Pulmonary circulation

Pressure parameters and gradient

Despite the fact that the right and left ventricles of the heart pump the exact same stroke volume, the pressure in the pulmonary circulation is 5–7 times lower than in the systemic circulation vessels of the same diameter. Blood movement through the pulmonary vessels is driven by a small pressure gradient, which is only 10–12 mm Hg.

The distribution of parameters in different parts of the vascular bed is as follows:

• In the pulmonary trunk: in the systolic phase it reaches 25–30 mm Hg, and in diastole it drops to 5–10 mm Hg.
• In the pulmonary capillary network: maintained at about 7 mm Hg.
• In the large pulmonary veins and left atrium: drops to 4–8 mm Hg.

To buffer sharp pressure increases in the pulmonary trunk, the pulmonary vascular bed is equipped with special arteriovenous anastomoses (shunts). They allow excess blood to be diverted, bypassing the capillary network when necessary.

Volumetric flow rate and dual blood supply

The volume of blood passing through the pulmonary circulation equals the cardiac output (CO) of the entire heart. At rest, from 3.5 to 5.5 L/min flows through the lungs. During intense physical exertion, this indicator can surge dramatically up to 30–40 L/min.

A unique feature of lung tissue is the presence of two circulatory systems performing fundamentally different tasks:

1. Pulmonary circulation — responsible exclusively for the specific gas exchange function.
2. Systemic circulation — provides trophic support (nutrition) for the lungs themselves. This role is assigned to the bronchial arteries branching from the thoracic aorta.

Interestingly, these two networks are not completely isolated from each other: bronchopulmonary anastomoses exist between the bronchial and pulmonary arteries.

Dependence of blood flow on ventilation and respiration

The intensity of pulmonary blood flow changes cyclically, closely responding to respiratory phases: it increases during inspiration and decreases during expiration.

The basis of mechanical (ventilatory) regulation is the ventilation-perfusion relationship. Blood is directed primarily to areas that are intensively ventilated. If ventilation in a particular lung zone is reduced or completely absent, blood flow there drops sharply or ceases.

The cascade of reactions during a forced inspiration is particularly illustrative:

1. Pressure in the thoracic cavities and venae cavae decreases significantly.
2. The resulting suction effect increases venous return (blood influx to the right heart).
3. Consequently, the systolic pressure in the right ventricle and pulmonary trunk increases.
4. The pressure gradient between the pulmonary trunk and the left atrium increases.
5. As a result, the total blood flow through the pulmonary circulation increases substantially.

Briefly: answers to topical questions

What else to study on this topic

• Detailed hemodynamic parameters in systole and diastole
• Functional significance of arteriovenous anastomoses in the lungs
• Hemodynamic effects of forced inspiration and venous return
• Role of bronchopulmonary anastomoses
• Mechanisms coupling pulmonary ventilation and local perfusion

Other topics in "Vascular Bed Hemodynamics"

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