Respiratory System Respiratory System To Accompany: Anatomy and Physiology Text and Laboratory Workbook, Stephen G. Davenport, Copyright 2006, All Rights Reserved, no part of this publication can be used for any commercial purpose. Permission requests should be addressed to Stephen G. Davenport, Link Publishing, P.O. Box 15562, San Antonio, TX, 78212 Respiratory System • The respiratory system consists of the – – – – – – – – – – nose, nasal cavity, pharynx, larynx, trachea, primary bronchi, and the lungs, which contain the smaller bronchi, the bronchioles, and the alveoli (air sacs). • The respiratory system consists of the organs that are involved in the delivery and exchange of gases between the air and the blood. • Functions of the respiratory system include the – – – – (1) delivery of air to and from the exchange surfaces, (2) protection of respiratory surfaces, (3) sound production, and (4) providing for the sense of smell (olfaction). Organization of Respiratory System • Upper respiratory tract – The upper respiratory tract consists of the nose, nasal cavity, pharynx, and trachea. • Lower respiratory tract – The lower respiratory tract consists of the bronchi, bronchiole, and the alveoli (air sacs). • The respiratory system is divided into the – upper and the lower respiratory tracts, the passage ways by which air enters and leaves the respiratory system. Figure 25.1 NOSE • The nose functions as a respiratory airway where incoming air is warmed, filtered, and moistened. Also, the nose serves as a resonating chamber for sound and houses the receptors for smell. It can be divided into two portions, the external nose and the nasal cavity. • External Nose – The external nose is the skin-covered portion that is supported by a bony and cartilaginous framework. • External nares (Nostrils) – The external nares (nostrils) are the openings formed by the external nose that open into the nasal cavity. 1 NASAL CAVITY NASAL CAVITY • Conchae • The nasal cavity is the space within and posterior to the external nose. It is lined with a mucous membrane. The external nares (nostrils) are the anterior entrances to the nasal cavity. The posterior openings of the nasal cavity, the internal nares, enter the nasal portion of the pharynx (nasopharynx). • Vestibule – The conchae are thin bony projections that extend inward from each side of the nasal cavity to almost reach the nasal septum. Each side of the nasal cavity has a superior, middle, and inferior concha. The conchae function to increase the surface area of the cavity. They form air passages called the meatuses. • Meatuses – The meatuses are the air passageways formed beneath the conchae. Each meatus is named in relation to its corresponding concha: the superior, the middle, and the inferior meatus. As air passes through the meatuses, it is moistened, warmed, and filtered. – A nasal vestibule is the short chamber that originates at each nostril and leads into the nasal cavity. • Nasal septum – The nasal septum is the vertical partition that divides the nasal cavity into right and left sides. The anterior portion is formed by cartilage and the posterior portion is formed by the perpendicular plate of the ethmoid bone and the vomer bone. • Internal nares – The internal nares are the posterior openings of each side of the nasal cavity into the nasopharynx. PHARYNX • • The pharynx is the tube that extends inferiorly from behind the nose to the base of the larynx (voice box). Nasopharynx – The nasopharynx is the uppermost portion of the pharynx. It communicates with the nasal cavity by the internal nares and the oropharynx at the level of the soft palate. Located in each of its lateral walls is the opening into the pharyngotympanic tube (also called auditory, or eustachian tube). The pharyngeal tonsil is located at its superior, posterior border. • Oropharynx – The oropharynx extends from the level of the soft palate to the level of the hyoid bone. Anteriorly, the oropharynx opens into the oral cavity. The uvula is a small Figure 25.2 process which hangs downward from the posterior border of the soft palate. Figure 25.2 PHARYNX LARYNX • The larynx (voice box) is positioned at the superior aspect of the trachea and consists of cartilages, extrinsic ligaments, muscles, and associated tissues. • Thyroid cartilage • Laryngopharynx – The laryngeal part of the pharynx extends from the level of the hyoid bone downward to the cricoid cartilage of the larynx and to the origin of the esophagus. – The thyroid cartilage is the upper and largest cartilage of the trachea. Its right and left anterior borders fuse to form the projection known as the Adam’s apple. • Cricoid cartilage – The cricoid cartilage is inferior to the thyroid cartilage and forms the lower part of the larynx. • Figure 25.2 Figure 25.2 Epiglottis – The epiglottis is a flap of elastic cartilage that is attached to the thyroid cartilage and projects upward. It closes the glottis, the opening of the larynx, during swallowing. 2 TRACHEA LARYNX • Vestibular folds (false vocal cords) • The trachea is the airway that originates from the larynx and extends downward to the level of the fifth thoracic vertebra where it branches into the right and left primary bronchi. • Mucosa – The vestibular folds (false vocal cords) are the upper pair of vocal folds. They are not used in the production of sound. They provide a protective function for the true focal folds. • – The vocal folds (true vocal cords) are the lower pair of vocal folds. Each is formed from an epithelial covered ligament that consists mostly of elastic tissue. The vocal cords function in the production of sound and are located inferior to the vestibular folds (false vocal cords). • – The mucosa is the innermost region of the trachea. Its lining is pseudostratified ciliated columnar epithelium. The epithelium contains numerous goblet cells that produce abundant mucus. The cilia function to move mucus to the pharynx. Vocal folds (vocal cords) Figure 25.2 Glottis • Submucosa – The outer region to the mucosa is the submucosa. The submucosa consists of a layer of connective tissue that contains numerous seromucous glands and blood vessels. The seromucous glands produce a combination of serous fluid and mucus that is emptied though ducts to the surface of the mucosa. – The glottis is the passageway between the vocal cords. TRACHEA Trachea – Lab Activity 2 • Adventitia – The adventitia is the outer region of the trachea. The adventitia consists of fibrous connective tissue and is supported by incomplete rings of hyaline cartilage. The cartilage rings are open at the posterior surface of the trachea and are described as shaped like the letter “C” (C-rings). The C-rings function to support and increase the flexibility of the trachea. A small band of smooth muscle, the trachealis muscle connects the posterior portions of the C-rings. Contraction of the trachealis muscle functions to Figure 25.3 Trachea – Lab Activity 2 Trachea – Lab Activity 2 Figure 25.5 Figure 25.4 3 LUNGS LUNGS • The lungs are located within the thoracic cavity on each side of the region called the mediastinum. • Each lung is surrounded by a microscopic cavity, the pleural cavity, which contains serous fluid produced by the serous membrane surrounding each lung. The portion of the serous membrane on the surface of the lungs is called the visceral pleura. The portion of the serous membrane external to the visceral pleura and surrounding each lung is called the parietal pleura. – The right lung consists of three lobes: the superior, middle, and inferior lobes. – The left lung consists of two lobes, the superior and inferior lobes. – The cardiac notch of the left lung is an indentation occupied by a portion of the heart. LUNGS LUNGS ELASTICITY AND PRESSURES • The elasticity of the lungs allows expansion and recoil during breathing. – Expansion of the lungs results because of decreased thoracic pressure produced by contraction of muscles external to the lungs, such as the diaphragm and external intercostal muscles. – Lung recoil is a passive event that results because of the recoil of the lung’s elastic fibers and the surface tension of alveolar fluid. Recoil tension from the lungs is transferred from the lung’s visceral pleura through the surrounding serous fluid to the parietal pleura and supporting tissues. Figure 25.6 LUNGS ELASTICITY AND PRESSURES • Thus, the pleural cavities (serous fluid) have an intrapleural pressure, a negative pressure produced by the tendency of the lungs to recoil. Intrapleural pressure is slightly less than atmospheric pressure and alveolar pressures (alveolar, or intrapulmonary pressure, varies with each inspiration and expiration). • Equalization of the intrapleural pressure and the intrapulmonary or the atmospheric pressure results in the recoil of the lung away from the parietal pleura, and the lung collapses. TRACHEA AND BRONCHIAL TREE Tracheobronchial Tree The branching pattern of the bronchi is often described as the bronchial or respiratory tree. 4 Tracheobronchial Tree • The trachea divides into the right and left primary bronchi, which begins the branching pattern called the bronchial tree. • Right Bronchus – The shorter, wider, and more vertical right bronchus branches into three secondary (lobar) bronchi. Each supplies one lobe of the right lung (three lobes). • Left Bronchus – The left bronchus branches into two secondary (lobar) bronchi. Each secondary bronchus supplies a lobe of the left lung (two lobes). – Secondary (lobar) bronchi – Tertiary (secondary) bronchi – Bronchioles – Respiratory bronchioles (with alveoli) – Alveolar ducts – Alveoli Structure of Trachea and Bronchial Tree • Trachea The trachea, the large respiratory airway that leads to the bronchial tree, – supported by cartilage C-rings, – has a limited amount of smooth muscle (the posterior trachealis muscle) and – is lined internally with pseudostratified ciliated columnar epithelium. Structure of Bronchial Tree • Bronchi show variations in structure as their size decreases. – Their supporting cartilage is formed by cartilaginous plates, which decrease in thickness and number as the bronchi become smaller. In the bronchiole cartilage is completely lacking. – The amount of smooth muscle increases as the bronchi become smaller. In the bronchiole, smooth muscle is abundant and functions in regulation of the diameter of the bronchiole. – The pseudostratified ciliated columnar epithelium lining the trachea and bronchi changes to ciliated cuboidal epithelium in the terminal and respiratory bronchiole. Simple squamous epithelium is present in the alveolar ducts and alveoli. Figure 25.7 Lab Activity 3 – Lung Tissue Figure 25.8 Bronchography is the x-ray examination of the tracheobronchial tree after exposure to a radiopaque substance. The image produced is called a bronchogram. bronchiole and alveoli are not shown in bronchograms because the radiopaque material is filtered out of the air before it reaches their locations. Figure 25.9 A scanning power photograph of a section of lung. The lung is extremely porous because of the presence of air sacs, the alveoli. 5 Lab Activity 3 – Lung Tissue Lab Activity 3 – Lung Tissue • Bronchi – The bronchi are the airways of the lungs. Bronchi are lined with pseudostratified ciliated columnar epithelium and their walls contain plates of supporting hyaline cartilage. • Bronchioles Figure 25.10 Low power photograph of a section of lung. This photograph shows several alveolar sacs associated with a bronchiole. Lab Activity 3 – Lung Tissue • Alveoli • The alveoli are the sites of gas exchange. Alveoli organized into groups are called alveolar sacs. Alveolar ducts connect alveolar sacs to bronchiole. The wall of an alveolus consists of alveolar epithelium and surrounded by a network of capillaries and elastic fibers. The alveolar epithelium and the capillaries function as sites of gas exchange between the air and the blood. Elastic fibers function in providing elasticity for alveoli expansion during inhalation and providing elastic recoil during the passive exhalation of air. Lab Activity 3 – Lung Tissue Alveolar epithelium • Simple cuboidal cells (type II cells) – The simple cuboidal cells, also called type II cells, function in the production of surfactant. Surfactant reduces the surface tension of water on the alveolar surfaces, thus, Fig 25.11 increasing lung compliance. Lung compliance refers to ability of the lungs to expansion. Having lung compliance means that the lungs can expand easily. – Bronchioles are the thin wall extensions of the bronchi. They contain abundant smooth muscle and are called the airways of resistance regulation because of their ability to undergo bronchoconstriction and bronchodilation. Lab Activity 3 – Lung Tissue Alveolar epithelium • Simple squamous cells (type I cells). – The simple squamous cells, also called type I cells, function in (1) gas diffusion between air in the alveolus and the capillaries that cover the alveoli and (2) produce the enzyme called angiotensin converting enzyme (ACE), which converts angiotensin Fig 25.11 I to angiotensin II. Angiotensin II is a powerful vasoconstrictor and functions in blood pressure regulation. Alveolar macrophages • Alveolar macrophages defend the alveoli from pathogens and other foreign substances. The alveoli lack ciliated epithelium and are not a part of the mucus escalator of the bronchiole, bronchi, and trachea. The mucus escalator continually moves inhaled substances to the thorax where they are swallowed. 6 Lab Activity 4 Alveolar Macrophages in Smoker’s Lung or Emphysema Figure 25.12 The alveoli are enlarged with thick fibrous (nonelastic) walls. The slide also shows abundant carbon in the tissue, which accumulated after years of inhalation of small carbon particles (probably from smoking). Figure 25.13 High power photograph of a macrophage from a pathology slide of emphysema and smoker’s lung. This macrophage is packed with small particulate matter, especially carbon particles. Respiratory membrane • The walls of the alveoli and the capillaries consist of simple squamous epithelia and their under lying basement membranes (basal lamina). – The two squamous epithelia (alveolar and capillary) are fused by their underlying basement membranes (basal lamina) to form the extremely thin respiratory membrane. The respiratory membrane forms the barrier between the air and the blood and functions in allowing gas exchange by diffusion. Oxygen diffuses into and carbon dioxide diffuses out of the blood. Respiratory membrane Pulmonary Ventilation Figure 25.14 Illustrations and high power photograph of an alveolus showing the structure of the respiratory membrane. Pulmonary (respiratory) ventilation is the exchange of air between the lungs and the atmosphere. Air Pressure (Driving Force for Ventilation) Air Pressure (Driving Force for Ventilation) • Air moves from an area of high pressure to an area of low pressure. The pressure of a gas is a result of the interaction between the molecules of air. Air pressure, or atmospheric pressure, is the pressure produced by the weight of the atmosphere. • At sea level atmospheric pressure is expressed as one atmosphere, or 760 millimeters (mm) of mercury (Hg.). • Boyle’s law states that the pressure and volume of a gas are inversely proportional. Decreasing the volume of a gas increases its pressure, and increasing the volume of a gas decreases its pressure. • Thus, pulmonary ventilation, or breathing, is based upon changing the pressure inside of the lungs, the intrapulmonary pressure, in reference to the pressure outside of the lungs. 7 Normal Quiet Breathing • Normal quiet breathing means that breathing is not forced. • In normal quiet breathing both diaphragmatic and costal breathing occur, with exhalation being passive by relaxation of the contracting muscles. • Diaphragmatic breathing accounts for most of the air exchange of normal quiet breathing. Diaphragmatic Breathing • In diaphragmatic breathing, contraction of the diaphragm moves the diaphragm downward, away from the thorax. This results in an increase in the vertical volume of the thorax. As volume increases, intrapulmonary pressure decreases and air flows into the lungs. Relaxation of the diaphragm results in the diaphragm moving upward decreasing the volume of the thorax. Costal Breathing In costal breathing, contraction of the external intercostal muscles moves the ribs upward and outward. This movement produces a horizontal increase in the volume of the thorax. Increased thoracic volume results in decreased intrapulmonary pressure and air flows into the lungs. Figure 25.15 Illustration showing normal quiet breathing involving diaphragm and intercostal muscles. Forced Breathing • Forced Inspiration Forced inspiration involves contraction of the – external intercostal muscles, – contraction of the diaphragm, and – contraction of the inspiratory accessory muscles, resulting in the maximal increased volume of the thorax. Forced Breathing • Forced Expiration Forced expiration involves contraction of the muscles of expiration, which include the – internal intercostal muscles and – abdominal muscles (such as the rectus abdominus), resulting in the maximal decreased volume of the thorax. 8 RESPIRATORY VOLUMES Air exchange must be adequate to maintain oxygen delivery to and carbon dioxide removal from the body. Figure 25.16 Forced inspiration and forced expiration involve maximal changes to the volume of the thorax. RESPIRATORY VOLUMES • Two variables in the exchange of air, – (1) the amount of air (respiratory volumes and capacities) and – (2) the rate of air exchange. Respiratory Volumes and Capacities • A respiratory volume is the amount of air in a single respiratory event. • A respiratory capacity is the sum of two or more respiratory volumes. • Spirometers – Spirometers are instruments used to measure the volume of air exchanged by the lungs. Respiratory Volumes and Capacities • Tidal volume, TV – The amount of air inhaled or exhaled in a normal quiet breath. • Inspiratory reserve volume, IRV – Inspiratory reserve volume is the amount of air inhaled above a normal quiet inspiration (tidal volume). • Expiratory reserve volume, ERV – Expiratory reserve volume is the amount of air exhaled after a normal quiet expiration (tidal volume). Respiratory Volumes and Capacities • Residual volume, RV – Residual volume is the amount of air remaining in the lungs after complete exhalation. This air remains in the airways and air spaces of the lungs. • Inspiratory capacity, IC – Inspiratory capacity is the amount of air that can be inhaled after a normal quiet expiration. IC = TV + IRV • Vital Capacity, VC – Vital capacity is the maximum amount of air that can be exhaled after a maximum inhalation. VC = ERV + TV + IRV or VC = ERV + IC 9 Respiratory Volumes and Capacities • Total Lung Capacity, TLC – Total lung capacity is the maximum amount of air contained in the lung after a maximum inhalation. TLC = RV + ERV + TV + IRV or TLC = RV + VC • Functional residual capacity, FRC – Functional residual capacity is the amount of air in the lungs after a normal quite expiration (tidal volume). FRC = RV + ERV Figure 25.17 Spirogram showing respiratory volumes and capacities. Measuring Respiratory Volumes Lab Activity 5 Hand-Held Spirometers • SAFETY PRECAUTIONS Measuring Respiratory Volumes Lab Activity 5 • Forced Expiratory Volume Timed (FEVT) – A respiratory function test called the Forced Expiratory Volume Timed (FEVT ) measures the total air exhalation (vital capacity) as a function of time intervals. – Seventy five percent (75%) of the forced vital capacity should be exhaled in the first one second interval (FEV1 ). – Consult the manufacture’s usage instruction sheets for information on cleaning and disinfection of equipment. – Be absolutely sure that your equipment has been adequately cleaned and disinfected before usage. – Insert a new sterile disposable mouthpiece and disposable bacterial filter before use. Determination of vital capacity percentage Determination of one second volume percentage Lab Activity 6 •EXHALE ONLY INTO THE SPIROMETER Figure 25.18 Spirogram from a recording spirometer. Determine the one second percentage by dividing the one second volume (4,000 ml.) by the vital capacity (4,600 ml.) and multiply by 100. The one second percentage is 4,000/4,600 = .869 X 100 = 87% • A person’s vital capacity should be at least 80% of his predicted vital capacity. • Consult the tables at the end of this chapter for the predicted vital capacities for males and females. • Determine your vital capacity percentage by dividing your vital capacity by your predicted vital capacity, and multiplying by 100. 10 Gas Movement and the Respiratory Membrane Gas Movement and the Respiratory Membrane Breathing allows air exchange for the lungs. • Alveolar air has a slightly higher concentration of carbon dioxide and a slightly lower concentration of oxygen than that found in atmospheric air. • Atmospheric air is a mixture of gases and at sea level is about 78.6% nitrogen, 20.9% oxygen, and 0.04% carbon dioxide, and water vapor. • In a mixture of gases, such as the atmosphere, each gas has its own partial pressure. The partial pressure of a gas in a mixture is the pressure that the single gas exerts, and is exerted as if it were the only gas in the container. Dalton’s Law of partial pressures Partial pressures at the Alveoli • • States that the total pressure of a gas, such as the atmospheric air, is the sum of the partial pressure of the individual gases. At sea level, atmospheric pressure is 760 mm Hg. • The partial pressure of each atmospheric gas at sea level (in mm Hg.) is determined by multiplying its percentage times the atmospheric pressure (in mm. Hg.). At sea level (760 mm Hg.), the partial pressure of nitrogen is 597 mm Hg. (78.6% X 760 mm Hg. =597 mm Hg.), oxygen is 159 mm Hg. (20.9% X 760 mm Hg.), and carbon dioxide is 0.3 mm Hg (0.04% X 760 mm Hg.). Comparing the partial pressures of air in the alveolus to the partial pressures of gases in the alveolar capillaries, – oxygen diffuses into the blood because the alveolar partial pressure of oxygen (PO2 is about 104 mm Hg.) is greater than the blood’s oxygen partial pressure (about 40 mm Hg.). – Carbon dioxide diffuses out of the blood capillaries because its blood partial pressure (PCO2 is about 45 mm Hg.) is greater than the carbon dioxide partial pressure (about 40 mm Hg.) of the alveoli. Partial pressures at the Tissues • Comparing the partial pressures of air in the tissue capillaries to the partial pressures of gases in the tissues, – oxygen diffuses into the tissues because the partial pressure of oxygen (about 104 mm Hg.) is greater than the tissue’s oxygen partial pressure (less than 40 mm Hg.). – Carbon dioxide diffuses out of the tissues because its partial pressure (more than 45 mm Hg.) is greater than the tissue’s blood capillaries carbon dioxide partial pressure (about 40 mm Hg.). Figure 25.19 Partial pressures at the alveoli. 11 Transport of Respiratory Gases Figure 25.20 Partial pressures at the tissue level. Transport of Respiratory Gases Oxygen Transport – Henry’s law takes into account the diffusion of gases into water. Henry’s law states that for a mixture of gases, more of each gas will diffuse into water as the partial pressure of the individual gas increases. – However, how much of the gas diffuses into the water is not just a function of the partial pressure of the gas, as the solubility of the gas in water is another important factor. Comparing the solubilities in water of the three major gases of atmospheric air, carbon dioxide is the most soluble, oxygen is much less soluble, and nitrogen is practically insoluble. • Oxygen transport by two methods, – (1) dissolved in plasma, and – (2) combined with hemoglobin. • Oxygen is mostly transported by the combination of oxygen with the heme units (binds with Fe++) of hemoglobin, forming oxyhemoglobin. About 98.5% of the body’s oxygen is carried as oxyhemoglobin, with the remaining, about 1.5%, being dissolved in plasma. Carbon Dioxide Transport • Carbon dioxide transport by three methods, – (1) dissolved in plasma, – (2) combined with hemoglobin, and – (3) bound in bicarbonate ions. Figure 25.21 The association and dissociation of hemoglobin with oxygen forming oxyhemoglobin (HbO2) and deoxyhemoglobin (HHb), respectively. • About 7% of the carbon dioxide is transported dissolved in the plasma. About 23% is transported combined with the protein portion of the hemoglobin molecule as carbaminohemoglobin. About 70% of the carbon dioxide is transported combined into bicarbonate ions. 12 Carbon dioxide and Oxygen Transport at the Tissues Figure 25.22 The reversible reaction showing the formation and dissociation of carbon dioxide from a bicarbonate ion. • Carbon dioxide diffuses into red blood cells where the enzyme carbonic anhydrase speeds the reaction between carbon dioxide and water and forms carbonic acid. • Carbonic acid dissociates into hydrogen ions and bicarbonate ions. The bicarbonate ions diffuse out of the red blood cells into the plasma and are transported to the lungs. The hydrogen ions bind to hemoglobin, which has released its oxygen for the oxidation of fuel molecules, and forms deoxyhemoglobin Carbon dioxide and Oxygen Transport at the Lungs • . At the lungs, bicarbonate ions diffuse into the RBCs, where they combine with hydrogen ions released from deoxyhemoglobin, and form carbonic acid. In the enzyme mediated reaction, carbonic acid is split into carbon dioxide and water. Carbon dioxide diffuses into the alveoli for removal from the lungs. • As deoxyhemoglobin releases its hydrogen ions, oxygen diffuses into the RBCs and binds to hemoglobin and forms oxyhemoglobin. Ninety eight percent of oxygen is transported to the tissues as oxyhemoglobin, and the remaining 1.5% is transported dissolved in the plasma. Figure 25.23 Illustration of the transport of carbon dioxide and oxygen at the tissue level. Mechanisms Controlling Respiration Figure 25.24 Illustration of the transport of carbon dioxide and oxygen at the lungs. 13 Mechanisms Controlling Respiration • Neural control of respiration is by respiratory centers in the medulla oblongata and pons of the brain stem. The medulla oblongata contains two respiratory centers, the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). Mechanisms Controlling Respiration • The respiratory center is mostly controlled by input from chemoreceptors, proprioceptors, and emotional (hypothalamic) and voluntary (cortical) centers of the brain. • Chemoreceptors are sensitive to changes in carbon dioxide and oxygen concentrations, and pH. – The peripheral chemoreceptors, includes the aortic bodies (in the aortic arch) and the carotid bodies (in the carotid sinuses). The peripheral chemoreceptors mostly monitor blood oxygen (PO2). – The central chemoreceptors are located in areas of the brain stem associated with the medulla and the pons. The central chemoreceptors are mostly sensitive to changes in pH and carbon dioxide concentration (PCO2). Mechanisms Controlling Respiration – Proprioceptors are receptors that monitor motion and are especially abundant in muscles, joints, tendons, and the inner ear. Increased stimulation of proprioceptors stimulates the respiratory center to increase rate of respiration. Brain centers such as those of the hypothalamus can trigger hyperventilation (anxiety attacks), and cortical (voluntary) control allows limited alterations in breathing rates. Central chemoreceptors (continued) • Hydrogen ions target the central chemoreceptors, which stimulate the respiratory center to increase the rate of respiration. The increased rate of respiration increases the pH (becomes more basic) as more carbon dioxide is removed from the blood. Carbon dioxide is derived from carbonic acid. Thus, removing more carbon dioxide, removes more carbonic acid (hydrogen ions), and increases pH back to normal levels. Central chemoreceptors • The central chemoreceptors are the dominate chemoreceptors and respond mostly to changes in pH of cerebrospinal fluid (CSF). – An increase of carbon dioxide results in a decreased pH of cerebrospinal fluid. An increase in carbon dioxide usually results from increased aerobic metabolism (oxidation of fuel molecules) or from hypercapnia, an increase of carbon dioxide caused by decreased and shallow breathing called hypoventilation. – A decreased pH (becomes more acidic) results because carbon dioxide combines with water and produces carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. Hypocapnia • a lower than normal concentration of carbon dioxide in the blood (or CSF) is usually caused by hyperventilation. Hyperventilation results in a increase of blood pH (becomes more basic) as increased carbon dioxide (thus, carbonic acid) is removed from the blood. • Hypocapnia and increased blood pH is reversed by hypoventilation, which results in conservation of carbon dioxide (thus, carbonic acid). • Medically, reversal of hypocapnia resulting from hyperventilation (increases pH) caused by anxiety, is often by conservation of carbon dioxide (decreases pH) by rebreathing into a bag . 14