Ventilation:Perfusion Relationship Flashcards
What is ventilation? What is perfusion?
Ventilation: volume of air going in and out of the respiratory system (L/min)
Perfusion: blood flow through the pulmonary circulation L/min
Ideally ventilation and perfusion match in L/min
How does height across the lung impact blood flow and ventilation?
Both blood flow and ventilation decrease with height across the lung
- At the base of the lungs blood flow is higher than ventilation as arterial pressure exceeds alveolar pressure
- Blood vessels push on the alveolar and compress them
- At the apex of the lung blood flow is low because arterial pressure is less than alveolar pressure (blood flow decreases faster than ventilation does)
- This compresses the arterioles in the apex of the lung
How does the ventilation:perfusion ratio change from the base to the apex of the lung?
75% of the healthy lung performs well in matching blood and air
o Perfectly matched – ventilation:perfusion ratio = 1.0
o Mismatch 1 (base) – ventilation:perfusion < 1.0
o Mismatch 2 (apex) – ventilation:perfusion > 1.0
Auto regulated to keep the ratio as close to 1
(Majority of mismatch in the apex)
What happens in a Ventilation:perfusion mismatch? How does it lead to a shunt?
If ventilation decreases in group of alveoli PCO2 increases and PO2 decreases
- Blood flowing past those alveoli doesn’t get oxygenated and blood going passed poorly ventilated region isn’t able to give up its CO2
- blood is taking O2 faster than it can be replenished = decrease in PO2
- leads to CO2 build up in this region
- this leads to the diluted blood going back to heart
- shunt = blood moved from right side of the left without undergoing gas exchange
How are shunts prevented?
- Blood vessels around the poorly ventilated areas respond to the decrease in PO2 by constricting
o Redirects the blood to the better ventilated areas
o This only happens in the pulmonary vessels not in systemic circulation
o The increase of PCO2 acts on the smooth muscle causing it to dilate - Improves ventilation
What is alveolar dead space?
Alveolar dead space = ventilation > blood flow
- (Opposite of a shunt)
o More air than blood flow = fresh air not participating in gas exchange
o Occurs at the apex
o Leads to pulmonary vasodilation + bronchial constriction.
o Common in pulmonary embolism (blood clot in vessel preventing blood flow to alveoli)
What is anatomical dead space?
air in the conducting zone of the respiratory tract unable to participate in gas exchange
What is the physiological dead space?
alveolar dead space + anatomical dead space
What is a shunt?
Shunt = ventilation < perfusion
- alveolar PO2 falls, PCO2 rises
- pulmonary vasoconstriction
- bronchial dilation
How is O2 carried in the blood?
Majority of oxygen wrapped up in haemoglobin in red blood cells (197mL/L out of 200mL)
Very little dissolved in plasma (3mL/L out of 200mL)
How is CO2 transported?
Bulk (77%) of CO2 transported in solution in plasma
23% stored within haemoglobin
What % of O2 is extracted by peripheral resting tissue?
25%
What type of haemoglobin is in adult blood? What is the structure of haemoglobin?
Haemoglobin A
4 haem groups - each associates with on molecule of oxygen
How does oxygen bind to haemoglobin?
Oxygen binds to the haem groups in a weak relationship that doesnt last very long
Cooperative binding:
- oxygen binding to haemoglobin causes conformational change making it easier for other oxygen molecules to bind
- when oxygen leaves the haemoglobin there is further conformational change making it easier for other oxygen molecules to be given up
How does haemoglobin maintain the partial pressure gradient between alveoli and plasma?
Red blood cells full of haemoglobin
Haemoglobin sucks the oxygen out of the plasma maintaining the partial pressure gradient between the alveoli and plasma
What is the haemoglobin saturation at PO2 100mmHg, 60mmHG, 40mmHg (PO2 at tissues)?
100mmHg: 100%
60mmHg: 90%
40mmHg: 75%
Therefore saying that venous blood is deoxygenated is misleading as still 75% oxygenated
What is anaemia?
- Anaemia – any condition where the oxygen carrying capacity of the blood is compromised
o E.g. Iron deficiency, haemorrhage, vitamin B12 deficiency
How does anaemia affect PaO2?
Completely normal - possible to have normal plasma PO2, while total blood O2 is low (assuming normal lung function)
- Red blood cells can be fully saturated in anaemia assuming PO2 in plasma normal
(o Not possible to have low plasma PO2 and normal total blood O2 content
- Plasma determines how much oxygen binds to haemoglobin
How does oxyhaemoglobin dissociation curve aid O2 loading in lungs?
Normal PO2 in plasma in lungs = 100mmHg —> fully saturated haemoglobin
Curve is dynamic so can slide along x-axis
- doesnt effect loading as still in top plateau phase
How does a decrease pH, PCO2 and increase in temperature affect the oxyhaemoglobin dissociation curve?
Reduces saturation of the haemoglobin
- this is Bohr effect = aids oxygen unloading at peripheral tissues
Example when exercising:
- production of lactic acid lowers pH
- higher O2 metabolism = more CO2 as waste
- increase in body temperature
How does a increase pH, PCO2 and decrease in temperature affect the oxyhaemoglobin dissociation curve?
Opposite effect
Hypothermia is so dangerous as haemoglobin wont give off oxygen to the peripheral tissues
How is CO2 carried in the blood?
CO2 is very soluble so travels readily in solution
- CO2 diffuses into plasma (down partial pressure gradient)
- 7% of CO2 remains in solution in plasma
- 93% moves into red blood cells
- 70% of this reacts with enzyme carbonic anhydrase to form carbonic acid
- dissociates into bicarbonate and H+ ions
- bicarbonate ions move back into plasma in exchange for Cl- ions
- therefore 70% of CO2 travels in systemic venous blood as bicarbonate ions
What factors favour CO2 unloading at the lungs?
Haemoglobin much rather associate with O2 than CO2
- PO2 increases —> haemoglobin wants to release the CO2 - haemoglobin releases H+ ions causing bicarbonate to shift back into red blood cells to neutralise H+ - carbonic anhydrase formed —> CO2 and water released from the carbonic acid formed
What is the difference between arterial partial pressure of oxygen (PaO2) and arterial O2 concentration
PaO2 refers purely to O2 in solution in the plasma and is determined by O2 solubility and the partial pressure of O2 in the gaseous phase that is driving O2 into solution
Partial pressure not the same as concentration as concentration varies on the form of the molecule (e.g. 30x more O2 molecules in 1L gas than 1L plasma)
