunity (FIGURE 21-6). Dust cells, also known as alveolar macrophages , patrol the sur-face of the alveolar epithelium and phagocytize any particles that have not already been “caught” by other methods.

Surfactant is important in keeping the alveoli open and reduces surface tension in the liquid that coats the alveolar surfaces. If there were no surfac-tant, surface tension would collapse the alveoli. Sur-factant forms a thin surface layer, interactive with H₂O molecules, to reduce surface tension and keep the alveoli open.

If the type II alveolar cells do not produce enough surfactant because of injury or genetic abnormalities, respiration is difficult. After every exhalation, the alve-oli collapse. This means that each inhalation must be strong enough to reopen the alveoli. The individual will soon be exhausted by these efforts. This condition is called respiratory distress syndrome.

At the respiratory membrane, there is a tiny distance separating the alveolar air and blood. This distance averages about 0.5 micrometer, but can be as small as 0.1 micrometer. Diffusion occurs quickly across the respiratory membrane since the distance is tiny, and the oxygen and CO₂ molecules are small and lipid-soluble. Oxygen and CO₂ are not prevented by the plasma membranes of the epithelial and endo-thelial cells from moving between the alveolar air and blood.

Blood Supply to the Lungs

The lung tissue is nourished by a circuit supply-ing the respiratory portion and one perfusing the conducting portion. Blood from the pulmo-nary circuit arteries is received by the respiratory exchange surfaces. Deoxygenated blood is carried by the ­pulmonary arteries, which enter the lungs at the hilum. They branch with the bronchi as they approach the lung lobules. Each lobule receives one arteriole and one venule. A network of capil-laries surrounds every alveolus and is part of the respiratory membrane. From the alveolar capillar-ies, oxygen-rich blood moves through the pulmo-nary venules. It enters the pulmonary veins, which deliver this blood to the left atrium.

While providing for gas exchange, the endo-thelial cells of the alveolar capillaries are also the main source of angiotensin- converting enzyme(ACE). This enzyme converts circulating angioten-sin I to angiotensin II. The enzyme is important in the regulation of blood volume and pressure. The conducting passageways of the lungs receive oxy-gen and nutrients from capillaries that are supplied by the bronchial arteries, which are branches of the thoracic aorta. Venous blood from these capillar-ies is emptied into bronchial veins or anastomoses. It then empties into the pulmonary veins. Outside blood flow bypasses the remainder of the systemic circuit. It dilutes oxygenated blood that is leaving the alveoli.

In the pulmonary circuit, blood pressure is usually low, about 30 mm Hg or less. The pulmonary vessels can, therefore, be easily blocked by masses of fat, small blood clots or air bubbles inside the pulmonary arter-ies. Since the lungs receive all the cardiac output, any objects moving through the blood will likely become trapped in the networks of pulmonary arteries or capillaries­. Tiny blood clots are occasionally formed in the veins. They are usually trapped inside the pul-monary capillaries where they quickly dissolve. Large emboli are, however, much more dangerous. Blood flow to a group of alveoli or lobules is stopped when a branch of a pulmonary artery becomes blocked. This condition is called a pulmonary embolism.

Innervation of the Lungs

Parasympathetic and sympathetic motor fibers along with visceral sensory fibers innervate the lungs. The visceral sensory fibers enter each lung through the pulmonary plexus, which is located on each lung root, and run along the bronchial tubes and lung blood vessels. The parasympathetic motor fibers constrict the air tubes (bronchoconstriction), whereas the sympathetic fibers cause them to dilate (bronchodilation).