Respiratory Physiology - Veterinary Physiology PDF

RESPIRATORY PHYSIOLOGY 5. Control of ventilation Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  Control of ventilation The nervous system normally adjusts the rate of...

RESPIRATORY PHYSIOLOGY 5. Control of ventilation Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  Control of ventilation The nervous system normally adjusts the rate of alveolar ventilation almost exactly to the demands of the body Po2 and Pco2 in the arterial blood are hardly altered Even during exercise and other types of respiratory stress VE = VT x f An increase in oxygen offer can only be acomplished through an increase in VT or f or both  2 Control of ventilation Breathing is controlled by centers in the brain stem medullaandPons There are 4 components of the entire controlling system CONTROL CENTERS FOR BREATHING IN THE BRAIN STEM CHEMORECEPTORS FOR O2 AND CO2 MECHANOREPCEPTORS IN THE LUNGS AND JOINTS RESPIRATORY MUSCLES WHOSE ACTIVITY IS DIRECTED BY THE BRAIN STEM CENTERS VOLUNTARY CONTROL CAN ALSO BE EXERTED BY COMMANDS the astute hinting FROM THE CEREBRAL CORTEX, WHICH CAN TEMPORARILY this brainstemcontrols OVERRIDE THE BRAIN STEM canoverridethis you though voluntary (Ex: breath holding or voluntary hyperventilation)  3 Brain stem control The respiratory center is composed by several groups of neurons located bilaterally in the medulla oblongata and pons It is divided into 3 major centers: DORSAL RESPIRATORY GROUP medulla VENTRAL RESPIRATORY GROUP THE PNEUMOTAXIC CENTER pons thePonsthatinhibittheactivityofneuronsinthedorsal within 4114ftp.teriniffsnftafefffrenafhffgns  4 more forcontrolofrespiration impo Dorsal respiratory group (DRG) The dorsal respiratory group of neurons plays a fundamental role in the control of respiration startofinspirationstartsw DRG Most of its neurons are located within the nucleus of the tractus solitarius (NTS) NTS is the sensory termination of both the vagal and the glossopharyngeal nerves, which transmit sensory signals into the respiratory center from: bringingphysiologicalinformation Peripheral receptors Baroreceptors Several types of receptors in the lungs  5 doneincontrolledway mainly Dorsal respiratory group (DRG) The basic rhythm of respiration is generated mainly in the dorsal respiratory group of neurons The nervous signals that are transmitted to the inspiratory muscles (mainly diaphragm) are not instantaneous bursts of action potentials The signal begins weak and increases steadily in a “ramp” manner, for about 2 secs in a normal inspiration It then ceases abruptly for the approximately next 3 secs That turns off the excitation of the diaphragm and allows elastic recoil of the lung to promote expiration passively There is a paradoxical increase in the inspiratory stimulation during the initial phase of expiration, to slow down the elastic The “ramp signal” causes a steady increase in the recoil volume of the lungs during inspiration, rather than inspiratory gasps Another cycle then begins and is repeated again and again, with expiration occurring in between the ramp signals  6 Dorsal respiratory group (DRG) Two characteristics of the inspiratory ramp are controlled The rate of increase of the ramp signal So that during heavy respiration, the ramp increases rapidly and therefore fills the lung more The limiting point at which the ramp suddenly ceases Which is the usual method for controlling the rate of respiration The earlier the ramp ceases, the shorter the duration of inspiration/expiration – increase in frequency incrdepthofrespiration thisisduetomorestimulation  7 Pneumotaxic center The pneumotaxic center is located in the pons, transmits signals to the inspiratory area The primary effect is to control the “switch-off” point of the inspiratory ramp Limit inspiration = control the duration of the filling phase of the lung cycle Consequently, increases the rate of breathing Limitation of inspiration also shortens expiration A STRONG PNEUMOTAXIC SIGNAL CAN INCREASE THE RATE OF BREATHING TO 30 TO 40 BREATHS PER MINUTE, WHEREAS A WEAK SIGNAL MAY REDUCE In att iii stain THE RATE TO ONLY 3 TO 5 BREATHS PER MINUTE  8 Ventral respiratory group The ventral respiratory group operates as an overdrive mechanism when high levels of pulmonary ventilation are required – i.e., heavy exercise The neurons are totally inactive during normal quiet respiration – do not participate in the basic rhythmical oscillation that controls respiration Action on inspiratory and expiratory muscles (turning both active) ntralrespiratorygroupgeneratesthebreathingrhythmandintegratesdatacomingtothemedulla VRGnotactiveduringeupeneal  9 Sensory information The brain stem controls breathing by processing sensory information and sending motor information to the respiratory muscles Of the sensory information arriving at the brain stem, the most important is that concerning: PaO2 PaCO2 pH THE SENSORY INFORMATION RELY ON PHERIPHERAL AND CENTRAL CHEMORECEPTORS  10 Central chemoreceptors The central chemoreceptors located in the brain stem are the most important for the minute-to-minute control of breathing These receptors communicate directly with the inspiratory center (dorsal respiratory group) They are very sensitive to increases in PaCO2 They indirectly detect increases in PaCO2 by detecting local decrease in pH (increase in H+ ions concentration) Don’t respond to changes in systemic pH  Central chemoreceptors How do central chemoreceptors detect changes in PCO2? CO2 in the blood combines reversibly with H2O to form H+ and HCO3-  hydration reaction BBB is relatively impermeable to H+ and HCO3-, these ions would be trapped in the blood (not affected by systemic pH) CO2, however, is quite permeable  cross BBB and enters the extracellular fluid of the brain and the CSF In the CSF, CO2 suffers hydration, being converted to H+ and HCO3- Thus, increases in arterial PaCO2 would increase CO2 in the CSF and decrease pH (increase in H+ concentration) This local decrease in pH is detected by the central chemoreceptors (in close proximity to CSF) Chemoreceptors then signals the inspiratory center to increase breathing rate and depth (hyperventilation)  12 Central chemoreceptors Central chemoreceptors also act on blood flow/pressure  13 Peripheral chemoreceptors The peripheral chemoreceptors are located in the carotid and aortic bodies Carotid and aortic bodies have high blood flow Information is relayed to the DRG via glossopharyngeal and vagus nerves DRG orchestrates the appropriate changes in breathing They are very sensitive to decreases in PO2 Increase in partial pressure of PCO2 Decrease in pH (increase of H+ concentration) onlyabletosense dissolved oxygenmoleculesnot theoxygenthatisboundtohemoglobin whendissolvedlevelsof0 drop hemoglobinreleasesoxygen  14 Peripheral chemoreceptors How peripheral chemoreceptors respond to changes in PaO2? When oxygen concentration in the arterial blood falls below normal, chemoreceptors become strongly stimulated The exact mechanism by which low PaO2 excites the nerve endings in the carotid and aortic bodies are still not completely understood This bodies have multiple highly characteristic glandular-like cells called glomus cells This cells synapse directly or indirectly with the nerve endings, functioning as a chemoreceptor  is Peripheral chemoreceptors When pO2 decreases below 60 mmHg, potassium channels close causing cell depolarization Voltage gated calcium channels open and the increase in cytosolic calcium stimulates transmitter release (ATP and acetylcholine) Neurotransmitter will activate afferent fibers that send signals to the central The exactly mechanism by which low pO2 nervous system and stimulates respiration influences K channels to close is still unclear  16 Peripheral chemoreceptors With the decrease in PaO2, the firing rate of the sensory neurons increase They are particularly sensitive to a PO2 range between 60 and 30 mmHg (when hemoglobin saturation decreases rapidly) Information is relayed to the DRG and the breathing rate and depth increases  n Peripheral chemoreceptors  18 Other receptors In addition to chemoreceptors, several other types of receptors are involved in the control of breathing: LUNG STRETCH RECEPTORS protectivereflex stretchtoo rate muchdearresp bicwanttopreventlesionsinalveoli Mechanoreceptors are present in the smooth muscle of the airways When stimulated by distension of the lungs and airways, they initiate a reflex decrease in breathing rate called the HERING-BREUER REFLEX This reflex decreases breathing rate by prolonging expiratory time Ensures that the lungs never over-inflates  19 Other receptors In addition to chemoreceptors, several other types of receptors are involved in the control of breathing: JOINT AND MUSCLE RECEPTORS incrcotosendbloodthere recall alsoneedincrventilationbcneedoxygensupply Mechanoreceptors located in the joints and muscles detect the movement of limbs and instruct the inspiratory center to increase breathing rate Information from the joints and muscles is important in the early (anticipatory) ventilatory response to exercise  N Other receptors In addition to chemoreceptors, several other types of receptors are involved in the control of breathing: IRRITANT RECEPTORS Irritant receptors for noxious chemicals and particles are located between epithelial cells lining the airways Responsive to chemical and mechanical stimuli Information from these receptors travels to the medulla and causes a reflex constriction of bronchial smooth muscle and an increase in breathing rate smoke ate Hiv tha Itat to's  u Response to exercise The response of the respiratory system to exercise is remarkable As the body’s demand for O2 increases, more O2 is supplied by increasing the ventilation rate An excellent matching occurs between O2 consumption, CO2 production and ventilation During moderate exercise there is no change in arterial PO2, PCO2 or pH Venous PCO2 usually increases  22 Breathing during exercise activehyperemia ghaltitude rateincrduetochemoreceptorsdetectingchange resp Barometric pressure (sea level) = 760 mmHg Response to high altitude Fraction of oxygen in air (FO2) = 0.21 PO2 = barometric pressure x fraction of O2 PO2 = 760 mmHg x 0.21 = 160 mmHg High altitude is one of the several causes of hypoxemia The respiratory responses to high altitude are the adaptive adjustments an individual must make to the decreased PO2 in At high altitude: same 21% of oxygen inspired and alveolar air Barometric pressure (at 8000m) = 267 mmHg The most significant short-term response is hyperventilation PO2 = 267 mmHg x 0.21 = 56 mmHg Levels of PO2 below 60 mm Hg stimulate peripheral chemoreceptors Medullary inspiratory centers are activated and increase the breathing rate Hyperventilation leads to RESPIRATORY ALKALOSIS (increase in pH) The increase in pH will inhibit central and peripheral chemoreceptors and offset the increase in ventilation rate These offsetting effects occur initially, but within hours to several days, bicarbonate excretion increases, and pH decreases toward normal  24 lessair havemoreRBC meansableto Response to high altitude mg If pin Long-term adjustments to hypoxia involves: Production of more erythrocytes under the influence of erythropoietin thiscomesfromkidney Decreased affinity of hemoglobin for oxygen because of increased concentration Of 2,3-diphosphoglycerate (2,3-DPG) Increased capillary density in muscle (angiogenesis) These adjustments are sufficient to restore maximal oxygen consumption to normal at moderate altitudes  25 Response to high altitude impo mayaskQonthis Measurement of HYPOXIC VASOCONSTRICTION can induce heart failure pulmonary artery pressure has allowed selection of breeding Hypoxic vasoconstriction is beneficial when there is localized alveolar hypoxia stock that are less susceptible When hypoxia is generalized, the vasoconstriction can have serious consequences Cattle grazing at high altitude (low O2) Edematous fluid accumulates in the Generalized pulmonary briskets hypoxic vasoconstriction Right-sided Increase in pulmonary Increase the workload of heart failure arterial pressure the right ventricle “brisket disease”  Dorsalrespiratoryinvolvedinmaintainingaconstant whenactivityinDraceases itnolonger stimulates to diaphragmandintercostals contract them allowing torela in resulting expiration vrainvolvedinforcedbreathingasthe neuronsintheurn theaccessorymusclesinvolvedinforcedbreathingto stimulate contract in resulting forcedi nspiration therespiratoryrateandthedepthofinspirationare regulated themedullaoblongataandPonshower by theregionsofthebraindosoinresponsetosystemic STIMULI mainlyinfluenced Co2by thegreaterthestimulusthegreaterthe Basically response increasingstimuli in results forced breathing of concentrations chemicals aresensedbychem cliffathemoreceptor andbrai locatedinbrain peripheralchemoreceptor I t.fi eaYehsarchan III leadto it increasedlevels ofhydrogenionsdecrea PH remember lowerpHlevel acidic sotheincr inHtionsinbraintriggersthecen chemoreceptorstostimulate respcenterstoinitia contraction ofdiaphragm intercostalmuscles result rate depthrespincreaseallowin moreco tobeexpelled which moreal brings ductionin6100 undslikemultiplefactors affect and rate ofrespiration depth 8 If476 Yite levelsofO2 Blood andotherbrainregions Hypothalamus