Integrated Control of Ventilation Flashcards
State what pH is proportional to in the body
log [HCO3-] / arterial pressure of carbon dioxide
Describe what regulates the two components that determine pH in the body
- The kidney regulates the [HCO3-] e.g. by regulating reabsorption / excretion in glomerular filtrate
- There is respiratory regulation of alveolar partial pressure of carbon dioxide e.g. through regulating ventilation
State what the cut off for acidosis and alkalosis is
- If the pH decreases below 7.35 then it is acidosis
- If the pH increases above 7.45 then it is alkalosis
Describe what can cause acidosis and what each cause is referred to
Can either be caused by CO2 being too high or HCO3- being too low:
- When CO2 is too high it is respiratory acidosis
- When HCO3- is low it is metabolic acidosis
Describe what can cause alkalosis and what each cause is referred to
Can either be caused by CO2 being too low or HCO3- being too high:
- When CO2 is low it is respiratory alkalosis
- When HCO3- is too high it is metabolic alkalosis
Describe briefly how alkalosis and acidosis occur
Alkalosis or acidosis occurs when a change in respiratory function causes a disturbance in pH -
CO2 + H2O -> H2CO3 -> H+ + HCO3-
- Accumulation of CO2 increases [H2CO3] which increases [H+] (acidosis)
- Removal of CO2 decreases [H2CO3] which decreases [H+] (alkalosis)
Describe how respiratory acidosis occurs and the body’s reflexes in place to reverse it
- Occurs when ventilation is insufficient relative to the metabolic demands of the body
- Ventilation has decreased
- Hence the alveolar partial pressure of carbon dioxide increases
- So carbon dioxide also builds up within the blood (hypercapnia)
- This then means that there is more reaction of CO2 and H2O so more carbonic acid and more H+ ions which decreases pH causing acidosis
- There is then chemoreceptor feedback which increases the respiratory rate to increase ventilation
- When this is ineffective due to pathology the renal system will kick in and decrease the HCO3- excretion from the kidney
Describe how respiratory alkalosis occurs and the body’s reflexes in place to reverse it
- Occurs when ventilation is excessive relative to the metabolic demands of the body
- Ventilation has increased
- Hence the alveolar partial pressure of carbon dioxide decreases
- This causes the carbon dioxide levels in the blood to decrease (hypocapnia)
- This then decreases [H+] which increases pH causing alkalosis
- There is then chemoreceptor feedback which decreases respiratory rate to decrease ventilation
- When this is ineffective due to pathology, environment or behaviour then again the kidney will increase the HCO3- excretion from the kidney
State and describe situations that may result in respiratory alkalosis
- Anxiety attacks - increased ventilation without increased metabolic demand which decreases the arterial carbon dioxide levels which leads to respiratory alkalosis
- At higher altitudes - the partial pressure of oxygen is lower so there is more alveolar ventilation to try and maintain normal alveolar and blood oxygen levels which increases ventilation and so decreases blood carbon dioxide levels leading to respiratory alkalosis
Describe how metabolic acidosis occurs and the body’s reflexes in place to reverse it
- Occurs when metabolic acid production is excessive and or [HCO3-] is deficient-
- When metabolic acid production increases the increased [H+] reacts with HCO3- which decreases the [HCO3-] (also decreased by reduced absorption or increased excretion of HCO3-)
- Unless the blood carbon dioxide levels change in proportion with [HCO3-] acidosis will occur
- Respiratory compensation will occur almost instantly to attempt to get pH into its normal range after feedback by chemoreceptors
What situations can cause metabolic acidosis?
- Lactic acidosis (sepsis) - infection leads to a pathological immune response which leads to septic shock- this then decreases oxygen delivery to tissues which increases lactic acid production and reduced pH
- Diabetic keto-acidosis - increased fatty acid release from the liver due to a deficiency in insulin which increases acidic ketone body production
- Renal failure - decreased HCO3- reabsorption in proximal tubule
- Diarrhoea - decreased HCO3- absorption in the colon
Describe how metabolic alkalosis occurs and the body’s reflexes in place to reverse it
- Occurs when metabolic acid excretion is excessive and or [HCO3-} becomes excessive -
- When there is less metabolic acid there is less [H+] available to react with HCO3- so [HCO3-] increases (also increased by increased absorption or reduced excretion of HCO3-
- Unless the blood carbon dioxide levels change in proportion with [HCO3-] alkalosis will occur
- Respiratory compensation occurs via chemoreceptor feedback and ventilation is decreased to attempt to return the pH back to a normal level
What situations cause metabolic alkalosis?
- Use of diuretics as it changes H+ and HCO3- reabsorption
- Vomiting - loss of H+ in stomach acid
- Antacids - increases HCO3- consumption
Describe how acidosis induced hyperkalemia occurs
- There is higher [H+] in the extracellular fluid which decreases the H+ gradient
- This means that less H+ ions enter the extracellular space from the intracellular space
- Hence less Na+ ions enter the cell via the H+/Na+ exchanger
- This then means that there is less Na+ ions in the cell so less Na+ leaves the cell via the Na+/K+ ATPase so there is a decreased influx of potassium ions into the cell
- K+ accumulation in serum then leads to hyperkalemia which can cause cardiac arrhythmias and muscle weakness
Describe how alkalosis induced cerebral vasoconstriction occurs
- Carbon dioxide (via H+) acts as a vasodilator in blood vessels (cerebral arteries particularly sensitive)
- Hyperventilation causes decreased carbon dioxide and decreased H+ (alkalosis)
- Alkalosis causes vasoconstriction of cerebral arteries
- Decreased cerebral blood flow causes headaches, lightheadedness, confusion and seizures
