Structural and Functional Unit
The microvascular bed is not just a chaotic network of small tubes, but a complex functional module. This module includes a strict sequence of microvessels:
- Arteriole — delivers arterial blood to tissues.
- Precapillary arteriole (precapillary) — controls entry into the capillary network. Blood flow here is regulated by vasomotions — slow, rhythmic oscillations of the lumen.
- Capillary — the site of primary transcapillary exchange.
- Postcapillary venule (postcapillary) — collects spent blood.
- Venule — drains blood from the tissue into the venous system.
Additionally, arteriovenous anastomoses exist, allowing blood to bypass the capillary bed and shunt directly into veins.
Biophysics of Microvessels: Laplace's Law
Despite having microscopic wall thickness, capillaries withstand high pressures without rupturing. This biophysical puzzle is explained by Laplace's law.
The equation is: T = Pr, where:
- T is vessel wall tension;
- P is transmural pressure;
- r is vessel radius.
The clinical significance of this law is immense: wall tension is directly proportional to the radius. Because the capillary radius is extremely small, its wall tension at normal arterial pressure is 12,000 times lower than that of the massive aorta.
Classification of Capillaries
Depending on endothelial wall structure and permeability, all capillaries are divided into three types:
- Continuous (Somatic). Endothelial cells form a solid, uninterrupted layer. These vessels permit the passage of only water and dissolved solutes. They predominate in skeletal and smooth muscle, skin, lungs, central nervous system, adipose tissue, and connective tissue.
- Fenestrated (Visceral). Their main feature is the presence of specific pores (fenestrae) that occupy up to 25% of the endothelial surface area. They allow the passage of large amounts of water with solutes, as well as macromolecules. They are localized in renal glomeruli, endocrine glands, and the intestinal mucosa.
- Sinusoidal. Characterized by a discontinuous endothelial lining and large intercellular gaps. Macromolecules and even formed blood elements pass freely through them. They are found in the liver, spleen, and bone marrow.
Physical Principles of Filtration and Reabsorption
Fluid exchange between the capillary lumen and the interstitial (intercellular) space is driven by pressure gradients (Starling forces). The driving forces are:
- Hydrostatic pressure ($P_{hd}$): the physical pressure of blood against the vessel wall, pushing fluid outward.
- Oncotic pressure ($P_{onc}$): generated by plasma and tissue proteins, acting like a "sponge" that retains water.
At the arterial end of the capillary, hydrostatic pressure is about 40 mmHg, which exceeds the plasma oncotic pressure (which remains stable at around 30 mmHg). Due to the predominance of $P_{hd}$, fluid moves out into the tissue—a process called filtration.
At the venous end, hydrostatic pressure drops to 10 mmHg. Now, the stable oncotic pressure of the blood predominates, "pulling" fluid back in—a process called reabsorption.
Interestingly, modern data challenge the classical view that both processes occur sequentially in every single vessel. Research shows heterogeneity: some capillaries specialize exclusively in filtration, while others specialize exclusively in reabsorption.
Capillary Bed Features in Gas Exchange
Capillaries involved in gas exchange (e.g., in the lungs) merit special mention. They are relatively short and wide (about 17 µm in diameter), which ensures extremely low resistance to blood flow.
The time blood contacts alveolar air at rest is approximately 0.75 seconds. However, during physical exertion, when blood flow accelerates significantly, gas exchange time may drop to 0.33 seconds.