S4) Chemical Control of Breathing Flashcards
What is the concentration of bicarbonate in the blood?
22–26mmol/L
What is the average PaO2 in the blood?
9.3–13.3kPa
What is the average PaCO2 in the blood?
4.7–6.0kPa
Identify 2 functions of the respiratory system
- Maintain oxygen and carbon dioxide partial pressure gradients to optimise transfer
- Regulate pH of ECF
What is hypoxia?
Hypoxia is a fall in pO2
What is hypercapnia?
Hypercapnia is a rise in pCO2
What is hypocapnia?
Hypocapnia is a fall in pCO2
What is hyperventilation?
Hyperventilation is when ventilation increases without change in metabolism
What is hypoventilation?
Hypoventilation is when ventilation decreases without change in metabolism
How is hypocapnia caused?
- pO2 changes without a change in pCO2
- correction of pO2 will cause pCO2 to drop
Why does a control system need to avoid marked hypoxia?
- Oxygen-Haemoglobin dissociation curve is flat from approx. 8kPa
- Hence, pO2 can fall considerably before saturation is markedly effected
What equation is used to make calculations in the carbonic acid-bicarbonate buffer system?
pH = pK + log [HCO3-] / [H2CO3-]
Demonstrate the effect of pCO2 on plasma pH if bicarbonate concentration doesn’t change
If [HCO3-] remains unchanged:
- pCO2 increase = pH falls (notably)
- pCO2 decrease = pH rises (notably)
What happens if the pH rises above 7.6?
Free calcium concentration drops which leads to tetany
What happens if the pH falls below 7.0 ?
Enzymes become denatured
In two steps, explain how respiratory acidosis occurs
⇒ Hypoventilation leads to an increase in pCO2
⇒ Hypercapnia leads to a fall in plasma pH
In two steps, explain how respiratory alkalosis occurs
⇒ Hyperventilation leads to a decrease in pCO2
⇒ Hypocapnia leads to a rise in plasma pH
How do the kidneys compensate for respiratory acidosis?
Respiratory acidosis is compensated by the kidneys increasing [HCO3-]
How do the kidneys compensate for respiratory alkalosis?
Respiratory alkalosis is compensated by the kidneys decreasing [HCO3-]
How long does it take the kidney to compensate for pH changes?
2-3 days
In two steps, explain how metabolic acidosis occurs
⇒ Tissues produce acid & this reacts with HCO3-
⇒ The fall in [HCO3-] leads to a fall in pH
How can metabolic acidosis be compensated for?
Change ventilation:
- Increased ventilation lowers pCO2
- pH is restored to normal
In two steps, explain how metabolic alkalosis occurs
⇒ If plasma [HCO3-] rises e.g. after vomiting
⇒ Plasma pH rises
How can metabolic alkalosis be compensated for?
Compensated to a degree by decreasing ventilation
State 3 principles which allow compensation for pH changes to happen?
- Plasma pH depends on the ratio of [HCO3-] to pCO2 and not their absolute values
- Respiratory driven changes in pH compensated by the kidney
- Metabolic changes in pH compensated by breathing
Compare and contrast the pH, pCO2 and HCO3- in the following conditions:
- Metabolic acidosis
- Respiratory acidosis
- Metabolic alkalosis
- Respiratory alkalosis
Where are the peripheral chemoreceptors located?
- Carotid bodies
- Aortic bodies
Explain how peripheral chemoreceptors are sensitive to large changes in pO2
Stimulates:
- Increased breathing
- Changes in heart rate
- Changes in blood flow distribution (increasing flow to brain & kidneys)
Compare and contrast peripheral and central chemoreceptors in terms of pCO2 sensitivity
- Peripheral chemoreceptors will detect changes but are relatively insensitive to pCO2
- Central chemoreceptors in the medulla of the brain are much more sensitive to pCO2
What do the central chemoreceptors do?
Detect changes in arterial pCO2:
- Small rises in pCO2 increase ventilation
- Small falls in pCO2 decrease ventilation
Ilustrate the negative feedback control of breathing by central chemoreceptors
Central chemoreceptors respond to changes in the pH of cerebro-spinal fluid (CSF).
Explain the pH control of the CSF
CSF separated from blood by the blood-brain barrier:
- CSF [HCO3-] controlled by choroid plexus cells
- CSF pCO2 determined by arterial pCO2
What determines the pH of CSF?
CSF pH is determined by ratio of [HCO3-] to pCO2
Why is [HCO3-] constant in the short term?
Blood Brain Barrier is impermeable to HCO3-
What is the signficance of this fixed [bicarbonate]?
- Falls in pCO2 lead to rises in CSF pH (decreases ventilation)
- Rises in pCO2 lead to falls in CSF pH (increases ventilation)
What is the role of [HCO3-] in pH of CSF?
- CSF [HCO3-] determines which pCO2 is seen as ‘normal’ CSF pH (‘sets’ the control system)
- It can be ‘reset’ by changing CSF [HCO3-] (choroid plexus cells)
Describe the events that lead to persistent hypoxia
- Hypoxia detected by peripheral chemoreceptors which increases ventilation
- But pCO2 falls further which decreases ventilation
- So, CSF composition compensates for the altered pCO2 and choroid plexus cells selectively add H+ / HCO3- into CSF
- Central chemoreceptors “accept” pCO2 as normal
Describe the events that lead to persistent hypercapnia
- Hypoxia and hypercapnia lead to respiratory acidosis
- Decreased pH of CSF cause both chemoreceptors to stimulate breathing
- Acidic pH is undesirable for neurons so choroid plexus adjusts pH of CSF by adding HCO3-
- Central chemoreceptors “accept” the high pCO2 as normal
