Mechanisms of blood propulsion

Main driving forces in the arterial bed

In the arterial limb of the vascular system, blood flow is initiated and maintained by three key factors.

First, there's the work of the heart, which performs the function of a pump. It is the systolic ejection of blood into the aorta that creates the main pressure gradient propelling blood forward (this mechanism is traditionally referred to by the term vis a tergo — "force from behind").

Second, the elasticity of the walls of large arteries is of tremendous importance. Elastic-type vessels (primarily the aorta) act as "compression" chambers. The so-called Windkessel effect occurs: stretching during systole, they store energy, and during diastole, they recoil, converting the pulsatile cardiac output into a continuous blood flow and maintaining blood pressure.

Third, the movement of blood portions is influenced by gravity (hydrostatic pressure and gravitational force). This factor is particularly relevant for the downward direction—it facilitates the flow of blood into the vessels of the trunk and lower extremities.

Mechanisms of venous return

The movement of blood in the venous limb, i.e., its return back to the heart, requires overcoming gravity and is provided by a whole cascade of auxiliary factors.

• "Muscle pump" and valve apparatus. In the lower extremities, the contraction of skeletal muscles mechanically compresses the thin-walled veins, squeezing blood upward. The presence of venous valves strictly prevents the retrograde (backward) flow of blood.
• "Respiratory pump" (suction force of the thoracic cavity). During inspiration, intrathoracic pressure decreases, becoming more negative, while intra-abdominal pressure, conversely, increases. This creates a powerful pressure gradient: blood is literally sucked from the venae cavae into the expanding intrathoracic veins.
• Abdominal organ pressure and peristalsis. An increase in intra-abdominal pressure, as well as rhythmic contractions of the intestine (peristalsis), exert a mechanical effect on the abdominal veins, additionally propelling blood toward the heart.
• Gravitational factor. For veins located above the level of the heart (from the head and neck, jugular veins), gravity acts as an ally, facilitating the flow of blood downward to the atria. In this case, venous pressure here decreases by the magnitude of the hydrostatic pressure and may even drop below atmospheric pressure. For veins below the heart, hydrostatic pressure is added to the pressure generated by the heart.
• Capillary forces. Surface tension forces at the level of the microvasculature make a certain contribution.

Suction action of the heart itself

The heart not only pushes blood but also actively "pulls" it toward itself. At the moment of right atrial diastole, the pressure within it drops sharply (down to –5 mm Hg), drawing blood along from the lower-lying venae cavae.

At the final stage of venous return, the "atrial pump" comes into play. The mechanism of its action is based on the fact that during ventricular systole, the plane of the atrioventricular septum shifts. This displacement creates an additional suction effect, known as vis a fronte ("force from the front"), which maximally facilitates the inflow of new portions of venous blood into the atria.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of the Windkessel effect in the aorta and large arteries
• Driving forces vis a tergo and vis a fronte
• Influence of hydrostatic pressure on veins above and below heart level
• Blood flow regimes through the vascular bed
• Function of the lower extremity "muscle pump"

Other topics in "Vascular Bed Hemodynamics"

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