Lecture 8 - Chemical control of breathing Flashcards
Functions of the respiration system
Maintain O2 and CO2 partial pressures to optimise transfer
Regulate pH of ECF
Hypercapnia
Rise of CO2
Hypoxia
Low O2
Hypocapnia
Decrease in CO2
Exercise
pO2 decreases due to increased metabolism
pCO2 increases
Therefore increased respiration rate to excrete more CO2 and absorb more O2
Hyperventilation
Increased respiration rate without an increased metabolic demand
O2 increases
CO2 decreases - Respiratory alkalosis
Hypoventilation
Decreased respiration rate without change in metabolic demand
Decrease O2
Increased CO2 - respiratory acidosis
Oxygen- Hb dissociation curve
Sigmoid
100% saturation at 8kPa
pO2 can fall considerably to 8kPa from 13 kPa before saturation is affected.
pH
pK+log ([HCO3-]/pCO2 x 0.23)
- H2CO3- quilibrates with CO2 so can be used interchangeably
pCO2 controlled by respiration
HCO3- controlled by kidneys
pH below 7
Denatures enzymes
pH above 7.6
Free calcium concentration drops leading to tetany
Kidney compensation for respiratory acidosis
Produce more HCO3-
Secrete more H+
Takes 2-3 days
Kidney compensation for respiratory alkalosis
Excrete more HCO3-
Reabsorb more H+
Takes 2-3 days
Metabolic acidosis
Tissues produce more H+
Reacts with HCO3- to produce CO2 which is breathed off
Decreasing HCO3- concentration in blood which decreases pH
Metabolic acidosis compensation
Increased respiratory rate
Breath off more CO2 which lowers [CO2] correcting ratio
Metabolic alkalosis
e.g. vomiting
Increases [HCO3-]
Increasing pH
Metabolic alkalosis compensation
Hypoventilation to an extent but can’t starve brain of oxygen therefore limited
Central chemoreceptors
In the CSF of the brain medulla
- Detect changes in pCO2
- Increased CO2, increased hyperventilation
- More CO2 breathed off to correct ratio
Peripheral chemoreceptors
In carotid and aortic body
Sensitive to:
H+
O2
CO2 - relatively insensitive
- Large fall in O2
- Stimulates: Hyperventilation
Increased heart rate
Increased blood flow to brain and kidneys
O2 peripheral chemoreceptors
Less sensitive to small changes in O2 as receives large blood flow
Central chemoreceptors sensitivity
More sensitive to pCO2 changes as less proteins so lower buffer capacity
Choroid plexus
CSF is separated from the blood by the blood brain barrier
CSF [HCO3-] and [H+] is controlled by the choroid plexus (normally impermeable).
CSF pCO2 is determined by arterial pCO2
COPD
- Increased pCO2 in CSF detected by the chemoreceptors in the medulla as pH decreases
- Increased hyperventilation to blow off CO2 restoring CSF pH
- Persistent high pCO2 causes the choroid plexus cells to become more permeable to [HCO3-]
- HCO3- diffuses into the CSF correcting the acid base set point in the CSF therefore a new set point is produced with a higher CO2
- pO2 becomes the new driving force for hyperventilation as low pO2 detected by the peripheral chemoreceptors
