Massi

• Respiration


  The entire process of gas exchange in the body
• Respiration


  1. Pulmonary ventilation (inspiration and expiration)

  2. External respiration (gas exchange between alveoli and pulmonary capillaries)

  3. Internal respiration (gas exchange between systemic capillaries and body tissues)
• Pulmonary ventilation is the movement of air in and out of the body
• External respiration refers to the gas exchange between the alveoli and the pulmonary capillaries
• Internal respiration is the gas exchange between the systemic capillaries and the body's tissues
• In health non-pregnant adults oxygen consumption is approximately 250ml/min at rest, increasing during pregnancy by approximately 20%
• The increase in oxygen consumption during pregnancy occurs mostly to maintain the additional metabolic requirements of pregnancy, uteroplacental circulation, and the additional work of maternal circulation
• There is a greater 40% increase in gas exchange during pregnancy
• The increase in gas exchange during pregnancy is achieved by increasing the tidal volume from 500 to 700 ml
• The driving force of the increased tidal volume during pregnancy is progesterone that stimulates the respiratory centre directly and increases sensitivity to carbon dioxide
• The expanding uterus decreases the residual volume and therefore the total lung capacity during pregnancy
• There is no change in respiratory rate during pregnancy, although many women complain of the sensation of shortness of breath
• Respiratory centre


  Clusters of neurons located bilaterally in the brain stem that control the 'autopilot' breathing our body performs when we are in a quiet state
• Factors influencing breathing rate

  • Cortical stimuli (e.g., voluntary changes in breathing)
  • Chemoreceptors (in the brain, the aorta and carotid arteries)
  • Limbic system input (e.g. anxiety)
  • Exercise
  • Temperature
  • Pain
  • Stretch receptors in the lungs
• All of these factors maintain blood gas homeostasis by way of a negative feedback loop
• Respiratory Control


  The capacity of the thorax changes when muscles contract as a result of nerve impulses transmitted from respiratory centres in the brain. These muscles then relax in the absence of nerve signals, and exhalation is, therefore, a passive activity.
• Respiratory centres


  • Located bilaterally in the brain stem
  • Divided into two principal areas: the medullary respiratory centre (medulla oblongata) and the pontine respiratory group (in the pons)
• Respiratory centre


  Controls the 'autopilot' breathing our body performs when we are in a quiet state
• Factors influencing breathing rate

  • Cortical stimuli (e.g., voluntary changes in breathing)
  • Chemoreceptors (in the brain, the aorta and carotid arteries)
  • Limbic system input (e.g. anxiety)
  • Exercise
  • Temperature
  • Pain
  • Stretch receptors in the lungs themselves
• All of these factors

  Maintain blood gas homeostasis by way of a negative feedback loop
• Click here to enlarge image
• Respiratory centre


  Clusters of neurons located bilaterally in the brain stem that send nerve impulses to the breathing muscles
• Components of the respiratory centre

  • Medullary respiratory centre
  • Pontine respiratory group
• Medullary respiratory centre


  Made up of the dorsal respiratory group (DRG) and the ventral respiratory group (VRG)
• Normal quiet breathing


  1. DRG generates impulses to diaphragm and external intercostal muscles

  2. Muscles contract, inhalation occurs

  3. DRG becomes inactive, diaphragm and intercostals relax, passive recoil of lungs and thoracic wall

  4. Cycle repeats
• Pre-Bötzinger complex


  Cluster of neurons in the VRG believed to be important in generating the rhythm of breathing
• The exact mechanism of the pacemaker cells in the pre-Bötzinger complex is unknown and is the topic of much ongoing research
• The pacemaker cells in the pre-Bötzinger complex are thought to provide input to the DRG, driving the rate at which DRG neurons fire action potentials
• Forceful inhalation


  1. DRG stimulates diaphragm and external intercostal muscles to contract

  2. DRG also activates VRG neurons involved in forceful inhalation to send impulses to accessory muscles of inhalation

  3. Accessory muscles contract, resulting in forceful inhalation
• Forceful exhalation


  1. DRG is inactive

  2. VRG neurons involved in forceful exhalation send nerve impulses to accessory muscles of exhalation

  3. Accessory muscles contract, resulting in forceful exhalation
• The remaining neurons of the VRG do not participate in normal quiet breathing
• The VRG becomes activated when forceful breathing is required, such as during exercise, when playing a wind instrument, or at high altitudes