pulm circ

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Pulmonary circulation

•The blood supply of the lung is derived from:- Bronchial arteries :

Arising from the aorta. It's blood returns to

pulmonary veins after supplying the bronchialtree and parietal pleura.

Pulmonary artery:

Where mixed venous blood flows to the lung andvisceral pleura.

The alveoli get their O2 from air inside them.

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2

Pulmonary and Bronchial Circulation

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Pulmonary Capillaries Near Alveoli

Basket like

capillary beds

surround the

.alveoli

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Blood supply to the lungs

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Pressures in the pulmonary

circulation

Pulmonary circulation is a low pressure

circulation.

In pulmonary arteries the pressure is 25/8

mmHg.

The mean pressure is about 15 mmHg.

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Pressures in the pulmonary

circulation

The pulmonary capillary pressure is

8 mmHg .

The mean pulmonary venous pressure is

5 mmHg .

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Capillary exchange of fluid in the

lungs

҉ Low pulmonary capillary pressure ( 8 mmHg).

҉ Hydrostatic force tending to push fluid out of

the capillary pores is low. ҉ Colloidal osmotic pressure (25 mmHg) is a

greater force tending to pull fluid into capillaries.

҉ This helps in keeping alveoli dry.

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Physiological shunts

Small amount of partially deoxygenatedblood mix with the fully oxygenated bloodthis leads to decrease partial pressure of O2

in arterial blood from 100 mmHg to 95mmHg.

These shunts are present in two sites:-

A) Pulmonary shunt.

B) Cardiac shunt.

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AlveolarSPACE

Venous blood

SIMPLE CONCEPT OF A SHUNT

Gas Exchange

CO2 O2

No Gas Exchange = SHUNT

AIR FLOW

Arterial blood

MixingLowered O2/l00 ml

No Gas Exchange

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Factors affecting pulmonary blood flow

1.Automatic local control (autoregulation).

҈ When alveolar PO2 (PAO2) becomes < 70mmHg,the adjacent blood vessels slowly constrict and the

vascular resistance is increased, this is called

hypoxic pulmonary vasoconstriction (HPV).

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Hypoxic vasoconstriction balances blood flow withventilation

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Mechanism of HPV

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Significance of HPV

It leads to redistribution

of blood flow to better

ventilated areas of thelung .

It prevents marked drop

of PaO2.

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2. Sympathetic stimulation reduces

pulmonary blood flow by as much as 30%.

3. Humoral agents such as nitric oxide (NO)

produce vasodilatation of pulmonary blood

vessels.

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4. Local accumulation of CO2

҈ leads to a drop in pH in the area, and produces

VC as opposed to the VD in other tissues .

. ҈ Reduction of the blood flow to a portion of the

lung lowers PACO2 in that area, and leads to

bronchoconstriction, shifting ventilation away

from the poorly perfused area.

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Factors affecting pulmonary blood

flow

5. Systemic hypoxia

Causes the pulmonary arterioles to

constrict, with a resultant increase inpulmonary arterial pressure.

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Effect of exercise on pulmonary

blood flow:

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Effect of exercise on pulmonary blood flow

With exercise, cardiac output increases and

pulmonary arterial pressure rises proportionatelywith little or no vasodilation.

More red cells move through the lungs without anyreduction in the O2 saturation of the hemoglobin

in them, and consequently, the total amount of O2 delivered to the systemic circulation is increased.

Capillaries dilate, and previously underperfusedcapillaries are “recruited” to carry blood.

The net effect is a marked increase in pulmonaryblood flow with few, if any, alterations inautonomic outflow to the pulmonary vessels.

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Effect of exercise on pulmonary blood

flow

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Pulmonary reservoir

Because of their dispensability, the pulmonary veins

are an important blood reservoir.

When a normal individual lies down, the pulmonary

blood volume increases by up to 400 ml.

When the person stands up this blood is discharged

into the general circulation.

This shift is the cause of the decrease in vital

capacity in the supine position.

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Pulmonary perfusion

Regional difference

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Pulmonary Perfusion

In the upright position

lung blood flow decreases

linearly from the bottom to the

top reaching very low level at

the apex.

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Pulmonary Perfusion

In the supine positionThe apical zone blood flow increases with the

result that, the distribution from the apex to

base becomes almost uniform.

But the blood flow in the posterior region of the

lung exceeds the flow in the anterior part.

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Pulmonary Perfusion

In mild exercise

both upper and lower zone blood flow increases

and the difference becomes less..

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Causes of the regional difference of the

pulmonary blood flow

1-Hydrostatic pressure:

It varies in the same way as pressure

in any vertical column of liquid. It

distends the vessels at the base of the

lung and allows those at the apex to

be narrow or even collapsed.

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2-The arterial - venous pressure difference

It drives blood through lung capillaries, but the

blood flows only when the hydrostatic pressure

keeps these capillaries open.

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Alveolar air pressure:

Lung capillaries are separated from the alveoli

by a very thin layer of tissues; therefore, thealveolar pressure which equals atmospheric

pressure at end of normal expiration can affect

blood flow through the capillaries.

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According to the relation between PA, Pa, and

PV the lung is divided into 3 perfusion zones

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Perfusion zones of the vertical lung

I- Zone 1(at the apex): Zone of no flow

PA > Pa > P v

It occurs when:1-Arterial pressure is reduced (hemorrhage).

2-Alveolar pressure is raised.

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PApa PvZone 1

PA>Pa>Pv

Low Flow

PA

Pa Pv

PA

Pa Pv

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II-Zone 2(middle zone)

Pa > PA > P v

The arterial pressure increases (because of hydrostatic effect). Therefore, the blood flow

becomes better than at the apex.

Water fall zone .

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III-Zone 3 (At the base of the lung)

Pa > P v > PA

Blood flow is continuous.

Vessels are distended.

More capillaries are opened with moreperfusion.

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36

Measurement of Gas Pressure

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Gravity and V-Q

•UprightLateral

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Regional difference in ventilation

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F l o w

o f B l o o d o

r A i r

VA / Q

R a t i o

Bottom TopDistance up Lung

Ventilation

Perfusion

VA /Q

1

2

3

.

.

. .

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Ventilation/perfusion

BY

Prof/

Hala Salah

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Ventilation/perfusion Relationship

(V/Q)

It is the ratio of alveolar ventilation to

pulmonary blood flow.

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Normal V/Q ratio

Alveolar ventilation is 4.2 L/ min.

Cardiac output (perfusion) is about 5L/min.

V/Q = 4.2 / 5 = 0.8(ideal lung)

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F l o w

o f B l o o d o

r A i r

VA / Q

R a t i o

Bottom TopDistance up Lung

Ventilation

Perfusion

VA /Q

1

2

3

.

.

. .

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Lung Ventilation/Perfusion Ratios

Alveoli at apex

underperfused

overventilated

V/Q=3.3

Wasted ventilation

(DEAD SPACE UNITS)

Insert fig. 16.243.33

0.6

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Lung Ventilation/Perfusion Ratios

Alveoli at the baseunder ventilated

Overperfused

V/Q=0.6

Wasted perfusion

(SHUNT UNITS)

Insert fig. 16.243.33

0.6

Contribution of Ventilation Perfusion Mismatch to

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Contribution of Ventilation Perfusion Mismatch to

Normal A-a Gradient

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Clinical importance of V/Q ratio

In many lung diseases, some areas of the lung

are well ventilated but not well perfused.

Other areas may have excellent blood flow but

no ventilation.

This severe mismatch impaired gas

exchange.

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VA/Q

decreased

Normal

increased

0 50 100 150

i i

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In bronchial obstruction

(Emphysema )

Alveolar ventilation is zero .

There is still perfusion .

V/Q = Zero .Blood leaves the alveolus unchanged.(Shunt).

PAO2 40mmHg .

PACO2 46mmHg . Mixed venous blood

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In pulmonary thrombosis

There is adequate ventilation .

Perfusion is zero (i.e. no blood flow due to

emboli blocking arterioles for example) .

V/Q=infinity(∞). (Dead space).

PAO2 =150mmHg.

PACO2 =0 mmHg.

/

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Pathological V/Q Mismatch and the Effect on

Arterial Oxygenation

Breathing 100% oxygen will correct the hypoxemia by greatly

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