Chemical Control of Breathing Flashcards
What is the gross function of the respiratory system?
Function of the respiratory system is to maintain oxygen and carbon dioxide partial pressure gradients to optimise transfer and to regulate pH of extracellular fluid.
What are hypo/hyperventilation?
Hypoventilation and hyperventilation are changes without a change in metabolism. Hyperventilation causes a rise in pO2 and a fall in pCO2. Hypoventilation causes a decrease in pO2 and an increase pCO2.
What happens if pCO2 or pO2 change independently of each other?
If pO2 changes without a change in pCO2 correction of pO2 will lead to hypocapnia.
Generally how low must oxygen partial pressure drop before problems begin to arise and why is this?
We can tolerate a significant drop in pO2 before saturation is effected, this is because of the sigmoid curve of the Hb dissociation curve. Our body only needs to have a control system for a large decrease in pO2.
How does pCO2 affect pH?
Our body has a very effective buffering system to pH – the carbonic acid-bicarbonate buffer system. CO2 is controlled by respiration and HCO3 is controlled by the kidneys. Remember that pH is dependent on the ratio of [HCO3]/pCO2. A small change in pCO2 will lead to a large change in pH.
How does hypo/hyperventilation effect pH?
So, hypoventilation and hyperventilation and the corresponding hypocapnia and hypercapnia results in respiratory acidosis/alkalosis.
How do the kidneys compensate for respiratory alkalosis and acidosis?
pH also depends upon the [HCO3] which is controlled by the kidneys. Respiratory acidosis is compensated by the kidneys increase [HCO3] and respiratory alkalosis is compensated by the kidneys decreasing [HCO3], but this process takes 2-3 days.
How do metabolic acidosis and alkalosis occur?
If tissues produce acid this reacts with HCO3. This fall in [HCO3] results in a fall in pH. This is called metabolic acidosis. Our body responds to this by increasing ventilation. Conversely if plasma HCO3 rises such as after vomiting plasma pH rises and is called metabolic alkalosis. This can be compensated (to only a small degree) by decreasing ventilation.
How does the body respond to metabolic changes in pH?
Changes in ventilation can correct metabolic disturbances of pH.
Sensors located in the CNS and periphery feedback information back to the control centre for processing and ventilation is adjusted as necessary.
Where are peripheral chemoreceptors found an what do they respond to?
Found in the carotid and aortic bodies, large fall in pO2 stimulate increased breathing, changes in heart rate and changes in blood flow and distribution i.e. increasing flow to brain and kidneys. These are relatively insensitive to pCO2.
Where are central chemo-receptors found and what do they respond to?
Are found in the medulla of the brain and are much more sensitive to pCO2. If there is a small increase in pCO2 then it stimulates a rise in ventilation and vice versa. Only CO2 can pass the blood brain barrier, H+ and HCO3- cannot because of their charge.
How do central chemo receptors detect changes in CO2?
The central chemoreceptors respond to changes in the pH of Cerebral-spinal fluid (CSF). The CSF is completely separated from blood by the blood brain barrier. The CSF [HCO3] is controlled by Choroid plexus cells as they can change the rate that they transport HCO3 into the CSF. CSF pCO2 is determined by arterial pCO2.
CSF pH is determined by ratio of [HCO3] to pCO2. [HCO3] is fixed in the short term because the BBB is impermeable to HCO3. Falls in pCO2 lead to rises in CSF pH. Rises in pCO2 lead to falls in CSF pH but persisting changes in pH corrected by choroid plexus cells which change [HCO3].
How can what the Central cheoreceptors perceive as the norm pCO2 change?
Choroid plexus
CSF [HCO3] determines which pCO2 is associated with the norm CSF pH. CSF [HCO3] therefore sets the control system to a specific pCO2. It can be reset by changing CSF [HCO3].
