Pressure Parameters and Gradient
Although the right and left ventricles of the heart pump the exact same stroke volume, the pressure in the pulmonary circuit is 5–7 times lower than in systemic vessels of the same diameter. Blood movement through the pulmonary vessels is driven by a small pressure gradient of only 10–12 mmHg.
Pressure distribution across different segments of the vascular bed is as follows:
- In the pulmonary trunk: systolic pressure reaches 25–30 mmHg, and diastolic pressure drops to 5–10 mmHg.
- In the pulmonary capillary network: maintained at around 7 mmHg.
- In large pulmonary veins and the left atrium: drops to 4–8 mmHg.
To buffer sharp pressure spikes in the pulmonary trunk, the pulmonary vascular bed is equipped with specialized arteriovenous anastomoses (shunts). These allow excess blood to bypass the capillary network when necessary.
Volumetric Flow Rate and Dual Blood Supply
The volume of blood passing through the pulmonary circuit equals the cardiac output (CO) of the entire heart. At rest, 3.5 to 5.5 L/min flow through the lungs. During intense physical exertion, this value can avalanche up to 30–40 L/min.
A unique feature of lung tissue is the presence of two circulatory systems performing fundamentally different tasks:
- Pulmonary circulation — exclusively responsible for specific gas exchange.
- Systemic circulation — provides trophic support (nutrition) to the lungs themselves via bronchial arteries branching from the thoracic aorta.
Interestingly, these two networks are not completely isolated: 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, responding sensitively to respiratory phases: it increases during inspiration and decreases during expiration.
The basis of mechanical (ventilatory) regulation is ventilation-perfusion coupling. Blood is directed primarily to well-ventilated areas. If ventilation in a specific lung zone is reduced or absent, blood flow there drops sharply or ceases.
The reaction cascade during a forced inspiration is particularly notable:
- Pressure in the thoracic cavity and venae cavae decreases significantly.
- The resulting suction effect increases venous return (blood inflow to the right heart).
- Consequently, systolic pressure in the right ventricle and pulmonary trunk increases.
- The pressure gradient between the pulmonary trunk and the left atrium increases.
- As a result, total blood flow through the pulmonary circulation increases substantially.