Flashcards in Pulnomary responses to exercise 2 Deck (22):
What is VCO2 determined by?
Substrate oxidation could fatty acids with lower RER (VCO2/VO2) or could be carbohydrate with higher RER
What is alveolar ventilation determined by?
The ratio of expired CO2 to the arterial carbon dioxide partial pressure VA= VCO2/PaCO2
What is total ventilation determined by?
- By the tidal volume, breathing frequency and physiological dead space
- VE = (VT x Fr) - (VD x Fr)
What happens above the lactate threshold?
- Lactic acid is produced and reacts with sodium bicarbonate
- Forms carbonic acid which dissociates into CO2 and H20 and is expelled as CO2
- Bicarbonate prevents fall in PH acting as a buffer
- What happens to PaCO2 above the lactate threshold and why?
- Decreases as total ventilation increases to clear metabolically produced CO2
Outline the Henderson hasslebach equation and how this is put under pressure during exercise?
-pH=pKa + log [HCO3-] / a x PCO2
- HCO3- drops as it reacts with extra protons produced (H+) above the lactate threshold - PCO2 must also therefore decrease as a consequence
What happens to breathing above the lactate threshold?
- Expiratory reserve volume stabilises and further increases in VT are due a decrease inspiratory reserve volume?
Outline the process of bicarbonate buffering
H+ + HCO3- H2CO3 H20 + CO2
What is the relationship between tidal volume and breathing rate during exercise
- Initially tidal volume increases up to a plateau where breathing frequency then begins to increase
- Breathing frequency increases through a reduced time of inspiration and expiration
What are the factors controlling ventilation during exercise?
- Higher brain centres - central neurogenesis
- Humoral - Central (medullary) chemoreception/ Peripheral (carotid body) Chemoreception
- Afferent feedback/mechanical receptors = Peripheral neurogenesis and Cardiogenic drive
Where do the factors controlling ventilation feed into?
- Respiratory control center (Medulla oblongata)
- Increased signalling to respiratory muscles
Describe the negative feedback loop controlling ventilation during exercise?
1) Sensor - Chemical composition of the blood detected by peripheral/central receptors
2) Medulla receive signal via afferent neurone
3) Increases outflow to respiratory volume
How as does breathing change during exercise?
Initially expiratory reserve volume decreases until it stabilises and inspiratory reserve volume decreases to increase tidal volume
Describe how central chemoreceptors respond to exercise and the causes of this
- Capillary by medulla is impermeable to chemoreceptors
- CO2 can freely diffuse
- CSF has low protein and therefore buffering capacity so H+ increases in response to small change in PaCO2
Where are central chemoreceptors located?
- Near ventral surface of medulla oblangata
Where are peripheral chemoreceptors located in?
Carotid bodies: At the bifurcation of common carotid arteries
Aortic bodies: Above and below aortic arch
How do carotid bodies respond to exercise?
- Sensitive to arterial PO2 <60 mmHg
- Highly vascularised easily detect reduction
- High blood flow so have small (a-v) O2 difference
- Respond to PH
- Exercise: PaO2 low, PaCO2, low pH
What is a potential mechanism for the true hyperventilation (respiratory compensation) above the lactate threshold? What is there to support
- Possibly from H+ stimulation of Carotid bodies
- When Carotid bodies are removed PH is increased and PaCO2 is increased at onset of exercise (Whipp et al)
What is the breathing reserve and what does it suggest?
- Breathing reserve is the difference between the MVV and max ventilation measured during the exercise test - normally 20 to 40% of MVV
- Suggests we are not mechanically limited
What is MVV and MVSC
- MVV is maximum voluntary ventilation
- MVSC is maximum sustainable ventilator capacity
What is the equation for flow that high breathing frequencies are dependent on? How are changes in flow brought about?
- Flow (V)= Change in transpulnomary pressure (P)/Airway resistance (R)
- Require large in P brought about by high respiratory muscle activity