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Respiratory System: Anatomy
The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove the waste product carbon dioxide, and help to maintain acid-base balance.
Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. Theconducting zoneof the respiratory system includes the organs and structures not directly involved in gas exchange; whilst the respiratory zone is involved in gas exchange.
The major functions of the conducting zone are to provide a route for incoming and outgoing air, remove debris and pathogens from the incoming air, and warm and humidify the incoming air. Several structures within the conducting zone perform other functions as well. The epithelium of the nasal passages, for example, is essential to sensing odours, and the bronchial epithelium that lines the lungs can metabolize some airborne pathogens.
In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles join arespiratory bronchiole, the smallest type of bronchiole, which then leads to an alveolar duct, opening into a cluster of alveoli. The trachea (windpipe) extends from the larynx toward the lungs. Thetracheais formed by C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. Thetrachealis muscleand elastic connective tissue together form thefibroelastic membrane, a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium.
trachea
The trachea (windpipe) extends from the larynx toward the lungs. Thetracheais formed by C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. Thetrachealis muscleand elastic connective tissue together form thefibroelastic membrane, a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium.
The trachea branches into the right and left primarybronchi. Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum, a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs. The bronchi continue to branch into bronchial a tree. Abronchial tree(or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens.
bronchiole
Abronchiolebranches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange. There are more than 1000 terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube.
alveolar ducts
Thealveolar ductsare tubes composed of smooth muscle and connective tissue, which opens into a cluster of alveoli. Analveolusis one of the many small, grape-like sacs that are attached to the alveolar ducts. Analveolar sacis a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbours byalveolar pores, which help maintain equal air pressure throughout the alveoli and lung.
The alveolar wall consists of three major cell types:
type I alveolar cells, type II alveolar cells, and alveolar macrophages.
Atype I alveolar cellis a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases.
Atype II alveolar cellis interspersed among the type I cells and secretespulmonary surfactant, a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli. Roaming around the alveolar wall is thealveolar macrophage, a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli.
The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries. Taken together, the alveoli and capillary membranes form arespiratory membranethat is approximately 0.5 μm (micrometers) thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and CO2to be released into the air of the alveoli.
Pulmonary ventilation comprises two major steps:
inspiration and expiration Inspirationis the process that causes air to enter the lungs, andexpirationis the process that causes air to leave the lungs.
respiratory cycle
Arespiratory cycleis one sequence of inspiration and expiration. In general, two muscle groups are used during normal inspiration: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity and more space for the lungs. Contraction of the external intercostal muscles moves the ribs downwards and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.
The process of normal expiration is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inspiration. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in intrapulmonary pressure. The intrapulmonary pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.
Pulmonary ventilation is dependent on three types of pressure:
atmospheric, intra-alveolar, and intrapleural.
Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body.
Intra-alveolar pressure (intrapulmonary pressure) is the pressure of the air within the alveoli, which changes during the different phases of breathing. Because the alveoli are connected to the atmosphere via the tubing of the airways, the intrapulmonary pressure of the alveoli always equalizes with the atmospheric pressure.
Intrapleural pressureis the pressure of the air within the pleural cavity, between the visceral and parietal pleurae. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure).
Transpulmonary pressure
the difference between the intrapleural and intra-alveolar pressures, and it determines the size of the lungs. A higher transpulmonary pressure corresponds to a larger lung.
Respiratory volume
is the term used for various volumes of air moved by or associated with the lungs at a given point in the respiratory cycle. There are four major types of respiratory volumes: tidal, residual, inspiratory reserve, and expiratory reserve).
Tidal volume (TV
is the amount of air that normally enters the lungs during quiet breathing.
Expiratory reserve volume (ERV)
is the amount of air you can forcefully exhale past a normal tidal expiration.
Inspiratory reserve volume (IRV)
is produced by a deep inhalation, past a tidal inspiration. This is the extra volume that can be brought into the lungs during a forced inspiration.
Residual volume (RV)
is the air left in the lungs if you exhale as much air as possible. The residual volume makes breathing easier by preventing the alveoli from collapsing. Respiratory volume is dependent on a variety of factors.
Respiratory capacity
the combination of two or more selected volumes, which further describes the amount of air in the lungs during a given time. For example,total lung capacity (TLC)is the sum of all of the lung volumes (TV, ERV, IRV, and RV), which represents the total amount of air a person can hold in the lungs after a forceful inhalation. TLC is about 6000 mL air for men, and about 4200 mL for women.Vital capacity (VC)is the amount of air a person can move into or out of his or her lungs, and is the sum of all of the volumes except residual volume (TV, ERV, and IRV), which is between 4000 and 5000 milliliters.Inspiratory capacity (IC)is the maximum amount of air that can be inhaled past a normal tidal expiration, is the sum of the tidal volume and inspiratory reserve volume. On the other hand, thefunctional residual capacity (FRC)is the amount of air that remains in the lung after a normal tidal expiration; it is the sum of expiratory reserve volume and residual volume.
Mechanisms of Respiration: Dead Space
In addition to the air that creates respiratory volumes, the respiratory system also containsanatomical dead space, which is air that is present in the airway that never reaches the alveoli and therefore never participates in gas exchange.Alveolar dead spaceinvolves air found within alveoli that are unable to function, such as those affected by disease or abnormal blood flow.Total dead spaceis the anatomical dead space and alveolar dead space together and represents all of the air in the respiratory system that is not being used in the gas exchange process.
Mechanisms of Respiration: Control
Breathing usually occurs without thought, although at times you can consciously control it, such as when you swim under water, sing a song, or blow bubbles. The respiratory rate is the total number of breaths, or respiratory cycles, that occur each minute. Respiratory rate can be an important indicator of disease, as the rate may increase or decrease during an illness or in a disease condition. The respiratory rate is controlled by the respiratory centre located within the medulla oblongata in the brain, which responds primarily to changes in carbon dioxide, oxygen, and pH levels in the blood.
Gas Exchange: Intro
Gas molecules exert force on the surfaces with which they are in contact; this force is called pressure. In natural systems, gases are normally present as a mixture of different types of molecules. For example, the atmosphere consists of oxygen, nitrogen, carbon dioxide, and other gaseous molecules, and this gaseous mixture exerts a certain pressure referred to as atmospheric pressure. Partial pressure(Px) is the pressure of a single type of gas in a mixture of gases. For example, in the atmosphere, oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen. Total pressureis the sum of all the partial pressures of a gaseous mixture.Dalton’s lawdescribes the behaviour of nonreactive gases in a gaseous mixture and states that a specific gas type in a mixture exerts its own pressure; thus, the total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases in the mixture.
Partial pressure is extremely important in predicting the movement of gases. Recall that gases tend to equalize their pressure in two regions that are connected. A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases.
Gas Exchange: Pulmonary Perfusion
The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries. These pulmonary capillaries create the respiratory membrane with the alveoli. As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen can dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes (red blood cells) and binds to a protein called haemoglobin, a process described later in this chapter. Oxygenated haemoglobin is red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. Some of the carbon dioxide is returned on haemoglobin, but can also be dissolved in plasma or is present as a converted form.
Gas Exchange: External Respiration
External respirationoccurs as a function of partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries. In external respiration, oxygen diffuses across the respiratory membrane from the alveolus to the capillary, whereas carbon dioxide diffuses out of the capillary into the alveolus.
Although the solubility of oxygen in blood is not high, there is a drastic difference in the partial pressure of oxygen in the alveoli versus in the blood of the pulmonary capillaries. This difference is about 64 mm Hg: The partial pressure of oxygen in the alveoli is about 104 mm Hg, whereas its partial pressure in the blood of the capillary is about 40 mm Hg. This large difference in partial pressure creates a very strong pressure gradient that causes oxygen to rapidly cross the respiratory membrane from the alveoli into the blood.
The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary. However, the partial pressure difference is less than that of oxygen, about 5 mm Hg. The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg. However, the solubility of carbon dioxide is much greater than that of oxygen—by a factor of about 20—in both blood and alveolar fluids. As a result, the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar.
Gas Exchange: Internal Respiration
Internal respirationis gas exchange that occurs at the level of body tissues. Like external respiration, internal respiration also occurs as simple diffusion due to a partial pressure gradient. However, the partial pressure gradients are opposite of those present at the respiratory membrane. The partial pressure of oxygen in tissues is low, about 40 mm Hg, because oxygen is continuously used for cellular respiration. In contrast, the partial pressure of oxygen in the blood is about 100 mm Hg. This creates a pressure gradient that causes oxygen to dissociate from haemoglobin, diffuse out of the blood, cross the interstitial space, and enter the tissue. Haemoglobin that has little oxygen bound to it loses much of its brightness, so that blood returning to the heart is more burgundy in colour.
Considering that cellular respiration continuously produces carbon dioxide, the partial pressure of carbon dioxide is lower in the blood than it is in the tissue, causing carbon dioxide to diffuse out of the tissue, cross the interstitial fluid, and enter the blood. It is then carried back to the lungs either bound to haemoglobin, dissolved in plasma, or in a converted form. By the time blood returns to the heart, the partial pressure of oxygen has returned to about 40 mm Hg, and the partial pressure of carbon dioxide has returned to about 45 mm Hg. The blood is then pumped back to the lungs to be oxygenated once again during external respiration.
Gas Exchange: Transport of Oxygen
Even though oxygen is transported via the blood, you may recall that oxygen is not very soluble in liquids. A small amount of oxygen does dissolve in the blood and is transported in the bloodstream, but it is only about 1.5% of the total amount. Most oxygen molecules are carried from the lungs to the body’s tissues by a specialized transport system, which relies on the erythrocyte—the red blood cell.
Erythrocytes contain a metalloprotein, haemoglobin, which serves to bind oxygen molecules to the erythrocyte. Heme is the portion of haemoglobin that contains iron, and it is heme that binds oxygen. Each hemoglobin molecule contains four iron-containing heme molecules, and because of this, one haemoglobin molecule can carry up to four molecules of oxygen. As oxygen diffuses across the respiratory membrane from the alveolus to the capillary, it also diffuses into the red blood cell and is bound by haemoglobin. The following reversible chemical reaction describes the production of the final product,oxyhaemoglobin(HbO2), which is formed when oxygen binds to haemoglobin.
Oxyhaemoglobin is a bright red-coloured molecule that contributes to the bright red colour of oxygenated blood.
In this formula, Hb represents reduced haemoglobin, that is, haemoglobin that does not have oxygen bound to it. There are multiple factors involved in how readily heme binds to and dissociates from oxygen, which will be discussed in the subsequent sections.
Haemoglobin is composed of subunits, a protein structure that is referred to as a quaternary structure. Each of the four subunits that make up haemoglobin is arranged in a ring-like fashion, with an iron atom covalently bound to the heme in the centre of each subunit. Binding of the first oxygen molecule causes a conformational change in haemoglobin that allows the second molecule of oxygen to bind more readily. As each molecule of oxygen is bound, it further facilitates the binding of the next molecule, until all four heme sites are occupied by oxygen. The opposite occurs as well: After the first oxygen molecule dissociates and is “dropped off” at the tissues, the next oxygen molecule dissociates more readily. When all four heme sites are occupied, the haemoglobin is said to be saturated. When one to three heme sites are occupied, the haemoglobin is said to be partially saturated. Therefore, when considering the blood, the percent of the available heme units that are bound to oxygen at a given time is called haemoglobin saturation.
Haemoglobin saturation of 100 percent means that every heme unit in all the erythrocytes of the body is bound to oxygen. In a healthy individual with normal haemoglobin levels, haemoglobin saturation generally ranges from 95 percent to 99 percent.
Gas Exchange: Oxygen Dissociation
Partial pressure is an important aspect of the binding of oxygen to and disassociation from heme. Anoxygen–haemoglobin dissociation curveis a graph that describes the relationship of partial pressure to the binding of oxygen to heme and its subsequent dissociation from heme. Remember that gases travel from an area of higher partial pressure to an area of lower partial pressure. In addition, the affinity of an oxygen molecule for heme increases as more oxygen molecules are bound. Therefore, in the oxygen–haemoglobin saturation curve, as the partial pressure of oxygen increases, a proportionately greater number of oxygen molecules are bound by heme. Not surprisingly, the oxygen–haemoglobin saturation/dissociation curve also shows that the lower the partial pressure of oxygen, the fewer oxygen molecules are bound to heme. As a result, the partial pressure of oxygen plays a major role in determining the degree of binding of oxygen to heme at the site of the respiratory membrane, as well as the degree of dissociation of oxygen from heme at the site of body tissues.
The mechanisms behind the oxygen–haemoglobin saturation/dissociation curve also serve as automatic control mechanisms that regulate how much oxygen is delivered to different tissues throughout the body. This is important because some tissues have a higher metabolic rate than others. Highly active tissues, such as muscle, rapidly use oxygen to produce ATP, lowering the partial pressure of oxygen in the tissue to about 20 mm Hg. The partial pressure of oxygen inside capillaries is about 100 mm Hg, so the difference between the two becomes quite high, about 80 mm Hg. As a result, a greater number of oxygen molecules dissociate from haemoglobin and enter the tissues. The reverse is true of tissues, such as adipose (body fat), which have lower metabolic rates. Because less oxygen is used by these cells, the partial pressure of oxygen within such tissues remains relatively high, resulting in fewer oxygen molecules dissociating from haemoglobin and entering the tissue interstitial fluid. Although venous blood is said to be deoxygenated, some oxygen is still bound to haemoglobin in its red blood cells. This provides an oxygen reserve that can be used when tissues suddenly demand more oxygen.
Gas Exchange: Oxygen Dissociation
Factors other than partial pressure also affect the oxygen–haemoglobin saturation/dissociation curve. For example, a higher temperature promotes haemoglobin and oxygen to dissociate faster, whereas a lower temperature inhibits dissociation. However, the body tightly regulates temperature, so this factor may not affect gas exchange throughout the body. The exception to this is in highly active tissues, which may release a larger amount of energy than is given off as heat. As a result, oxygen readily dissociates from haemoglobin, which is a mechanism that helps to provide active tissues with more oxygen.
The pH of the blood is another factor that influences the oxygen–haemoglobin saturation/dissociation curve. TheBohr effectis a phenomenon that arises from the relationship between pH and oxygen’s affinity for haemoglobin: A lower, more acidic pH promotes oxygen dissociation from haemoglobin. In contrast, a higher, or more basic, pH inhibits oxygen dissociation from haemoglobin. The greater the amount of carbon dioxide in the blood, the more molecules that must be converted, which in turn generates hydrogen ions and thus lowers blood pH. Furthermore, blood pH may become more acidic when certain byproducts of cell metabolism, such as lactic acid, carbonic acid, and carbon dioxide, are released into the bloodstream.
