8 Chemical Control of Breathing Flashcards
What is hypoxaemia?
Fall in arterial pO2
What do hypocapnia and hypercapnia mean?
- Hypercapnia: Rise in alveolar (hence arterial) pCO2
- Hypocapnia: Fall in alveolar (hence arterial) pCO2
Hypoventilation can result in respiratory acidosis. Explain why?
pH has decreased and ventilation rate decrease
Removal of CO2 from lungs= less rapid
pCO2 in alveoli has increased
[dissolved CO2] increases relative to [HCO3-] so ratio not 20:1
(Opposite for hyperventilation)
How do the kidneys respond to persistent respiratory acidosis?
Reduce excretion of HCO3-
(Opposite for hyperventilation)
What happens to the HCO3- concentration in the blood if there is excess metabolic production of acid (eg diabetic ketoacidosis)? What is the consequence of this?
Excess acid- buffered by HCO3- so used up–> concentration decreases
pH falls- metabolic acidosis
What are the 2 ways in which disturbances in the pH of the blood can be countered?
- Alteration of alveolar pCO2 - alter ventilation rate
- Corrected by kidneys (excretion of HCO3-)
What effect will these changes have on ventilation rate?
Drop in pO2
Large increase in pCO2
Drop in pH
How are the ventilation changes brought about (as a result of changes in pO2/pCO2/pH)?
Chemoreceptors- detect changes in arterial pO2/pCO2/pH
Send impulses to respiratory centres in brainstem–> modulate rate and depth of ventilation
What are the 2 types of chemoreceptors that respond to changes in blood pO2/pCO2/pH?
Peripheral and central
What can peripheral chemoreceptors detecting changes in pO2/pCO2/pH be found and what nerves carry the impulses to the brainstem respiratory centre?
- Carotid bodies
- Glossopharyngeal nerves (IX)
- Aortic bodies
- Vagus nerves (X)
What changes in the blood pO2 do peripheral chemoreceptors detect and what changes do the bring about? (Carotid and aortic bodies)
- Changes detected: Respond to large falls in pO2
- Changes made: Increase tidal volume, rate of respiration, direct more blood to brain and kidneys, increased pumping of blood by the heart
(Respiratory drive from hypoxia remains in persitent hypoxia)
What changes in the blood pCO2 and pH do peripheral chemoreceptors detect and what changes do the bring about? (Carotid and aortic bodies)
- Changes detected:
- Directly activated by changes in pH
- (respond to LARGE changes in pCO2)
- Changes made:
- Low pH: increased respiratory rate and tidal volume
Where can central chemoreceptors be found?
Ventral surface of medulla
Exposed to cerebro-spinal fluid
What do the central chemoreceptors respond to and what changes do they bring about?
Respond to: rise in arterial pCO2 (pH decrease)
Changes: impulses from chemoreceptors travel to brainstem respiratory centres - change respiration
Why is CSF pH corrected more quickly than blood pH?
Small volume of CSF
Lower protein content- overall buffering capacity is less
CSF is separted from the blood by a blood-brain barrier. How is its pH therefore determined?
Free passage of CO2 across blood-brain barrier (NOT HCO3-)
pH determined by its own HCO3- conc to dissolved CO2 buffer system
[HCO3-] determined by activity of choroid plexus cells in CSF
What happens to the central chemoreceptors if pCO2 remains elevated for any length of time? (eg with lung disease)
Choroid plexus cells - reset central chemoreceptors–> no longer sensitive to existing pCO2
(So in long standing hypocapnia- drive from increased ventilation is due to hypoxia (hypoxic drive) via peripheral chemorecpetors)
Define hyperventilation.
Ventilation increase without change in metabolism
Write out the equation that demostrates how pH of the blood is determined by the concentration of dissolved CO2 in the blood and [HCO3-]
What happens in the body if pH rises above 7.6?
Free calcium concentration drops (tetany: condition marked by intermittent muscular spasms)
What may cause metabolic alkalosis? How can it be compensated for?
Vomiting (loss of protons)
Plasma [HCO3- rises]
Decrease degree of ventilation
For info:
What will happen to total oxygen content of the blood if a normal individual breathes air at twice normal atmospheric pressure?
Fill in the missing gaps in the table:
