Blood Rheology in Capillaries
In the microvasculature, blood behaves as a heterogeneous multi-component suspension consisting of formed elements suspended in a solution of lipids, proteins, and electrolytes. Blood viscosity is the primary factor determining the quality of tissue perfusion.
Unlike large arteries, viscosity in small vessels is directly dependent on flow velocity. A distinct phenomenon of significant viscosity reduction occurs here due to the formation of a parabolic flow profile: because of transverse pressure gradients, erythrocytes shift from the walls toward the center. This creates an axial stream, allowing cells to move through the capillary even faster than whole blood.
Rheological parameters are also influenced by:
- Hematocrit: direct relationship (more cells equal thicker blood).
- Acid-base balance: inverse relationship.
- Hypercapnia: an excess of 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 specialized properties of erythrocytes. The absence of a nucleus grants them high deformability, allowing them to squeeze through capillaries with a diameter smaller than the cell itself. Additionally, their membranes maintain a zeta potential (approximately 35 mV), which causes erythrocytes to repel each other and the vascular wall, preventing aggregation.
The vessel wall itself, specifically the endothelium, functions as a massive active organ generating vasoactive substances.
The endothelial secretory profile 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 vessel wall.
Four main transport mechanisms are recognized:
- 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 net pressure drops, driving the reuptake (reabsorption) of molecules back into the blood.
- Diffusion. Based on concentration gradients, this is the primary pathway for water and oxygen exchange.
- Micropinocytosis. An active process in which endothelial cells engulf substances using vesicles. It is essential for transporting large compounds such as glycogen, myoglobin, and $\gamma$-globulins.
Vascular Wall Permeability
The ability of substances to cross the barrier is described by the reflection coefficient ($\sigma$), which illustrates the degree of restriction to molecular passage. For example, water encounters no resistance ($\sigma = 0$), whereas large proteins like albumin are practically unable to leave the vascular lumen ($\sigma = 1$).
Capillary permeability is not constant and is regulated by various humoral factors:
- Increase permeability: bradykinin, histamine, serotonin.
- Decrease permeability: catecholamines, calcium ions, vitamins C and PP.