Control of breathing

• Origin of Breathing: 

  • The respiratory system has no intrinsic driving system like the heart  

  • It is therefore totally dependent on an external neural drive  

  • Origin of breathing occurs in respiratory control centres in brainstem  

  • In health breathing occurs automatically without conscious effort 

   
• Purpose of Breathing: 

  • Purpose of breathing is to provide adequate alveolar ventilation (amount of air used for gaseous exhange) 

  – O2 delivery  

  – CO2 excretion 

    

  • Alveolar ventilation alters in response to changing environmental (altitude) or metabolic demands (Exercise)  

   

  • Adequate alveolar ventilation is essential to maintain a neutral acid-base balance 

   
• Respiratory Control Centres (RCC): 

  • Respiratory control centres are situated in brainstem (medulla and pons)  

  • Medullary respiratory centre – Provides output to respiratory muscles  

  • Pontine respiratory centres – Influence output from medullary respiratory centre  

  • Interaction of these centres establish and modify respiration 

   
• Higher Brain Centres: 

  • Other brain centres can influence RCC  

  • Cerebral cortex  

  – Cortical input can over-ride intrinsic breathing pattern from RCC  

  – Voluntary pathways synapse with AHC in spinal cord  

  – Directly stimulate respiratory muscles  

  – Necessary for speech eating coughing 

   

   • Hypothalamus / Limbic systems – Pain and emotional state can modulate RCC 

   
• Clinical Failure of RCC: 

  • Hypoventilation or absent ventilation  

  – Brainstem CVA or tumour  

  – Raised intracranial pressure  

  – Decreased level of consciousness  

   

  • Alcohol  

  • Drugs – Pharmacology,  suppress drive to breath 

  • Central sleep apnea  

  • Hyperventilation – Breathing dysregulation syndrome 

   
• Clinical Failure Effectors: 

  • Neuromuscular disease  

  – Nerves (phrenic nerve)  

  – Neuromuscular junction (myasthenia gravis)  

  – Muscles (muscular dystrophy)  

   

  • Chest wall dysfunction  

  – Kyphoscoliosis  

  – Obesity 

   
• Sensors: 

  • Chemoreceptors  

  – Central  

  – Peripheral  

   

  • Lung  

  – Stretch receptors  

  – Irritant receptors  

  – J-receptors - responsive to fluid accumulation around the alveoli  

   

  • Other – Proprioceptors  

  • All sensors feedback to RCC to ↑or ↓VT or RF  

   
• CO2 is an acidic gas which must be kept within normal range to allow cellular function CO2 levels are constantly sampled and monitored by chemoreceptors 

   
• If co2 is too high – respiratory acidosis  

  If co2 is too low – respiratory alkalosis  

   
• Advantages of CO2 based ventilatory system:  

  • CO2 production is related to O2 consumption 

  • CO2 production is related to pH  

  • CO2 has linear relationship to ventilation