Main Driving Forces in the Arterial Bed
In the arterial segment of the vascular system, blood flow is initiated and maintained by three key factors.
First, there is the action of the heart, which functions as a pump. Systolic ejection of blood into the aorta creates the primary pressure gradient that pushes blood forward (a mechanism traditionally designated by the term vis a tergo — "force from behind").
Second, the elasticity of large artery walls is of immense importance. Elastic-type vessels (primarily the aorta) act as "compression" chambers. The so-called Windkessel effect occurs: by stretching during systole, they store energy, and during diastole, they recoil, converting pulsating cardiac output into a continuous blood flow and maintaining arterial blood pressure.
Third, blood movement is influenced by gravity (hydrostatic pressure and the force of gravity). This factor is especially relevant in the downward direction, facilitating blood delivery to the vessels of the trunk and lower extremities.
Mechanisms of Venous Return
Blood movement in the venous segment—its return back to the heart—requires overcoming the force of gravity and is supported by a cascade of auxiliary factors.
- "Muscle pump" and valve apparatus. In the lower extremities, skeletal muscle contraction mechanically compresses thin-walled veins, squeezing blood upward. The presence of venous valves strictly prevents retrograde (backward) blood flow.
- "Respiratory pump" (thoracic suction). During inspiration, intrathoracic pressure decreases, becoming more negative, whereas intra-abdominal pressure increases. This creates a powerful pressure gradient: blood is literally sucked from the caval veins into the expanding intrathoracic veins.
- Abdominal cavity pressure and peristalsis. Increased intra-abdominal pressure, as well as rhythmic contractions of the intestines (peristalsis), exerts mechanical pressure on the abdominal veins, additionally propelling blood toward the heart.
- Gravitational factor. For veins located above the heart level (jugular veins of the head and neck), gravity acts as an ally, facilitating blood flow downward toward the atria. Here, venous pressure decreases by the magnitude of hydrostatic pressure and can even drop below atmospheric pressure. For veins below the heart, hydrostatic pressure adds to the pressure generated by the heart.
- Capillary forces. Surface tension forces at the level of the microcirculation contribute to a certain extent.
Suction Action of the Heart
The heart not only pushes blood but also actively "pulls" it. During right atrial diastole, pressure drops sharply (down to –5 mmHg), drawing blood from the underlying venae cavae.
The "atrial pump" is activated during the final stage of venous return. Its mechanism is based on the displacement of the atrioventricular plane during ventricular systole. This displacement creates an additional suction effect, known as vis a fronte ("force from in front"), which maximally facilitates the entry of new portions of venous blood into the atria.