Control of Ventilation Flashcards
What is the hierarchy of control of ventilation?
- Brainstem automatic
- Brainstem response to stimuli
- Higher brainstem
- Learned responses
- Voluntary
- Input from multiple sources
Describe neural respiratory control with respect to the medulla.
- Medulla generates rhythm - inspiration then expiration
- Ventilation ceases - section below medulla
- Retained - section above medulla
What is believed to generate the breathing rhythm?
- Network of neurons - Pre-Botzinger complex
- Located near upper end of medullary respiratory complex
What gives rise to inspiration neurally?
- Rhythm generated by Pre-Botzinger complex
- Excites dorsal respiratory group neurones (inspiratory)
- Contraction of inspiratory muscles
- Firing stops - passive expiration
Describe the Pre-Botzinger complex.
- Ventral group of neurones expressed bilaterally
- Brain slice studies - rhythmic discharge produces respiratory pattern
Describe muscle contraction during inspiration.
- Thoracic volume increased - diaphragm contraction and flattens
- External intercostal muscle contraction lifts ribs and moves out sternum - ‘bucket handle’ mechanism
How is active expiration during hyperventilation controlled neurally?
- Increased firing of dorsal neurones excites ventral respiratory group neurones
- Excites internal intercostals, abdominals causing forced expiration
- Expiratory muscles not activated in normal breathing
How do neurones in the pons generate rhythms in the medulla?
- Pneumotaxic centre stimulated when dorsal respiratory neurones fire
- Inspiration inhibited
- APNEUSIS occurs when PC not present - prolonged breathing
Describe the apneustic centre.
- Impulses from neurones excite inspiratory area of medulla
- Prolonged inspiration
Describe some regions that can send stimuli that influence the respiratory centres.
- Higher brain centres e.g cerebral cortex, hypothalamus
- Stretch receptors in walls of bronchi and bronchioles
- Juxtapulmonary receptors - stimulated by pulmonary oedema
- Baroreceptors - increased ventilatory rate in response to reduced BP
- Central and peripheral chemoreceptors
- Nose and pharynx
Describe feedback loop control in respiratory rhythm.
- Inflation of lung stops inspiration
- Deflation induces inspiration
- Dependent on vagal inputs
Describe pulmonary stretch receptors in reflex modification of breathing.
- Activated during inspiration
- Afferent discharge inhibits inspiration - Hering-Breuer reflex
Describe joint receptors.
- Impulses from moving limbs reflexly increases breathing
- Probably contribute to increased ventilation during exercise
- Work alongside tendon/muscle/intracellular pH receptors
How might ventilation increase during exercise?
- Reflexes originating from body movement
- Adrenaline release
- Impulses from cerebral cortex
- Increase in body temperature
- Accumulation of CO2 and H+ generated by active transport
Describe the cough reflex and how it is controlled.
- Activated by irritation of airways
- Afferent discharge causes short intake of breath, followed by closure of larynx, contraction of abdominals (raised intra-alveolar pressure).
- Larynx opens and expulsion of air at high speed.
What do peripheral chemoreceptors detect?
- Oxygen and CO2 tension
- [H+] in blood
Describe peripheral chemo-sensitive areas. PART 1
- Occurs in carotid and aortic bodies
- Receives high amount of blood flow
- Oxygen sensitive K+ channels, haem based mitochondrial cytochrome enzymes - responsive to local PO2 concentration
- Nerve output from carotid body goes to brainstem through vagus
Describe peripheral chemo-sensitive areas. PART 2
- Carotid body responds to arterial PO2 of <60mmHg. Above this, little response
- Ventilatory response occurs within seconds
- Ventilation increases for 5-10 minutes. Some PCO2 and pH responses also occur
Describe central chemoreceptors and the nature of the substance that they detect.
- Situated near surface of medulla of brainstem
- Responds to [H+] of CSF
- CSF separated from blood by blood-brain barrier
- Barrier relatively impermeable to H+ and HCO3-, CO2 diffuses readily
- CSF - less protein than blood so less buffered
Describe the effects of CO2
- If in CSF, increased [H+] and acidosis
- Effect mediated via pH effect on muscarinic receptors to ACh or on other enzymes
- Increased PCO2 increases rate, volume of breathing within 5 minutes
- Increases PCO2 5mmHg doubles ventilation
- Hypoxia - increased sensitivity to PCO2
Describe the variation in CO2 drive. PART 1
- Response to PCO2 reduced during sleep
- Patients with persistently raised PCO2 - insensitive to PCO2 changes, depend on hypoxic drice - common in COPD
- If no response and excess oxygen delivered so PO2 raised, PCO2 rises and ventilation falls
Describe the variation in CO2 drive. PART 2
- CO2 drive reduced with increasing age
- Some patients without lung disease may have reduced PCO2 response, depend on hypoxia
- ASTHMATICS WHO DEVELOP INCREASED PCO2 - abnormal CO2 response even when asthma improves
Describe the hypoxic drive of ventilation
- Occurs via peripheral chemoreceptors
- Stimulated when arterial PO2 falls to low levels
- Not important in normal respiration
- Important at high altitudes/patients with chronic CO2 retention i.e COPD
What is hypoxia at high altitudes caused by and what is the main response to this hypoxia?
- Caused by decreased partial pressure of inspired oxygen
- Response - hyperventilation and increased cardiac output
What is the effect of pH on ventilation?
- Raised PCO2 increases ventilation
- Increased acidosis due to renal failure, diabetic ketoacidosis, lactic acidosis and Kussmaul respiration
- Slow diffusion of H+ into CSF - pH slowly stabilises
- Alkalosis leads to rising PCO2 - small effect
What are the chronic adaptations to high altitude hypoxia?
- Polycythaemia - increased oxygen carrying capacity
- Increased 2,3-BPG produced within RBC - easier offloading of oxygen to tissues
- Increased number of capillaries and mitochondria
- Kidneys conserve acid so arterial pH falls
What is the influence of the following chemical factors on peripheral (PC) and central chemoreceptors (CC)?
- ARTERIAL PCO2
- ARTERIAL PO2
- ARTERIAL H+
- PCO2 - weak stimulation of PC, opposite for CC
- PO2 - PC stimulated when PO2 falls below 8 kPa, severe hypoxia depresses respiratory centre for CC
- H+ - PC stimulated, H+ doesn’t cross blood-brain barrier for CC
Describe hypothalamic control of breathing. PART 1
- Pain, temperature changes and emotions affect ventilation. Temperature increases ventilation
- Sudden cooling causes apnoea or gasping
Describe hypothalamic control of breathing. PART 2
- Increased activation of oestrogen dependent progesterone receptors increases ventilation during pregnancy/luteal phase of menstrual cycle
- More hypoventilation during sleep in some obese patients
How is control of ventilation different in transplanted hearts and lungs?
- No vagal input to brain and no response to lung stretch receptors
- No Hering Bruer reflex but normal breathing
- Normal response to irritants in trachea but not smaller airways (which can sometimes appear dilated)
- Normal yawning, sighing
Describe the evidences proving cortical control is involved in ventilation.
- Learned patterns allow people to drink, breathe and talk at same time
- Babies/patients with cerebral diseases - can choke upon swalling due to poor upper airway control
- Singers alter their own airflow accurately to hit right notes.
Describe Cheyne Stokes respiration.
- Alternating hypo- and hyperventilation with repetitive pattern of breathing
- Present in stage 1 sleep, neonates, altitude hypoxia, cerebrovascular and neurodegenartive diseases
- Occurs - increased sensitivity to PCO2, prolongation of circulation from lungs to brain
Describe hyperventilation
- In metabolic acidosis, ventilation increases. PCO2 falls, pH increases
- Tetany (muscle spasms) occur with rises in pH
- Low brain PCO2 causes cerebral vasoconstriction and unconsciousness
