Hemodynamics and the Mechanics of Blood Vessel Stability
Factors Determining Blood Flow
The factors that determine blood flow include:
- Fluid properties: Density
- Forces: Pressure
- Vessel structure: Variations in the radius
Structure of Blood Vessels
The role of blood vessel components is to monitor blood flow and achieve vascular stability by varying the radius. They consist of three parts:
- Elastic fibers: Formed by elastin and collagen.
- Smooth muscle fibers: Responsible for contraction and dilation.
- Endothelial tissue: A thin layer located between the vessel wall and the blood that absorbs energy loss caused by friction.
Elasticity of Blood Vessels
Variations in the radius of the vessel are the result of the relationship between two forces:
Transmural Pressure and Radial Forces
Transmural pressure acts from the inside to the outside of the vessel. These are distensive radial forces (FD). Because of the higher pressure inside, each vessel is under tension. There must be a balance between the two forces. The tension (T) (F / L) offsets the transmural pressure to maintain balance. Vascular balance is expressed as Fp = T + T = 2T.
Radial Compression Forces
These forces are due to tension in the walls of blood vessels and act parallel to the circumference of the vessel walls. There are two types of radial compressive forces:
Elastic Tension
This involves parietal elastic fibers. The elastic fiber is subjected to tensile deformation and follows Hooke's law. When a vessel undergoes enlargement, it acquires elastic strain.
Active Tension and Vasomotor Tone
In physiology, this is known as vasomotor tone. Tension (T = F / S) represents the force per unit length. It originates in the smooth muscle fibers surrounding the blood vessel walls and depends on:
- Nervous System (SN) activity: This involves the constant consumption of metabolic energy.
- It does not depend on the radius of the association or the elastic properties of the material.
Stability of the Blood Vessels
Vascular balance is required to offset the forces Fp and Fc. We can analyze two specific scenarios:
- Vessels formed only by elastic fibers: A vessel of this type tends to restore its equilibrium position after suffering a contraction or expansion.
- Vessels formed only of smooth muscle: Without elastic strain, the vessel would collapse or break due to the absence of elastic fibers.