Other Factors Contributing to Non-Uniform Distribution of Perfusion

• Right pulmonary artery comes off at an acute angle. Therefore, there is more flow to the left lung.

Factors Contributing to Non-Uniform Distribution of Ventilation

• Diaphragm and ribcage association with lungs provides for• greater expansion at the base than at apex

• outer parencyma expands more than inner

• Weight of the lung and blood at the base• greater starting volume at apex (less ventilation upon

inhalation)

• results in less negative pleural pressure at base, favoring greater expansion at apex

• Net result: more ventilation at the base than at the apex.

Powder Dye (x mg)

Concentration (x mg / total ml)

V/Q Matching

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

V/Q Matching

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

V/Q Matching

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

Powder Dye (V)

Fluid Flow (Q)

Concentration (V/Q)

2345 TOPBOTTOM

Rib Number

0.15

0.10

0.05

L/min or

% lung volume

Ventilation

Blood Flow

3

2

1

VA/QC

VA

/ Q

C

60

40

20

0

PC

O2

(mm

Hg

)

40 80 100 120 14060

PO2 (mmHg)

Low VA/QHigh V

A /Q

V/Q Matching

mixed venousblood

Tracheal air

VA

(L/min)Q

0.24 0.07

0.82 1.29

VA/Q

3.3

0.63

PO2 PCO2

mmHg

132 28

89 42

pH

7.51

7.39

V/Q Matching

Why is arterial PO2 lower than average (ideal) alveolar PO2?

• When pulmonary veins joins combining volumes of blood that have different oxygen concentrations, the final concentration of oxygen is a simple, linear function of• Oxygen content of each volume

• Sum of the volumes (final volume)

pv

V/Q=10/10

O2 Concentration inVol%

V/Q=1/10 V/Q=10/1

20.5

18.0 X 10 = 18020.0 X 10 = 20020.5 X 1 = 20.5

400.5 / 21 = 19.1

19.1

V/Q Matching

mv(pa)

2018

60 X 10 = 600100 X 10 = 1000150 X 1 = 150

1750 / 21 = 83.3

(60) (150)(100)(PO2)(70)

Hb

Sat

ura

tion

(%

)100

80

60

40

20

0

2

20 40 60 80 100 600

PO (mmHg)

O2 C

onten

t

(ml O

2 /dl b

lood)

2

6

10

14

18

22Total O2

OxygenTransport

Why is arterial PO2 lower than average (ideal) alveolar PO2?

• When pulmonary veins joins combining volumes of blood that have different oxygen concentrations, the final concentration of oxygen is a simple, linear function of• Oxygen content of each volume

• Sum of the volumes (final volume)

• PO2 is not a simple linear function of oxygen content or concentration.

• The PO2 of blood leaving the lungs of an upright individual will be slightly less than ideal (average) alveolar PO2 as a result of

• the largest quantity of blood coming from regions with the lowest V/Q ratios, coupled with the non-linear characteristics of the oxyhemoglobin dissociation curve, skews the PO2 of arterial blood toward the alveolar PO2 values at the base

• shunts

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