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VENTILATION-PERFUSION RATIO

INTRODUCTION:

- V/Q ratio is d ratio of alveolar ventilation topulmonaryblood flow

- Normal value is 0.84[4.2(L/min)/5.0(L/min)]VENTILATION: it is d movement of air between d

atmosphere and alveoli and the distribution of air within dlungs to maintain appropriate concentrations of O2 and CO2.

- Air moves btw the atmosphere and d lungs viainspiration and expiration.

- At the alveoli there is gaseous exchange - O2diffuses down it concentration gradient from the

alveoli to the blood; CO2 diffuses along its

concentration gradient from blood to the alveoli.

- The amount of air that is used for this gaseousexchange is called alveolar ventilation (VA). VA = VT

(tidal volume = 500ml) anatomical dead spaces(150ml) = 350ml per breadth.

o Not all d air inspired reaches properlyfunctioning alveoli; they r trapped in the

anatomical dead space [conducting zones

of d respiratory airways and non-

functioning alveoli collapsed alveoli and

not well perfused alveoli where [O2] is

lower than normal and [CO2] is higher than

normal].

o Air that reaches functioning alveoliundergoes alveolar ventilation, gaseous

exchange.

DISTRIBUTION OF AIR IN THE LUNGS IN AN UPRIGHT

INDIVIDUAL

Air @ d apex of d lungs is more than air @ d base of d lungs;

but alveolar ventilation is lesser @ d apex than at d base.

This is due to d effect of the gravity on the normal

intrapleural pressure of the lungs (-5cm H2O).

- AT THE APEX:o The intrapleural pressure here reduces from

-5cmH2O to abt -10cmH2O (becomes more

negative) due to gravity.

o This represents a large force that expands dalveoli so dat it accommodates more air to

become large and stiff. The alveoli are thus

said to be less compliant.

o During inspiration intrapleural pressfurther reduces from -10cmH20 to -13cmH

meaning a larger force to further exp

the alveoli. But the alveoli are already fi

with air and stiff. As such little or no air

enter the apical alveoli and there will

little or no expansion of the alveoli dur

inspiration.

o Thus, the alveoli at the apex is said topoorly ventilated.

- AT D BASE;o The intrapleural pressure increases fro

5cmH2O to -2cmH2O (becomes more +

also, due to gravity.

o This represents a small force expandinalveoli so dat the alveoli at d base

smaller dan those at d apex and are also

stiff (more compliant).

o During inspiration, intrapleural pressdecreases from -2cmH2O to -8cmH

Because the alveoli here are m

compliant and have more room for air,

will be able to enter the alveoli.

o Thus, the alveoli at d base of the lungsaid to be well ventilated.

PERFUSION: is d movement/flow of blood through

pulmonary capillaries.

DISTRIBUTION OF BLOOD WITHIN THE LUNGS IN

UPRIGHT INDIVIDUAL

Due to gravity, the pulmonary bld pressure at the ape

lower than @ the base of the lungs. Because of gravi

effect on pulmonary blood pressure, there is more blood

d base than @ the apex of an upright lung:

Based on these regional differences in the pulmonary

pressure and pulmonary blood flow, the lungs are divid

into 3 perfusion zones (zones 1, 2 and 3)

In Zone I, there is no blood flow to this region and this zo

is absent in normal breathing.

In Zone II, blood flows to this region during systole

In Zone III, blood flow to this region during both systole

diastole (the entire cardiac cycle).

- In zone I Pa (arterial pulmonary blood pressureless than PA (alveolar pressure). As such, the blo

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capillaries are collapsed and there is no blood flow

in this zone. This means that the alveoli in this zone

does not participate in gas exchange and are part of

the lungs alveolar dead space.

- In healthy subjects under normal perfusionpressures, zone I is not present because arterial

pressures is just sufficient to raise blood to the top of

the lung and exceed alveolar pressure.

- Zone I may be present if pulmonary arterialpressure is reduced (following severe

hemorrhage or general anaesthesia) or if alveolar

pressure is raised (during positive pressure

ventilation).

- Zone I disappears when lying down as this reduces dvertical height of d lungs.

- During exercise and speech alveolar pressureincreases, meaning d appearance of zone I in an

individual.

- In zone II the pressure in the alveoli is less than thepulmonary systolic pressure and more than the

pulmonary diastolic pressure. Because of this there

is blood flow to this region during systole. So, this

zone is called the area of intermittent flow.

- In zone III, the pulmonary arterial pressure is morethan the alveolar pressure both during systole and

diastole. So the bld flow is continuous. Hence this

part of the lungs is called area of continuous blood

flow.

NOTE: The zones are not static; their borders vary

VENTILATION-PERFUSION RELATIONSHIP

- AT THE APEX OF THE LUNG:o The Alveoli here have relatively higher

volume of air than those at the base of the

lungs.

o Alveolar ventilation (0.24L/min) is lowerthan normal (0.42L/min); and pulmonary

blood flow (0.07L/min) is lower than

normal.

o Consequently, V/Q ratio (3.40) is higher thannormal (0.8) and the alveoli said to be over-

ventilated and under-perfused

- AT THE BASE OF THE LUNG:o The Alveoli have relatively lower volume of

air than those at the apex of the lungs.

o Alveolar ventilation (0.82L/min) is higthan normal (0.42L/min); pulmonary blo

flow (1.29L/min) though lower than norm

is still higher than at d apex.

o Consequently, V/Q ratio (0.63) is lower tnormal; hence the alveoli is said to be ov

perfused and under-ventilated

THINGS TO NOTE:

- FICKS LAW = P.A.S/T.Mw. where P = pressgradient; A = surface area of permeable membra

S = solubility; T = thickness of permeable membra

Mw = Molecular weight.

- NUMBER OF ALVEOLAR AIR SACS = ~300 milsacs

- ALVEOLI SURFACE AREA = 60-80 m2- Mycobacterium tuberculi thrives well at the apex

the lungs because of high V/Q ration there.