introduction
Even at rest, the human body requires a large amount of oxygen to fuel the metabolic pathways that produce ATP. The respiratory system facilitates the exchange of gases between the atmosphere and the blood to oxygenate tissues while removing one of the end products of metabolism, carbon dioxide. The interface between air and blood in the respiratory system is made up of fluid and tissue (cells and extracellular matrix). Gases must dissolve in this liquid and pass through the tissue barrier to move between the air and the blood.
Gases passively diffuse between air and blood, and knowing some of the parameters that determine the rate of diffusion between air and blood helps to understand the structure of the respiratory system. Lucky for us, a formula shown below describes the rate of diffusion of gases between air and blood.
Although there are several terms, the formula is quite simple and contains two terms related to the structure of the respiratory system. The formula states that the rate of diffusion between air and blood across a tissue barrier depends on the surface area of the barrier (A) and the thickness of the barrier (a). Based on the formula, the respiratory system facilitates gas exchange by creating a tissue barrier with a large surface area, but very thin.
To create a large surface area, the respiratory system consists of a highly branched network of tubes that end in thin sacs that separate air from blood. The respiratory system begins with a single tube called the trachea, which is about 2 cm in diameter and through 23 generations of branches produces millions of small sacs called alveoli, which form an internal surface of about 50 to 75 m2(approximately the size of a tennis court). Furthermore, the diffusion rate is inversely proportional to the distance. To facilitate gas exchange, the walls of alveoli have a very thin layer of tissue that separates air from blood.
The tube network of the respiratory system is structurally and functionally divided into conducting airways and airways.
- The conducting airways include the nose, pharynx, larynx, trachea, bronchi, and bronchioles. These segments serve to direct, clean, heat and humidify the air. This exercise deals with conducting the airway from the trachea.
- Airways (exchange) include respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. These segments facilitate gas exchange and are located entirely within the lungs.
Keep these questions in mind as we describe the structure of the conducting and breathing segments of the airways. How does segment epithelium perform segment function? What structures keep the airways open or regulate their diameter? How does the respiratory system deal with the large amount of foreign particles that we breathe?
Main airways - sections
The structure of the conducting airways performs its primary functions of facilitating flow into and out of the respiratory portion of the airways and conditioning and purifying the air entering the lungs. Conditioning usually involves humidifying the air, creating a layer of fluid on the surface of the conducting airway. Air purification is mainly accomplished by a layer of mucus on the surface of the conductive airways that traps inhaled particles. Cilia on the apical surface of some of the epithelial cells in the conducting airways move mucus upward through the airways.
Conductive airways begin with the nose, throat, and larynx. We begin our discussion with the trachea.
trachea
The trachea connects the larynx above and the two main bronchi below. The trachea is easily recognized by its large C-shaped rings of hyaline cartilage. These rings prevent the tracheal lining from collapsing during inspiration and are located in front of the trachea. The ends of the rings are joined together by the tracheal muscle, which is made of smooth muscle fibers at the back of the trachea. Note the presence of glands in the submucosal layer of the trachea. These glands contain cells that secrete mucus and others that secrete fluid approximately isotonic with plasma. Fluid is released onto the surface of the epithelium. The tracheal epithelium is pseudostratified.
The image below shows, at higher magnification, the components found in the wall of the trachea and large bronchi. The epithelium facing the lumen of the trachea is clearly pseudostratified with cilia and will be discussed in more detail below. The epithelium rests on a thick basement membrane. Beneath the basement membrane is the lamina propria, which contains connective tissue and blood vessels. The epithelium, basement membrane, and lamina propria form the mucous layer.
The submucosa is below the mucosa and contains a large number of seromucin glands. The glands secrete fluid and mucus onto the surface of the epithelium. The vapor from the fluid in the epithelium humidifies the air, which helps to prevent the alveoli from drying out when the air reaches this point. Note also the hyaline cartilage in the submucosa. In the windpipe, cartilage forms a C-shaped ring at the front of the windpipe. In the bronchi, cartilage forms plaques.
Pseudostratified epithelium of the trachea and bronchi
The trachea and bronchi are lined with pseudostratified epithelium. The main function of the pseudostratified epithelium is to trap and remove foreign particles and airborne pathogens that enter the lungs during inhalation. The image below shows a section of the wall of a bronchus at higher magnification. Note the characteristic pseudostratified epithelium containing ciliated cells and mucus-secreting cells. Mucus is a mixture of water, ions, a variety of glycoproteins called mucins, and various other proteins. Mucins form a gel-like structure on the surface of the epithelium that traps foreign particles and pathogens. In addition, mucus components contain antioxidant, antimicrobial, and antiprotease activities. Cilia on the apical surface of the pseudostratified epithelium undulate to move mucus through the airways. Cilia are filled with microtubules. The motor protein dynein is located at the base of cilia and pushes ciliary microtubules against each other to produce cilia motility.
bronchi
Two main bronchi branch off the trachea, supplying the left and right lungs. The main bronchi branch again to form the secondary bronchi, and the branching process continues for another 9 generations to form smaller, more numerous bronchi. The larger bronchi contain a pseudostratified epithelium with a cellular composition similar to that of the trachea, but as the bronchi decrease in size, the epithelium becomes ciliated and columnar and the number of goblet cells decreases. The basement membrane in the bronchi is much thinner than in the trachea. Glands similar to those of the trachea are still found in the submucosa.
The bronchi are distinguishable in sections of lung tissue by the cartilaginous plaques in their walls. As the bronchi enter the lungs, the C-shaped cartilages that characterize the trachea and primary bronchi are replaced by irregular plates of cartilage that surround the cylindrical muscular tube of the airways. As in the trachea, the cartilage prevents the bronchial tubes from collapsing. The bronchi also contain a layer of smooth muscle arranged in a spiral. The significance of the spiral arrangement is unclear, but muscle contraction reduces the diameter of the bronchi.
Conduction of the bronchioles
The conductive bronchioles are the final segment of the conductive airways and form the 11th to 16th generations of the respiratory system. The diameter of the conducting bronchioles is usually 1 millimeter or less. The epithelium of the conducting bronchioles is ciliated columnar with very few goblet cells. Bronchi lack cartilaginous plates and glands, which distinguishes them from bronchi.
The lack of cartilage would cause the bronchioles to collapse during expiration and subsequently become difficult to open during inspiration. To keep the bronchioles open during the respiratory cycle, the tissue that surrounds the bronchioles and is attached to the walls creates tension primarily through its extracellular matrix components. Note that most of the tissue around the bronchioles is alveolar. When the alveoli expand during inspiration, the tension in the walls of the alveoli is transferred to the walls of the bronchioles to expand them. During expiration, when transmural pressure drops, the rigidity of the alveolar walls prevents the bronchioles from collapsing. Diseases that damage the walls of the alveoli reduce tension in the bronchioles, making them more difficult to open and increasing resistance to airflow through the bronchioles.
The bronchioles also have a layer of smooth muscle that, under certain conditions, can change the diameter of the bronchioles to decrease or increase resistance to airflow. Parasympathetic nerves release acetylcholine to the smooth muscle cells of the bronchioles to stimulate contraction and decrease the diameter of the bronchioles. In contrast, sympathetic nerves release norepinephrine, which relaxes the smooth muscle in the bronchioles and expands the diameter of the bronchioles. Epinephrine, produced in the adrenal gland, has a similar but stronger effect to norepinephrine.
The size of the smooth muscle layer can also change in certain pathological processes. Both asthma and other inflammatory responses have been shown to increase the thickness of the smooth muscle layer in the bronchi and conducting bronchioles through hypertrophy and hyperplasia. The thicker layer of muscle increases airway resistance and decreases airflow in the airways.
The smallest conducting bronchioles are called terminal bronchioles, which merge into the airways.
respiratory tract
The airways extend from the respiratory bronchioles to the alveoli. The primary function of this section of the airway is to facilitate gas exchange.
The respiratory bronchioles branch from the terminal bronchioles and form the 17th to 19th generation of branches. Respiratory bronchioles are about 0.5 mm in diameter and contain a few alveoli scattered along their walls. Each respiratory bronchiole branches into 2 to 11 alveolar ducts, which retain a cubical epithelium and still contain smooth muscle fibers in their walls. In the walls of the alveolar ducts there are single alveoli and numerous alveolar sacs containing 2 to 4 alveoli. The space at the entrance of the alveolar duct to an alveolar sac is called the atrium.
The epithelium of the respiratory bronchioles is cuboidal with a mixture of hair and club cells. Club cells perform several important functions, including producing surfactant-like material, detoxifying inhaled chemicals, and absorbing ions and water from the lumen of the airways to control the amount of fluid in the airways. The club cells also serve as stem cells capable of replacing the other epithelial cells in the airway bronchioles.
alveoli
Alveoli facilitate gas exchange between inhaled air and blood by forming a thin layer of tissue between air and blood. The tissue layer consists of airway epithelium, basement membrane, and capillary endothelium. The combination of a thin barrier between air and blood and a large alveolar surface area allows for rapid diffusion of gases between air and blood.
When looking at histological images of alveoli and thinking about the process of gas exchange, it is important to remember that the surface of the alveoli facing the airspace is covered by fluid. This fluid is usually not visible on histological images, but its presence has important consequences for gas exchange and the structure of the alveoli. First, oxygen from the air must be dissolved in the liquid before it can diffuse through the walls of the alveoli. At 37 °C, the solubility of oxygen in water is about 0.0013 mM/mm Hg. Second, the amount of fluid in the alveoli also affects the rate of diffusion of gases between the airways and the blood. The more liquid there is, the greater the distance that the gases must travel between the air and the blood. The epithelial cells of the alveoli reduce the amount of fluid in the lungs by actively reabsorbing sodium and chloride from the fluid in the lumen. The osmotic gradient created by the reabsorption of sodium and chloride draws water from the airways into the alveoli and into the interstitium. Finally, the fluid affects the architecture of the alveoli. The interface between the liquid and the air creates a surface tension strong enough to collapse the alveoli. The surfactant described below lowers surface tension to prevent alveoli from collapsing.
alveolar cells
Alveoli contain several different types of cells that can be divided into resident cells and transient cells. Resident cells are those that form the structure of the alveoli and participate in gas exchange. Resident cells include pneumocytes, endothelial cells, and occasionally fibroblasts. Transient cells include dust cells (macrophages) and other immune cells, the number of which varies depending on the presence of infectious agents and foreign particles.
pneumocytosis
The superficial (air-facing) epithelium of the alveoli contains two developmentally related but functionally distinct cells known as pneumocytes. Type I pneumocytes form a simple squamous epithelium that covers most of the surface of the alveoli. Type I pneumocytes surround a basement membrane and the endothelial cells of capillaries to form the air-blood barrier through which gases diffuse between air and blood.
Type II pneumocytes are larger cubical cells. They produce and excrete surfactants in the fluid facing the airspace. The surfactant reduces surface tension along the fluid-air interface, preventing alveoli from collapsing. The surfactant contains molecules that are similar to lipids in that they contain both hydrophilic and hydrophobic (amphiphilic) domains. Hydrophobic domains interact with air while hydrophilic domains associate with liquid. Surfactant also contains protein components that play a role in innate immunity, in addition to its structural role in surfactant formation.
Type II pneumocytes are often found at the junctions between two alveolar walls. After damage to the alveoli, type II pneumocytes are able to proliferate and differentiate into type I pneumocytes during the repair process.
Although type II pneumocytes are more numerous than type I pneumocytes, type I pneumocytes occupy about 95% of the surface of alveoli because of their scaly shape.
Pneumocyte EM
Electron micrographs show the structural differences between type I and type II pneumocytes and the thin barrier that separates the airways from the circulatory system. Note the thin cytoplasm of type I pneumocytes surrounding a capillary. The capillary is defined by a continuous endothelium. Between the endothelial cell and the type I pneumocyte there is a thin basement membrane that is shared by both cells. The size of red blood cells in the capillary makes it possible to estimate the distance that gases diffuse between the airways and the blood.
Type II pneumocytes are significantly larger and more cubical than Type I pneumocytes. Lamellar bodies are distinguishing features of Type II pneumocytes and contain the phospholipid precursors of lung surfactant, which is released into airway fluid to reduce surface tension between the fluid and the air.
air-blood barrier
This electron micrograph shows the three layers of the air-blood barrier through which gas exchange takes place. The type I pneumocyte is part of the simple squamous epithelium of the alveoli and the endothelial cell represents the capillary epithelium. The two cells share a fused basement membrane that allows the barrier through which gas exchange takes place to be minimized. Oxygen from the air dissolves in the surface fluid of the alveoli and then diffuses through the cytoplasm of type I pneumocytes, through the basement membrane, and then through the endothelial cell cytoplasm to reach the blood. Carbon dioxide follows the reverse path from blood to air.
Absorption of fluid by pneumocytes
Because the fluid along the surface of pneumocytes in alveoli is a barrier to gas diffusion, pneumocytes actively absorb fluid via vector transport to maintain a thin layer of fluid and optimize gas diffusion. Type I and type II pneumocytes are involved in fluid absorption. Both cells transport sodium and chloride from the respiratory fluid into the interstitium to create an osmotic gradient that drives water absorption. Sodium (ENaC) and chloride (CFTR) channels on the apical surface mediate diffusion to pneumocytes, and the sodium-potassium pump drives sodium into the interstitium. In type I pneumocytes, water can flow paracellularly or through aquaporin channels.
alveolar macrophages
Alveolar macrophages reside in the air spaces of the alveoli and serve to remove particles such as dust and pollen. Alveolar macrophages are also called dust cells. Alveolar macrophages are derived from monocytes and are also found in the connective tissue of the lungs. An increase in macrophages in the airways is often an indicator of a pathological condition.
pulmonary circulation
The lungs are supplied by two types of circulatory systems: the pulmonary and the bronchial. The bronchial circuit carries oxygen-rich blood to the main branches of the airways, and its blood vessels are similar to the blood vessels of other systemic circulatory systems.
The pulmonary system brings deoxygenated blood from the right ventricle to the lungs to mediate gas exchange in the alveoli. The pulmonary circulatory system has characteristics that differ from other systemic circulatory systems. First, the pulmonary arteries carry deoxygenated blood and the veins carry oxygenated blood. Second, the pulmonary system has a significantly lower resistance compared to the systemic resistance (about 1/10).
The lower resistance in the pulmonary circulation is created by a structural change in the walls of the arteries and arterioles. First, arteries in the pulmonary circulation have relatively more elastic fibers and less smooth muscle compared with similarly sized arteries in the systemic circulation. Second, arterioles in the pulmonary system either contain only a partial layer of smooth muscle cells or lack smooth muscle. Remember that in most systemic circulatory systems, arterioles create the most resistance.
lung section
Now that you are familiar with the histological structure of the lung, this image shows a section of the lung that contains the bronchi, bronchioles, alveolar ducts, and sacs. Note that the bronchi and bronchioles are similar in structure, but bronchioles lack cartilaginous plates. Also appreciate the close juxtaposition of the airways with the pulmonary blood vessels. The pulmonary arteries run alongside the bronchi and the conducting bronchioles. When a main bronchus or bronchiole branches, the pulmonary artery that accompanies the airway also branches, sending out a smaller artery that runs alongside each of the new airways.
Note that the walls of the bronchioles are connected to the alveoli. The tension in the walls of the alveoli prevents the bronchioles from collapsing during expiration. Cartilaginous plates around the bronchial tubes keep the bronchial tubes open during expiration.
Structure of the alveoli and respiratory rate
The structure and composition of alveoli are not only the site of gas exchange, but also essential for ventilation in and out of the lungs. Recall from physiology class that the movement of air in and out of the lungs is largely driven by pressure differences within the lungs and the atmosphere. As the alveoli occupy the largest volume of the lungs, changes in their dimensions create the pressure differences between the lungs and the atmosphere. Increasing the volume in the alveoli decreases the pressure in the lungs to pull air into the lungs. Decreasing the volume in the alveoli increases the pressure in the lungs to push air out.
Two parameters reduce the volume of alveoli. Most important is the surface tension at the air-water interface. This tension is somewhat reduced by the surfactant. The second parameter is the connective tissue in the walls of the alveoli. Connective tissue contains a mixture of elastic fibers and collagen fibers. During normal inspiration and expiration (tidal volumes), elastic fibers are the most important component in determining alveolar volume. Remember that elastic fibers stretch under pulling forces and then bounce back when those forces are removed. Thus, the recoil of the elastic fibers reduces the volume of the alveoli during expiration. Collagen fibers are coiled around the walls of the alveoli and resist expansion of the alveoli during deep inspiration only when the walls of the alveoli elongate enough to unwind the fibers. Remember that collagen fibers are much more rigid than elastic fibers and are more of a restriction on the expansion of alveoli.
To increase its volume during inspiration, the lungs must overcome the repulsive forces of surface tension and tension created by elastic fibers and collagen fibers in the walls of the alveoli. These three parameters largely determine the compliance of the lungs. Compliance is a measure of how much force is required to increase lung volume. The greater the compliance, the less force is required to increase lung volume. The force to increase lung volume comes from the expansion of the rib cage and diaphragm, which creates negative pressure in the fluid that separates the pleural surface of the lung from the pleural surface of the chest cavity. The negative pressure in the intrapleural fluid creates the force to expand the volume of the lungs. In the image above, notice how the alveoli are attached to the pleural wall of the lungs. Tension on the lung wall (created by negative pressure in the intrapleural fluid) pulls on the walls of the alveoli to expand the alveolar space and increase the volume in the lungs. Elastic recoil forces on the alveoli are usually sufficient to reduce the alveolar space during expiration.
Dealing with inhaled particles
At rest, the average person takes in 400 to 500 ml of air with each breath. This volume increases as the metabolic demand for oxygen increases. In addition to gases, air contains varying concentrations of small particles (10 µm and smaller) that can penetrate deep into the lungs. Particles smaller than 2.5 µm (PM2.5) are of most concern as they have been found to reach the alveoli and enter the interstitial tissue and blood.
The concentration of PM2.5 particles varies by geography. In the United States, air with PM2.5 concentrations below 35 µg/m3it is considered safe. Many places around the world have much higher concentrations of PM2.5. For example, measurements from the US Embassy in Beijing found PM2.5 concentrations greater than 100 µg/m33on more than half of winter days between 2010 and 2014 and maximum values of 744 µg/m33.
The concentration of PM2.5 particles in the United States has been slowly decreasing in recent years, particularly in the eastern states and during the winter months in the western states. Unfortunately, PM2.5 particle concentrations have not decreased in western states during the summer months. One explanation for this discrepancy is the increase in the number of large wildfires in western states in recent years. The increase in wildfires is linked to climate change, which has increased average daily spring and summer temperatures in western states.
PM2.5 particles have been shown to cause multiple pathologies. Experiments in animal models have shown that PM2.5 particles generate free radicals in lung tissue, which lead to cell and DNA damage. Cell damage triggers inflammation, which compromises this structure of the alveoli and narrows the diameter of the conducting bronchioles. Additionally, inhaled particles appear to aggravate several pre-existing conditions, such as asthma, by activating inflammatory pathways and increasing airway resistance.
The lungs have two mechanisms for eliminating inhaled particles. The first is to trap the particles in the mucus and remove it through the action of cilia on the epithelium. In the alveoli, dust cells (macrophages) patrol the airspace and can eliminate inhaled particles through phagocytosis. While macrophages can clear tracts and interstitial tissues of inhaled particles, excessive activation of macrophages triggers inflammation, leading to tissue damage and narrowing of the airways.
