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Regulation of Ventilation, Ventilation/ Perfusion Ratio, and Transport of Gases

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Regulation of Ventilation, Ventilation/ Perfusion Ratio, and Transport of Gases MM07_LIMM3516_01_SE_C07.indd07_LIMM3516_01_SE_C07.indd PPageage 3377 11/28/11/28/11 44:38:38 PPMM user-s146user-s146 //Users/user-s146/Desktop/Merry_X-Mas/NewUsers/user-s146/Desktop/Merry_X-Mas/New TOPIC Standard Pathophysiology Competency Applies fundamental knowledge of the pathophysiology of respiration and perfusion to patient assessment and management. 7 Regulation of Ventilation, Ventilation/ Perfusion Ratio, and Transport of Gases INTRODUCTION he pathophysiology included in this topic is among the most formative and challenging changes to the new T education standards. Previously, EMTs had to know only about the air moving in and out of the body—and, quite frankly, in very simple terms and concepts. It was the belief, in b The body’s regulation of normal ventilation through creating the standards that include this information, that EMTs blood chemistry values and the receptors that control them: would understand the internal processes of perfusion and gas – Central chemoreceptors. transport that would make assessment and care performed – Peripheral chemoreceptors. more intuitive and effective. – Hypoxic and hypercarbic drive. b How normal ventilation is affected by lung REGULATION OF VENTILATION receptors and respiratory control centers in the Although breathing can be altered voluntarily, it is primarily brainstem. controlled involuntarily by the autonomic nervous system. A b The ventilation/perfusion ratio and how it pertains to large part of the regulation is related to maintaining normal gas oxygenation of the bloodstream: exchange and normal blood gas levels. Receptors within the – Illustration of how changes to the ventilation/ perfusion ratio affect other bodily processes. body constantly measure the amount of oxygen (O2 ), carbon dioxide (CO2 ), and hydrogen ions (pH) and signal the brain to b The role blood, red blood cells, and hemoglobin play in adjust the rate and depth of respiration ( Ī Figure 7-1 ). Centers cellular oxygenation and the removal of carbon dioxide responsible for ventilatory control are the chemoreceptors, lung from the cells. receptors, and specialized centers in the brainstem. b Alveolar and cellular gas exchange in the blood. Chemoreceptors Chemoreceptors are specialized receptors that monitor the number of hydrogen ions (pH) and the carbon dioxide and oxy- there is an association between CO2 and the level of acid in the gen levels in the arterial blood. There are two different types of body as follows: chemoreceptors: central and peripheral. An increase in the amount of CO2 in the blood will increase the amount of acid in the blood. CENTRAL CHEMORECEPTORS The central chemoreceptors A decrease in the amount of CO in the blood will decrease are located near the respiratory center in the medulla. These 2 the amount of acid in the blood. receptors are most sensitive to changes in the amount of car- bon dioxide in arterial blood and the pH of cerebrospinal fluid The central chemoreceptors are highly sensitive to the amount (CSF). The pH of CSF is directly related to the amount of carbon of hydrogen in the CSF. After the CO2 and H2 O molecules com- + dioxide in the arterial blood. Carbon dioxide readily crosses the bine to form H2 CO3 , the hydrogen ions (H ) disassociate from blood–brain barrier and moves into the CSF. In the CSF, the CO2 the H2 CO3 , enter the CSF, and stimulate the central chemore- + combines with water (H2 O) to form carbonic acid (H2 CO3 ). Thus ceptors. Small changes in the H level in the CSF will stimulate MM07_LIMM3516_01_SE_C07.indd07_LIMM3516_01_SE_C07.indd PPageage 3388 11/28/11/28/11 44:38:38 PPMM user-s146user-s146 //Users/user-s146/Desktop/Merry_X-Mas/NewUsers/user-s146/Desktop/Merry_X-Mas/New Voluntary and higher centers Pneumotaxic center Central A chemoreceptor Apneustic center Ventral respiratory Vagus nerve group B A) Carotid body C D Dorsal respiratory B) Aortic bodies group E C) Stretch receptors D) Irritant receptors E) J-receptors Intercostal nerve Phrenic nerve (to diaphragm) Figure 7-1 Respiration is controlled by the autonomic nervous system. Receptors within the body measure oxygen, carbon dioxide, and hydrogen ions and send signals to the brain to adjust the rate and depth of respiration. a change in respirations. Because CO2 is PERIPHERAL CHEMORECEPTORS The increase in the rate and depth of needed to produce H2 CO3 , the changes peripheral chemoreceptors are located in ventilation aimed at increasing the in the breathing rate and depth are the aortic arch and carotid artery bodies arterial oxygen content. geared toward increasing or decreasing in the neck. These chemoreceptors are Stimulation of both the central and the CO level in the arterial blood. The also sensitive to CO and pH; however, 2 2 peripheral chemoreceptors have a response of ventilation can be summa- the arterial oxygen level is the strongest greater influence on changing the rate rized as follows: stimulus. Thus, a change in the arterial oxy- and depth of ventilation than either alone. gen level is what stimulates the brain to An increase in arterial CO will 2 increase or decrease ventilation. It takes a increase the number of hydrogen ions Hypoxic Drive significant decrease in the arterial oxygen in the CSF, stimulating an increase in content to trigger the peripheral chemore- A person’s ventilation is normally control- the rate and depth of respiration to ceptors to stimulate the respiratory center led by the strong stimulus provided by blow off more CO . 2 to increase rate and depth of respiration. the amount of CO2 in the arterial blood. A decrease in arterial CO2 will The activity of the peripheral chemorecep- This is referred to as a hypercapnic drive decrease the number of hydrogen tors can be summarized as follows: or hypercarbic drive . However, some ions in the CSF, causing a decrease patients with chronic obstructive pulmo- in the rate and depth of respiration to A significant decrease in the arte- nary disease (COPD), such as emphysema blow off less CO2 . rial oxygen content will result in an or chronic bronchitis, have a tendency 38 www.bradybooks.com MM07_LIMM3516_01_SE_C07.indd07_LIMM3516_01_SE_C07.indd PPageage 3399 11/28/11/28/11 44:38:38 PPMM user-s146user-s146 //Users/user-s146/Desktop/Merry_X-Mas/NewUsers/user-s146/Desktop/Merry_X-Mas/New to retain carbon dioxide in their arterial 2 seconds, followed by 3 seconds with no bases of the lungs, air In a typical blood from poor gas exchange. Because stimulation, resulting in respiratory muscle travels upward to the the CO2 level is chronically elevated, the relaxation. apexes (tops) of the respiratory central chemoreceptors become desen- The ventral respiratory group (VRG) lungs and increases cycle, the DRG sitized to fluctuations that typically would has both inspiratory and expiratory neu- the residual volume. stimulate a change in the rate or depth rons. However, the VRG is basically inac- Interestingly, the alve- stimulates the of ventilation. Because of the desensitiza- tive during normal quiet breathing. The oli in the apexes of the respiratory tion of the central chemoreceptors, the VRG becomes active when accessory lungs have a greater muscles to peripheral chemoreceptors become the muscles are needed to assist in inspira- residual volume of primary stimulus to control ventilation. tion or expiration. The VRGI , in which air, are larger, and contract for Thus, hypoxia, rather than CO2 , becomes the I subscript indicates inspiratory VRG have a higher surface 2 seconds, the stimulus for the person to breathe; neurons, stimulates the pectoralis minor, tension, but they are this is referred to as a hypoxic drive . scalene, and sternocleidomastoid mus- fewer in number com- followed by cles to force inspiration. The VRGE , in pared with other areas 3 seconds Lung Receptors which the E subscript indicates expira- of the lungs. These with no stim- Three different types of receptors are tory VRG neurons, stimulates the internal larger alveoli in the found within the lungs: irritant receptors, intercostal and abdominal muscles to apexes have a higher ulation. stretch receptors, and J-receptors. The force exhalation. surface tension, which irritant receptors are found in the airways The apneustic center does not con- makes them less compliant and harder to and are sensitive to irritating gases, aero- trol the rhythm of respiration; however, inflate during ventilation. Thus, the tidal sols, and particles. Irritant receptors will it provides stimulation to the DRG and volume is shifted to the lower lobes, where cause a cough, bronchoconstriction, and VRGI to intensify the inhalation effort. The the lung is more compliant and there is an increase in the rate of ventilation. apneustic center may prolong inspiration, less surface tension. The stretch receptors are located within increasing the ventilatory volume. Because gravity pulls the blood the smooth muscle of the airways. These The pneumotaxic center sends inhibi- downward, less pressure is required to are responsible for measuring the size and tory impulses to the apneustic center to perfuse the lower lobes of the lungs, as volume of the lungs. To prevent overin- cease inhalation before the lungs become compared with the apexes, which are flation when stimulated by high tidal vol- overinflated. It can promote passive exha- above the level of the heart. As a result, umes, these receptors decrease the rate lation both by shutting off the DRG and the bases of the lungs receive a greater and volume of ventilation when stretched. VRGI and by activating the VRGE . amount of blood and are much better J-receptors are located in the capillar- perfused than the apexes. This is a desir- ies surrounding the alveoli and are sen- able condition, as the greatest amount VENTILATION/ sitive to increases in pressure within the of ventilation also exists in the base of capillary. When activated, these receptors PERFUSION RATIO the lungs. stimulate rapid, shallow respiration. The ventilation/perfusion (V/Q) ratio The V/Q ratio is never at an ideal state describes the dynamic relationship in any zones of the lungs.
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