control of ventilation Flashcards
which nerves stimulate the muscles required for inspiration?
the phrenic nerves innervate the daiphragm causing it to contract and bring about inspiation.
the intercostal nerves innervate the external interostal muscles which contract to bring about inspiration
what controls ventilation/
ill defined centres in the pons and the medulla
which stimulation of ventilation has the “last word”?
the brainstem, it’s stimulation can be overidden by voluntary signals from the brain but not completely (you cant hold your breath until you die)
what is the function of the repiratory centres in the pons and medulla?
- set an automatic rhythm for breathing
2. adjust this rhythm according to stimuli
what is the function of the DRG (dorsal respiratory group) ?
controls the inspiratory muscles
what is the function of the VRG (ventral respiratory group)?
controls the expiratory muscles and to a lesser extent some inspiratory muscles and the pharynx, larynx and tongue muscles
what inputs to the respiratory centres adjust respiraory rate?
- emotion (via limbic system in the brain)
- voluntary over-ride (via higher centres in the brain)
- Mechano-sensory input from the thorax ( e.g. stretch reflex)
- chemical composition of the blood (PCO2, PO2 and pH) by chemoreceptors
what is the purpose of stretch receptors in the intercostal muscles?
they prevent damage to the lungs by over inflation as they send an inhibitory signal back to the brain when the external intercosal muscles are contracting too much
how do the central chemoreceptors in the medulla control carbon dioxide levels in blood?
detect changes in the carbon dioxide concentration in blood, it does this by detecting the hydrogen ion concentration in the CSF. A high hydrogen ion concentration is indicative of a high CO2 level. receptors detect this rise and send signals to the respiratory control centres which causes an in crease in ventilation. it is the opposite for a decrease in carbon dioxide concentration
how do peripheral chemoreceptors in the aortic and carotid bodies control oxygen levels in the blood?
cells in the chemoreceptors contain oxygen receptors on their surface, binding of oxygen to these receptors closes potassium channels. This causes depolarising the cell. This causes exocitosis of dopamine which binds to dopamine receptors on a sensory neuron, if the amount of dopamine is large enough an action potential is sent to the respiratory receptors which causes an increase in ventilation.
these chemoreceptors also respond to hydrogen ion concentration.
when do peripheral chemoreceptors increase ventilation?
when PP of oxygen is low and concentration of hydrogen ions is high (low pH)
What happens during alkalosis?
the peripheral chemoreceptors detect the decreases in hydrogen ion concentraion and inhibit ventilation
what happens during acidosis?
the peripheral chemoreceptors detect the increase in hydrogen ion concentration and stimulate ventilation
are central chemoreceptors more sensitive to changes in carbon dioxide level than peripheral chemoreceptors are to oxygen level?
yes
do peripheral chemoreceptors respond to changes in oxygen content of blood?
no, just oxygen PP
what happens to chemoreceptors in patients with chronic advanced lung disease?
the central chemoreceptors become desensitised to carbon dioxide as it is always at such a high level. they become dependent on peripheral receptors.
why is it not possible to voluntarily hyperventilate?
ventilation is reflexly inhibited by an increase in arterial PP of oxygen or a decrease in arterial PP of carbon dioxide or hydrogen ion conc.
what is the action of opiods and barbiturates?
they depress the respiratory centre
what do gaseous anaesthetic agents do?
increase respiratory rate and decrease tidal volume so decrease alveolar ventilation
why is nitrous oxide dangerous in sufferers of chronic advanced lung disease?
they have a hypoxic drive and nitrous oxide blocks the action of the peripheral chemoreceptors so they cannot detect a decrease in PO2 or an increase in PCO2. Giving oxygen aggravates the situation
what is hypercapnea
driven by a raised level of PCO2 (as in the central chemoreceptors)
