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Lecture 18

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Michelle French

Lecture 18  An experiment  Hold your breath. Time how long it takes before you must breathe. Now take 10 deep breaths. Hold your breath again. Record the time before you must breathe.  Is there a difference? Why?  PCO2 is reduced and also [H+] is reduced.  Less “drive” to breath, block of chemoreceptors  Pulmonary ventilation matches O2 uptake/consumption and CO2 production/elimination  Pulmonary Ventilation  Is the rate at which gas enters or leaves the lung  Feedback Regulation of Breathing  To have the levels of CO2 and O2 at a relatively constant level, we have chemoreflexes which monitor the levels and alter ventilation, thereby allowing for gas exchange  As ventilation increases, arterial PCO2 drops: The Metabolic Hyperbola  Alveolar PCO2 = CO2 production / Alveolar Ventilation  As ventilation (L/min) is increased, Arterial PCO2 mmHg is dropped  As breathing is increased, more CO2 is “blown off” and PCO2 falls. If breathing is reduced, PCO2 rises  The and the position of the metabolic hyperbola is determined by the rate of production of CO2 by metabolism  How is CO2 and O2 monitored  Central Chemoreceptors  Located in medulla oblongata (brain stem) and scattered in other brain tissue  Located also in Pons  Detect [H+] by binding in cerebrospinal fluid  Not sensitive to PO2 levels  Peripheral Chemoreceptors  Located mainly in the Carotid artery and also in Aortic bodies  Detect [H+] in blood – sensitivity is increased when PO2 falls  The carotid bodies are most sensitive to changes in partial pressure of arterial oxygen and pH. The aortic bodies are most sensitive to the content of arterial oxygen  Detection is sent to the medulla via sensory neurons  How do chemoreceptors influence ventilation  The H+ constituents of CO2 levels are the determinants that are detected by the medullary oblongata and pons chemoreceptors directly and by the periphery chemoreceptors via the afferent sensory neurons  From the medullary oblongata and pons, information is sent through the somatic motor neurons to either stimulate inspiration or expiration via their associated muscles  Inspiration  External Intercostal  Diaphragm  Scalene and Sternocleidomastoid  Expiration  External Intercostal  Abdominal Muscle  Central Chemoreceptors  Central chemoreceptors detect H+ in the cerebrospinal fluid  CO2 present in the cerebral blood will diffuse in the cerebrospinal fluid. CO2 will bind water to form Carbonic Anhydrase that dissociated to protons (H+) and bicarbonate  H+ closes K+ channels causing depolarization of chemoreceptors  Takes about 5 min to respond “fully” to changes arterial PCO2, due to the diffusion of CO2 into the cerebrospinal fluid  The higher the [H+], the more increase in ventilation  The Central Chemoreflex  Ventilation increases linearly with PCO2  Central chemoreflex provides most of our drive to breath
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