BIOM2012 Respiratory Physiology L5 PDF

BIOM2012 - Systems Physiology: Respiratory System L5 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonar...

BIOM2012 - Systems Physiology: Respiratory System L5 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonary Ventilation and Respiratory mechanics Gas exchange Gas transport Blood pH regulation Control of Respiration Learning Objectives Understand the neuronal control of respiration Factors that regulate ventilation and how Arterial pH Arterial partial pressure of Oxygen (PaO2) PaCO2 Imagine if respiration is regulated like… heart beat? skeletal muscle?  Control of Respiration Breathing is normally an involuntary, rhythmic phenomenon that can be overridden by voluntary control. There are separate anatomical pathways for automatic and voluntary breathing, but the pathways are highly integrated. There is evidence to a support the idea of a central pattern generator (CPG) located in the brainstem that drives respiration. The CPG is composed of various respiratory related neurons.... inspiratory versus expiratory neurons interneurons premotor neurons The CPG drives motor neurons (phrenic & intercostal motor neuron pools) Two main ideas around origin of respiratory rhythm a synaptic network intrinsic membrane properties (pacemakers) exist  Control of Respiration There are three important brainstem respiratory centres that integrate signals from the periphery and send efferent signals to control breathing. The pontine respiratory group (PRG) in the dorsal lateral pons The dorsal (DRG) and ventral respiratory groups (VRG) in the medulla oblongata. Respiratory centers of the brain.' BCcampus, Pressbooks, Creative Commons Licence (CC BY 4.0) Dorsal Respiratory Group (DRG) of the medulla oblongata These neurons are mainly involved in inspiration (ie. discharge in inspiration), although they have some expiratory roles. They are important in involuntary, rhythmic, quiet breathing. Constant stimulation by DRG to the diaphragm and external intercostal muscles results in inspiration. Interruption of constant stimulation (via inhibitory impulses), causes inspiration to cease, the muscles relax and the lungs recoil, and exhalation occurs. Remember that expiration occurs passively owing to the elastic recoil of the lungs. Ventral Respiratory Group (VRG) of the medulla oblongata Both inspiratory and expiratory neurons in the VRG are responsible for the generation of respiratory rhythm. Also evidence supports the notion that this group of neurons is involved in voluntary breathing by stimulating the internal intercostal and accessory respiratory muscles. eg. forced expiration during exercise. (see later voluntary control) Pontine Respiratory Group (PRG) Above the medulla lies the pons and the other respiratory control centre, the PRG. PRG neurons are not essential for respiratory rhythm generation, but rather the modification and fine control of respiratory rhythm. The PRG primarily connects with the medullary DRG respiratory neurons and modulates respiratory frequency. Interesting the pons receives neural input from the hypothalamus which modulates changes in respiration related to emotions, arousal and pain. It’s all complex….. Long, slow, deep breaths are controlled by the apneustic centre in the pons, inhibiting stretch receptor input from the pulmonary muscles, allowing for bigger breaths (increase tidal volume). Precise, fine control of breath is governed by signals from the pneumotaxic centre in the pons, inhibiting the inhibitory signals from the apneustic centre to decrease breath size (decrease tidal volume).  Control of Respiration So (from the above slides) neurons in the medulla oblongata and the pons control unconscious, involuntary breathing. but also voluntary control of breathing is mediated by the motor cortex, over-riding involuntary control. The rate and rhythm of breathing can be influenced by the forebrain, as observed during voluntary hyperventilation or breath-holding. Altogether the two types of control (involuntary and voluntary) over breathing are highly integrated, as observed during speaking and singing, when automatic breathing is suppressed. Marieb Fig 22.27  Will be covered in CVS lectures Chemoreceptor Reflex Mainly regulates ventilation, but also has CVS effects Central chemoreceptors Central chemoreceptors Within medulla oblongata Detect changes in cerebral spinal fluid Carotid Respond to high pCO2 - low pH chemoreceptors Peripheral chemoreceptors Arterial, not Venous Carotid and aortic bodies Aortic chemoreceptors Respond to low pO2 in blood CVS response: increase CO and peripheral resistance Please don’t mix up between baroreceptors and chemoreceptors. Arterial partial pressure of Oxygen (PaO2) When oxygen is low O2 deprivation decreases activity levels in all nervous tissues EXCEPT peripheral chemoreceptors  Arterial partial pressure of CO2 Most important factor in regulation ventilation PCO2 and H+ are closely linked Peripheral chemoreceptors Blood as buffer Rely on Central chemoreceptors! Arterial partial pressure of CO2 PCO2 diffuses across BBB to ECF No blood = no buffer Significant changes in ECF pH What about pH? H+ cannot cross blood-brain barrier Rely on peripheral chemoreceptors Body tries to increase pH Increase frequency and depth of respiration  Sensory inputs: Chemoreceptor Reflex Peripheral chemoreceptors Located in the carotid and aortic bodies and are innervated by the glossopharyngeal and vagus nerve, projects to the NTS (medulla). Are primarily activated by hypoxia, but also (less so) by increased arterial pCO2, decreased pH and hypoperfusion. Remember the peripheral chemoreceptors are responsible for sensing large changes in blood oxygen levels. If blood oxygen levels become quite low, pO2 < 60 mm Hg then peripheral chemoreceptors stimulate an increase in respiratory activity. Recall O2-Hb dissociation curve.  Sensory inputs: Chemoreceptor Reflex Central chemoreceptors Located in the locus coeruleus (pons) and NTS, midline (raphe) of the ventral medulla, and ventrolateral quadrant of medulla ie. close to the respiratory control centres. Respond primarily to high pCO2 mediated through the detection of a fall in the pH of the cerebrospinal fluid (CSF) Recall Note these receptors are crucial for adequate breathing in sleep! Marieb Fig 22.27  Stretch Receptor Input Hering-Breuer reflex Ever notice that when you inhale (unconsciously), it gets to a certain point before you naturally exhale? This involuntary control of breathing is governed by the Hering-Breuer reflex. Beside chemoreceptors, there are specialised mechanoreceptors pulmonary stretch receptors in the thorax that exert a profound impact on breathing. Localised to the smooth muscles of bronchi and bronchioles in the lung and the visceral pleura. These receptors provide afferent sensory neural input to the respiratory centre via the Vagus nerve. In essence the sensory input prevents over-inflation. Inspiratory neurons in the respiratory CPG are inhibited, stopping inhalation, allowing for muscle relaxation and exhalation.  Other Sensory Input Propioreceptors The intercostal muscles and diaphragm contain muscle spindles that respond to stretch. Muscle contraction stimulates a positive feedback loop via the spinal cord that increases motor drive to the inspiratory muscles. This response ensures that an increase in the resistance to inhalation is met with a compensatory increase in muscle recruitment. Receptors in the muscles and joints of the locomotor system also provide positive feedback signals to the medullary controller, stimulating hyperpnoea. Irritant receptors within the lungs Within the epithelial cells of the airways are specialised nerve endings that respond to irritants such as dry and/or cold air, smoke, dust, pollen, chemical fumes and excess mucus. Stimulation of these receptors triggers protective reflexes. Bronchoconstriction occurs, breath holding occurs before coughing strongly prevents irritants from going deeper into the airways before expelling them Function Location Influencing factors Ventilation Airways Airway resistance Lung compliance Alveolar surface tension Gas exchange Respiratory membrane Gas characteristics Pressure gradient Diffusion Coefficient Membrane characteristics V/Q matching Gas Transport Blood PO2, PCO2 Haemoglobin level and saturation Respiratory rate chemoreceptors PaO2 PaCO2 Arterial pH