Pulmonary circulation

Objectives and anatomical background

Pulmonary circulation has historically and physiologically formed to fulfill two key tasks in the human body. First, it is the creation of the most efficient gas exchange between the air entering the alveoli and the blood flowing through the capillaries. Second, it is the provision of natural prevention, i.e., the performance of a specific protective function.

The functioning of this system has a crucial physiological prerequisite related to the structure of the organ itself. The porous, airy tissue of the lung creates virtually no natural mechanical support for the vascular wall. If hemodynamics here operated according to the laws of the systemic circulation, high pressure would easily lead to capillary rupture. Precisely to avoid the rupture of delicate vessels, the pulmonary circulation has acquired a number of unique hemodynamic features.

Features of lesser circulation hemodynamics

Pulmonary circulation hemodynamics differs radically from systemic blood flow. The main feature lies in the specific pressure-dependent resistance. When intravascular pressure begins to rise, the lumen of the pulmonary vessels compensates by increasing. Due to this general expansion of the vascular bed, the total resistance to blood flow in the lungs naturally decreases.

This feature forms the basis of the crucial adaptive mechanism during physical exercise. When a person starts moving actively, blood flow increases sharply. In response to this, a simultaneous and rapid decrease in lesser circulation vascular resistance occurs. Such a reaction reliably prevents an excessive and potentially dangerous pressure rise both in the pulmonary trunk and in the smaller vessels of the lungs.

Also, gravity has a huge impact on pulmonary blood flow. In a person in an upright position, the magnitude of blood flow is distributed unevenly: it gradually decreases in the direction from bottom to top. That is, the basal (lower) parts of the lungs are perfused much more abundantly than their apices.

Parin's reflex and metabolism

With a sharp, critical increase in pressure in the pulmonary trunk, the protective Parin's reflex is triggered. This reflex reaction includes three mandatory components aimed at unloading the lesser circulation:

• The onset of pronounced bradycardia (decreased heart rate).
• Emergency redistribution of blood volumes into the systemic circulation.
• Pooling of excess blood.

As for the organ's own needs, lung tissue metabolism is characterized by high efficiency. To maintain its vital activity and the operation of all described mechanisms, the lung tissue itself consumes very few resources—no more than 10% of the total volume of oxygen inhaled by a person.

Blood flow regulation and the effect of nitric oxide

Adequate blood flow in the lesser circulation is strictly controlled and depends on three main regulatory factors:

1. Mechanical (so-called ventilation) regulation.
2. Nervous regulation.
3. Metabolic regulation.

Among metabolic factors, nitric oxide (NO) occupies a special place. It can be produced directly in the vascular endothelium or come from outside (inhaled NO). The mechanism of its action is that nitric oxide penetrates the smooth muscles of the pulmonary vessels and increases the content of cyclic guanosine monophosphate (cGMP) there. The physiological effect of this biochemical cascade is sustained dilation of the pulmonary vessels, which improves perfusion and reduces the load on the right side of the heart.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanical (ventilation) regulation of pulmonary vessels
• Nervous regulation of lesser circulation hemodynamics
• Metabolic regulation of pulmonary circulation
• Natural preventive (protective) function of the lungs

Other topics in "Vascular Bed Hemodynamics"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.