Pulmonary blood flow is measured by applying Fick principle.


Pulmonary blood flow is regulated by the following factors:

1. Cardiac output

2. Vascular resistance

3. Nervous factors

4. Chemical factors

5. Gravity and hydrostatic pressure.


Pulmonary blood flow is directly proportional to cardiac output. So, any factor that alters the cardiac

output, also affects pulmonary blood flow. Cardiac output is in turn regulated by four factors:

i. Venous return

ii. Force of contraction

iii. Rate of contraction

iv. Peripheral resistance.


Pulmonary blood flow is inversely proportional to the pulmonary vascular resistance. Pulmonary vascular

resistance is low compared to systemic vascular resistance. Pulmonary vascular resistance is altered

in different phases of respiration. During inspiration, pulmonary blood vessels are distended because

of decreased intrathoracic pressure. This causes decrease in vascular resistance resulting in increased

pulmonary blood flow. During expiration, the pulmonary vascular resistance increases resulting

in decreased blood flow. During the conditions like exercise, the vascular resistance decreases and blood flow increases. It is influenced by the exercise-induced hypoxia and hypercapnea.


Stimulation of sympathetic nerves under experimental conditions increases the pulmonary vascular resistance by vasoconstriction and the stimulation of parasympathetic, i.e. vagus nerve decreases the vascular resistance by vasodilatation. However, under physiological conditions, it is doubtful whether autonomic nerves play any role in regulating the blood flow to lungs.


Excess of carbon dioxide or lack of oxygen causes vasoconstriction. The cause for pulmonary vasoconstriction by hypoxia is not known. But it has some significance. If some part of lungs is affected by hypoxia, there is constriction of capillaries in that area. Thus, blood is directed to the alveoli of neighboring area where gaseous exchange occurs.


Normally in standing position, blood pressure in lower extremity of the body is very high and in upper parts above the level of heart, the pressure is low. This is because of the effect of gravitational force.

A similar condition is observed to some extent in lungs also. Pulmonary vascular pressure varies in

different parts of the lungs:

i. Apical Portion – Zone 1

Normally, in the apical portion of lungs, pulmonary capillary pressure is almost same as alveolar pressure.

So, the pulmonary arterial pressure is just sufficient for flow of blood into alveolar capillaries. However, if

pulmonary arterial pressure decreases or if alveolar pressure increases, the capillaries are collapsed. This prevents flow of blood to alveoli. So, this zone of lung is called area of zero blood flow.

Under these conditions, there is no gaseous exchange in this zone of lungs. So, it is considered as the

part of physiological dead space, which is ventilated but not perfused. And, the ventilation-perfusion ratio

increases. It may lead to growth of bacteria, particularly tubercle bacilli making this part of lungs susceptible for tuberculosis.

 Midportion – Zone 2

In the midportion of lungs, the pressure in alveoli is less than pulmonary systolic pressure and more than

the pulmonary diastolic pressure. Because of this, the blood flow to the alveoli increases during systole and decreases during diastole. So, this zone of the lung is called area of intermittent flow. Ventilation-perfusion ratio is normal.

iii. Lower Portion – Zone 3

In the lower portion of lungs, the pulmonary arterial pressure is high and it is more than alveolar pressure

both during systole and diastole. So the blood flows continuously. Hence, this part of lungs is called area

of continuous blood flow. Ventilation-perfusion ratio decreases because of increased blood flow

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