Control of Ventilation: Voluntary Control and Chemoreceptors

Voluntary Control of Ventilation

• The respiratory centers in the medulla communicate with zones in the pons and higher brain areas.
• These higher brain areas can override the respiratory centers by inhibiting or stimulating inspiration or expiration, providing fine-tuning of breathing.
• Medulla coordinates processes to maintain homeostasis by influencing the autonomic nervous system or creating hormones.
• The motor cortex in the cerebrum allows voluntary control over breathing, overriding medullary control.

Breath Holding and Carbon Dioxide

• The urge to breathe is primarily driven by an increase in the arterial partial pressure of carbon dioxide (P{CO2}), not a lack of oxygen.
• The breaking point, where the urge to breathe becomes overwhelming, typically occurs when P{CO2} reaches approximately 50 mmHg.

Hyperventilation and Breath Holding

• Hyperventilation lowers the partial pressure of carbon dioxide (P{CO2}) before breath-holding, extending the time until the urge to breathe arises.
• Normal P{CO2} is about 40 mmHg, but hyperventilation can decrease P{CO2} to approximately 15 mmHg.
• This creates a larger diffusion gradient for CO_2 to move from the blood into the alveoli. Swimmers and divers use this technique to extend breath-holding capacity.

Risks of Hyperventilation and Hypoxic Blackout

• Hyperventilation can lead to a dangerous situation where oxygen levels drop critically low before P{CO2} reaches the threshold to stimulate breathing.
• This can cause a hypoxic blackout, where an individual loses consciousness due to oxygen deprivation before feeling the urge to breathe.
• Low partial pressure of O2 and high partial pressure of CO2 can lead to a hypoxic blackout.

Respiratory Centers and Receptors

• During rest, the medulla houses the inspiratory and expiratory centers (more technically, ventral and dorsal respiratory centers).
• During exercise, the body monitors internal environment changes via central chemoreceptors, peripheral chemoreceptors, and stretch receptors in muscles and lungs.
• Information from receptors is sent to the medulla's respiratory centers, which then stimulate the muscles of inspiration (external intercostals and diaphragm), increasing thoracic volume and drawing air into the lungs.
• Stretch receptors in the lungs signal to stop inspiration, initiating expiration.
• Expiration involves the internal intercostals and abdominal muscles, decreasing thoracic volume and forcing air out of the lungs.

Chemical Monitoring

• The body constantly monitors partial pressure of oxygen (P{O2}), partial pressure of carbon dioxide (P{CO2}), and pH (acidity from CO_2 buffering).
• These factors also influence the cardiovascular system, affecting cardiac output and blood vessel dilation/constriction.

Chemoreceptors and Muscle Feedback

• Central chemoreceptors are located within the brain; peripheral chemoreceptors are in the aortic arches and carotid arteries.
• Muscle and joint receptors monitor movement, and temperature is also monitored, contributing to overall control.

Integration and Response to Stress

• The respiratory system requires coordinated activation of inspiratory and expiratory centers to seamlessly transition and increase ventilation as needed.
• Increased ventilation can be triggered by exercise, pain, or stress.