Respiratory Physiology 4.1-Ventilation: perfusion relationship Flashcards
WWhat does ventilation refer to?
the amount of air that is getting into the lungs
(air getting to alveoli L/min)
What does perfusion refer to?
refers to blood flow through the pulmonary circulation
(local blood flow L/min)
Where is most ventilation and perfusion in the lung?
at the base of the lung
Why at the base of the lung- is blood flow higher than ventilation?
because arterial pressure exceeds alveolar pressure and therefore the blood vessels effectively push on the alveoli and compress them.
Comes about due to gravity causing blood to flow more to the base of the lung than to the apex of the lung.
Why at the apex of the lung is blood flow low?
because arterial pressure is less than alveolar pressure. Compresses the arterioles.
Where is the only part of the lung where perfusion and ventilation is equal?
rib 3.
V/Q ratio is 1.
Describe how V/Q changes at base and apex of lung?
ratio much smaller at the base of the lung
biggest ventilation- perfusion mismatch occurs at the apex
blood flow declines faster than ventilation from base to apex of lung
Give example of ventilation and perfusion ratios?
perfectly matched- ventilation:perfusion ratio=1.0
mismatch 1 (base) - ventilation<perfusion= <1.0
mismatch 2 (apex) ventilation>perfusion= >1.0
Describe autoregulation when blood flow > ventilation?
blood going past the poorly ventilated region is now no longer able to give up its carbon dioxide and no longer able to pick up oxygen.
poorly ventilated part of the lung is getting carbon dioxide from the blood flowing past it faster than it can blow off.
End up with build up of carbon dioxide in that poorly ventilated region of the lung. And blood is taking oxygen out of that region of the lung faster than it is being replenished and there is fall in partial pressure of oxygen.
Why do we need a big partial pressure gradient for gas exchange?
to get oxygen to diffuse from the alveoli into the blood
What would happen if carbon dioxide started to build up in the alveoli?
lose the partial pressure gradient that would tend to pull carbon dioxide out of the blood and therefore can’t get rid of the carbon dioxide that is in the pulmonary arterial blood.
Neither can we replenish the blood with oxygen.
Results in dilution of the oxygenated blood from the better ventilated areas of the lung.
Describe “shunted”?
where blood is shunted - moved from the right side of the heart to the left side of the heart without undergoing gas exchange.
Describe the body’s mechanism to try and minimise the effects of shunt? In response to hypoxia
the smooth muscle in the blood vessels going to that area constrict so they reduce the amount of blood that flows to that area.
Redirects that blood to the better ventilated regions of the lung
How does body react to hypoxia in systemic circulation?
systemic vessels job is to deliver oxygen
systemic vessels dilate in hypoxic region so they can deliver more
How does alveolar space come about?
more ventilation than we have blood flow
occurs to small extent at apex of normal lung
occurs pathologically in pulmonary embolus
Describe what is the outcome when ventilation > perfusion?
Decrease in partial pressure of carbon dioxide because we’re blowing off carbon dioxide faster than adding to alveoli because of that reduction in blood flow.
Increase in partial pressure of oxygen leads to pulmonary vasodilation.
Decrease in partial pressure of carbon dioxide brings about a more mild bronchial constriction
What is alveolar dead space?
refers to alveoli that are ventilated but not perfused
What is anatomical dead space?
refers to air in the conducting zone of the respiratory tract unable to participate in gas exchange as walls of airways in this region (nasal cavities, trachea, bronchi and upper bronchioles) are too thick
What is physiologic dead space?
Alveolar DS + Anatomical DS
What is respiratory sinus arrhythmia?
describes the normal change in heart rate during the breathing cycle
Why is it important that heart rate changes during breathing cycle?
If heart rate stayed constant during inspiration- end up having an increase in alveolar dead space as would have more ventilation that you have perfusion through your lungs.
And during expiration - would end up with shunt as have more perfusion than ventilation.
How does respiratory sinus arrythmia come about?
due to changes in activity of parasympathetic vagus nerve- that innervates the heart.
Vagal nerve slows down heart rate.
How does activity of vagal nerve change during breathing cycle?
during inspiration - decreased activity and therefore heart rate increases
during expiration- increase in vagal activity slows heart down
