Ventilation/perfusion relationship Flashcards
What does ‘ventilation’ refer to?
ventilation refers to the air that reaches the alveoli - the mechanical act of moving air in and out of lungs
what is ‘pulmonary ventilation’?
minute ventilation = voulme of air breathed in and out in one minute =
what is more important? Pulmonary or alveolar ventilation?
alveolar = becuase it takes out the ‘dead space’ that is in the upper airway that doesn’t exchange gas
How do you determine alveolar ventilation?
what is ‘perfusion’?
The blood that reaches the alveoli
- The process by which CO2 from tissues is brought to lungs for removal
- The process by which deoxygenated blood passes through the lung and becomes oxygenated.
what are the waste products of respiration?
water, CO2, nitrogen?
what type of epithelial cells make up the alveolus?
type 1 alveolar cells
what is the only barrier between gas and blood?
the structual matrix
*when people have cystic fibrosis, they feel breathless b/c this region has increased in thickness therefore they are not properly exhanging
how do RBCs pass through the capillary component>?
single file
how does gas exchange occur in the lungs?
by simple diffusion from high to low concentration
what makes up the partial pressure of the gas leaving your lungs?
N2, O2 and CO2 (and H2O)
does the oxygen content in the alveoli fluctuate?
no, O2 arriving in alveoli replaces O2 diffusing out of alveoli to pulmonary capillaries - it doesn’t accumulate but remains the same
GAS DIFFUSES FROM HIGH TO LOW CONCENTRATIONS
how many alveoli are in the lungs?
•The lungs contain around 300-500 million alveoli and have a surface area of around 70 square metres (half a tennis court)
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Major function : Gas Exchange
•The Lungs are ideally structured for gas exchange
what is the law of gas exchange?
Ficks Law of diffusion
The greater the surface area
and the shorter the distance ;
the greater the rate of diffusion
what occurs during rest and exercise to our gas exchange?
- During exercise the area available for exchange can be physiologically increased. decreasing the ‘thickness’ of membrane and allowing more gas exchange
- During rest ; some pulmonary capillaries closed
–Low pressure pulmonary circulation
•Increased cardiac output ; opening of capillaries.
pulmonary oedema reduces gas exchange how?
it increases the thickness of the barrier between capillaries and the alveoli - therefore you have a decreased gas exchange
the same thing happens in pulmonary fibrosis - but there is scar tissue etc between alveoli and the capillaries
what is the ventilation/perfusion ratio?
•The ratio of the amount of air reaching the alveoli (V)
–To the amount of blood reaching the alveoli (Q)
This ratio is very important - but the numbers themselves are useless
- 1L Air contains approx 200ml O2
- 1L Blood can hold approx 200ml O2
- our bodies have managed to take in as much oxygen as our blood can absorb
- Efficiency and Adequacy
if your V/Q is high what happens?
you’re taking in more oxygen than you’re absorbing and you’ll be saturated with oxygen
(high Pressure O2, low pressure CO2)
what is the cause of hypoxemia in cardiopulmonary disease?
the mismatch of the ration between V and Q is the most frequent cause of hypoxemia (reduced blood PO2 and low V/Q ratio)
does normal V/Q relationship indicate that ventilation and perfusion is normal?
no- sometimes it could be normal, but there are other clincial symptoms
•In lobar pneumonia,
–ventilation to the affected lobe is decreased.
–This produces hypoxic vasoconstriction in the pulmonary capillary bed supplying the lobe
–resulting in decreased perfusion.
–
•Therefore, the V/Q ratio is normal but both ventilation and perfusion are reduced.
how does the V/Q ratio differ between regions of the lung itself?
apex = very good ventilation here, but poor diffusion b/c of gravity
at base= perfusion is great, but ventilation is poor b/c we often do not breath deep enough
what is arterial hypoxemia?
–Arterial PO2 less than 80 mm Hg in an adult breathing room air at sea level.
what is hypoxia?
–Insufficient O2 to carry out normal metabolic functions – usually when arterial PO2 is less than 60 mm Hg
–
–Low V/Q (well perfused, under ventilated)
what is hypercapnia?
–Increase in arterial PCO2 above the normal range (40 mm Hg)
–Hypoventilation (low V/Q))
what is hypocapnia?
•Hypocapnia
–Abnormally low arterial PCO2 (usually less than 35 mm Hg)
–Hyperventilation (high V/Q)
*give them a paper bag b/c they are bringing their own CO2 back into their blood*
at a high altitude, would you hyperventilate or hypoventilate?
hyperventilate b/c at a high altitude you’re always a little hypoxic
hypercapnia is associated with what illnesses?
associated with COPD
what causes hypocapnia?
hyperventilation
- arterial PCO2 falls
- leads to vasoconstriction and less flow to the brain
how would you get a V/Q = 0?
only if there is no ventilation
how do you get a V/Q approaching infinity?
When profusion is equal to 0 = wasted ventilation = you’re breathing but not exchanging
How does V/Q change in respiratory disease?
- Increased (overventilation/underperfusion)
- decreased (underventilation/overperfusion)
•
- An increased V/Q ratio means an increase in alveolar dead space and “wasted ventilation”
- Good Ventilation; bad/no perfusion
•
- A decreased V/Q ratio means “shunting”
- deoxygenated venous blood bypasses the exchange area and enters the left heart causing arterial hypoxaemia
- Bad/No Ventilation ; Good Perfusion
what is the result of good ventilation but no perfusion?
V/q= infinity
wasted ventilation
what is the result of an obstructive respiratory disease of v/Q ?
obstructive = low ventilation
normal profusion
therefore V/Q = very small
*this is called shunting b/c the deoxygenated blood is brought to better regions of the lung where it’s possibly less obstructed*
what happens to the airway smooth muscle and pulmonary arterioles in hypoxic pulmonary vasoconstriction?
- Relative underventilation/overperfusion
- decreases alveolar PO2 and increases PCO2
•
- A decrease in PAO2 and an increase in PACO2 causes relaxation of airway smooth muscle but contraction of pulmonary arterioles\
- Hypoxic pulmonary vasoconstriction causes diversion of blood away from poorly ventilated/overperfused alveoli and therefore limits “shunting”
- However, long-term alveolar hypoxia (altitude, some respiratory diseases) will cause pulmonary hypertension
what occurs to V/Q in pulmonary vasodilation?
What is hyperpnea?
- Increased ventilation that matches increased metabolic demand
- During exercise
•Explain how airway obstruction would cause hypoxaemia.
•
•What is shunting ?
•
Describe the pulmonary circulation network.
