Integrated control of breathing Flashcards
do the kidneys or the lungs take longer to change bicarbonate levels?
kidneys take longer, takes hours to days
lungs take seconds to minutes
how do the lungs and the kidneys change bicarbonate levels?
lungs - changing the level of ventilation which will increase the amount of co2 removed
kidneys - excrete more bicarbonate ions in the urine, less are reabsorbed
what may excessive pH changes result from?
may result from respiratory (CO2) or metabolic (HCO3-) dysfunction
when do respiratory acidosis or alkalosis occur?
when a change in respiratory function causes a disturbance in pH
high CO2 = respiratory acidosis
low HCO3- = metabolic alkalosis
and vice versa
metabolic vs respiratory acidosis/alkalosis
respiratory is to do with CO2 levels
metabolic is to do with HCO3- levels
what is pH equivalent to?
[HCO3-]/PaCO2 = pH
unless [HCO3-] changes in proportion with PaCO2, acidosis or alkalosis will occur
what causes respiratory acidosis?
- Hypoventilation, because the respiratory system isn’t meeting the demand of the body
- Increase in co2 both in the alveolar space and arterial blood
- Hypercapnia, resulting in a decrease in pH and acidosis
what compensatory mechanism occurs as a result of the decrease in pH?
increased activation of central chemoreceptors, leading to an increase in ventilation as you get chemoreceptor feedback
Normally with good health this would normally naturally happen almost instantaneously as chemoreceptors are very sensitive to changes in pH
what must happen in order for a real acidosis to occur?
there has to be some pathology eg. where the individual can’t ventilate their system properly because their airways are obstructed
when a real acidosis occurs because the rest system is compromised, what must the kidneys do to restore pH?
the renal system will start to compensate for the change in pH, so as a result the excretion of bicarbonate ions will decrease, as you want to pH to go back to normal
more bicarbonate, increase in pH (more basic)
In the event of chronic hypo-ventilation what happens?
HCO3- excretion must decrease to maintain normal pH.
name some causes of hypoventilation:
- airway obstruction
2. problems with initiation of breathing (airway is clear)
how does airway obstruction cause hypoventilation?
- airways are blocked completely or start to constrict in terms of their diameter
- this increases the level of airway resistance and creates an opposing force that makes it harder to get air in
-at a certain point, it becomes difficult to generate sufficient airflow in order to get enough air in and out of the lungs at a sufficient rate
- so, you get situations where the person chokes
- e.g. obstruction of the upper airways like in obstructive sleep apnoea
- e.g. obstruction of the lower airways like in asthma or COPD, which cause increased airway resistance
how do problems with initiation of breathing cause hypoventilation?
anything that causes insufficient signals to be sent from the brain stem to the respiratory muscles
- drug overdose (typically opioids)
- stroke
- neuromuscular problems
- motor neuron disease
- respiratory muscle wasting
What causes respiratory acidosis?
hypoventilation
What causes respiratory alkalosis?
hyperventilation
name some causes of hyperventilation:
- anxiety
- altitude
how does hyperventilation cause respiratory alkalosis?
a person is breathing at an excessive level relative to the metabolic demands of their body resulting in excess co2 being expelled and pH increasing, meaning less acidity.
chemoreceptors stop being activated and stop sending signals to the brain, ventilation decrease
kidneys excrete more bicarbonate ions as you want the pH to get back to normal
how does anxiety cause hyperventilation, and therefore respiratory alkalosis?
- anxiety trigger leads to a panic attack
- increased ventilation without increased metabolic demand
- hyperventilation leads to decreased PaCO2
- this causes respiratory alkalosis
how does altitude cause hyperventilation, and therefore respiratory alkalosis?
- a higher altitudes, the fraction of gases in the atmosphere doesn’t change much (still 21% oxygen etc) but the overall barometric pressure decreases
- this means there’s less O2 going into the alveoli
- so, there’s increased alveolar ventilation to increase PAO2 and PaO2 (via hypoxic drive)
- increased ventilation decrease PaCO2
- causes respiratory alkalosis because you are getting rid of excess CO2
what is metabolic acidosis?
- occurs in situations where you get large amounts of acid production, or where the kidneys are excreting too much HCO3- ions/not reabsorbing enough HCO3- ions
- if the pH becomes too acidic, ventilation will increase as you want to increase breathing so more CO2 is removed
using the equation with PaCO2 and [HCO3-] explain why acidosis will occur
[HCO3-]/PaCO2 = pH
- there will be a decrease in [HCO3-]
- unless PaCO2 changes in proportion with [HCO3-] (in the opposite direction), acidosis (pH) will occur
name some causes of metabolic acidosis, other than lactic acidosis:
diabetic ketoacidosis
- increased FA release from liver, due to insulin deficiency
- increased ketone body production (acidic)
diarrhoea
-decreased HCO3- reabsorption in the colon
renal failure
-decreased HCO3- reabsorption in the proximal convoluted tubule
explain how lactic acidosis (sepsis) causes metabolic acidosis:
- infectional sepsis
- pathological immune response
- septic shock
- decreased o2 delivery to tissue
- large amount of lactic acid production
- leads to decreased pH
- patients breathing at a faster rate (tachycardia) to try and get rid of CO2.
what why metabolic alkalosis and why does it occur?
-occurs when metabolic acid excretion is excessive or due to excess [HCO3-]
–unless PaCO2 changes in proportion with [HCO3-], alkalosis (pH) will occur
-respiratory compensation typically occurs almost instantly, however it may not be sufficient to prevent disturbance in pH
why might we want hypoventilation to occur when we have metabolic alkalosis?
- carbon dioxide is acidic and if the pH is high we want more CO2 to balance out the pH
- so, hypoventilation occurs where we decrease the rate of breathing (in order to retain more CO2)
Causes of metabolic alkalosis
- diuretics affect ion exchange and balance within the tubules of the kidney resulting in insufficient excretion/increased reabsorption of HCO3-, also affects H+ reabsorption
- vomiting, causes loss of H+ in stomach acid
- antacids, which increase HCO3- consumption
in healthy individuals, why does PaCO2 not increase during exercise?
-this is because ventilation increases in order to remove CO2, before the extra CO2 produced by muscles can increase PaCO2
