Microcirculation and transvascular exchange

Blood rheology in capillaries

In the microcirculatory bed, blood behaves as a heterogeneous multicomponent suspension. It consists of formed elements suspended in a solution of lipids, proteins, and electrolytes. The primary factor determining the quality of tissue perfusion is blood viscosity.

Unlike large arteries, in small vessels viscosity directly depends on the flow velocity. A specific phenomenon of its significant reduction is observed here. This is due to the formation of a parabolic flow profile: owing to the transverse pressure gradient, erythrocytes shift from the walls to the center. An axial stream is formed, allowing cells to move through the capillary even faster than whole blood.

Rheological parameters are also influenced by:

• Hematocrit: direct correlation (the more cells, the thicker the blood).
• Acid-base balance: inverse correlation.
• Hypercapnia: excess carbon dioxide increases viscosity, which is why venous blood is always more viscous than arterial blood.

Role of erythrocytes and endothelium

Unimpeded blood movement in the microvasculature is impossible without the special properties of erythrocytes. The absence of a nucleus provides them with high deformability, allowing them to squeeze into capillaries whose diameter is smaller than the cell itself. In addition, a zeta potential (about 35 mV) is maintained on their membrane, causing erythrocytes to repel each other and the vessel wall, preventing aggregation.

The vascular wall itself, or more precisely the endothelium, acts as a massive active organ that generates vasoactive substances.

The secretory profile of the endothelium includes:

• Vasodilators (widen vessels): nitric oxide (NO), endothelium-derived hyperpolarizing factor, prostacyclins, and prostaglandins.
• Vasoconstrictors (narrow vessels): endothelin, angiotensin II, superoxide anion, and vasoconstrictor prostanoids.

Mechanisms of transvascular exchange

The exchange of molecules and fluid between the capillary lumen and tissue cells occurs continuously. Hemodynamic analysis shows that the direction of fluid movement is determined by the vectors of forces acting on the vascular wall.

Four main transport mechanisms are distinguished:

1. Filtration and reabsorption. At the arterial end of the vessel, high pressure promotes the extrusion (filtration) of substances into the tissues. At the venous pole, the resulting pressure drops, which ensures the reuptake (reabsorption) of molecules back into the blood.
2. Diffusion. Based on concentration gradients. This is the main pathway for water and oxygen exchange.
3. Micropinocytosis. An active process in which endothelial cells capture substances using vesicles. Essential for transporting large compounds: glycogen, myoglobin, $\gamma$-globulins.

Vascular wall permeability

The ability of substances to cross the barrier is described by the reflection coefficient ($\sigma$). It illustrates the degree of difficulty for a molecule to pass through. For example, water encounters no resistance ($\sigma = 0$), whereas large proteins such as albumin are practically unable to leave the bloodstream ($\sigma = 1$).

Capillary permeability is not constant; it is regulated by a number of humoral factors:

• Increase permeability: bradykinin, histamine, serotonin.
• Decrease permeability: catecholamines, calcium ions, vitamins C and PP.

Class and exam questions

What else to study on this topic

• Mathematical analysis of force vectors determining fluid filtration and reabsorption.
• Dependence of blood rheology on shifts in the acid-base balance.
• Mechanisms of synthesis and action of the endothelium-derived hyperpolarizing factor.
• Physicochemical nature of zeta potential formation on the erythrocyte membrane.

Other topics in "Vascular Bed Hemodynamics"

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