Control Of Respiration Flashcards
What is the medulla
A major rhythm generator
Neural control of respiration
Breathing rhythm is generated by a network of neurones called the Pre-Botzinger complex
Display pace maker activity
Located near upper end of medullary respiratory centre
What gives rise to inspiration
Rhythm generated by Pre-Botzinger complex
Excites Dorsal respiratory group neurones (inspiratory)
Fire in bursts
Firing leads to contraction of inspiratory muscles - inspiration
When firing stops - passive expiration
What happens during active expiration
Increased firing of dorsal neurones excites a second group: ventral respiratory group neurones
These excite internal intercostals, abdominals etc and cause forceful expiration
How do neurones in the pons affect the rhythm
Rhythm generated in medulla can be modified by neurones in the pons Pneumotaxic centre (PC) is stimulated when dorsal respiratory neurones fire and it terminates inspiration
Without PC, breathing is prolonged - APNEUSIS
How do pulmonary stretch receptors modify breathing
Activated during inspiration, afferent discharge inhibits inspiration - Hering-Breuer reflex
Only activated at large tidal volumes
May prevent over inflation in lungs during hard exercise
How do joint receptors modify breathing
Impulses from moving limbs reflexly increase breathing
Contribute to increased ventilation during exercise
Factors which may increase ventilation during exercise
Reflexes originating from body movement Adrenaline release Impulses from the cerebral cortex Increase in body temperature Later, accumulation of CO2 and Hc generated by active muscles
Explain the cough reflex as a reflex modification of breathing
Vital part of body defence
Helps clear airways of dust, dirt or excessive secretions
Activated by irritation of airways or tight airways (e.g. asthma)
Centre in the medulla, afferent discharge stimulates: Short intake of breath followed by closure of the larynx and contraction of Abdominal muscles and finally opening of the larynx and expulsion of air at a high speed
Explain the chemical control of respiration
Example of a negative feedback control system
Controlled variables are the blood gas tensions, especially CO2
Chemoreceptors sense values of the gas tensions
Explain chemoreceptors
Peripheral chemoreceptors sense tension of oxygen and carbon dioxide in the blood
Central chemoreceptors are situated near surface of medulla and répond to [H+] of cerebrospinal fluid CSF
CsF is separated from the blood by the blood-brain barrier and contains less protein than blood and hence is less buffered than blood
Explain the hypoxaemia drive of respiration
Effect is all via the peripheral chemoreceptors
Stimulated only when arterial PO2 falls to low levels (<8.0kPa)
Not important in normal respiration but is in patients with COPD
Important at high altitudes
Explain the controlled oxygen therapy for COPD patients with hypoxaemia and chronically elevated CO2 (hypercapnia)
Giving excessive oxygen can lead to worsening of hypercapnia and acidosis
Aim is to give oxygen to raise PAO2 without worsening hypercapnia
Target is the maintain a lower SaO2 of 88-92%
Explain hypoxia at high altitudes
Caused by decreased partial pressure of inspired oxygen (PiO2)
Acute response: hyperventilation and increased cardiac output
Symptoms: headache, fatigue, nausea, tachycardia, dizziness, SOB etc
What are chronic adaptations to high altitudes hypoxia
Increased RBC production (polycythaemia)
Increased number of capillaries
Increased number of mitochondria
Kidneys conserve acid
