Vascular Plasticity: Response to Load and Gravity
The vascular system responds dynamically to environmental changes. There are two primary models of functional adaptation:
- Response to reduced load (hypofunction): Occurs, for example, during prolonged muscle immobilization. Adaptation proceeds in two stages. The first is the functional phase, in which smooth myocytes contract, causing arteriolar spasm and temporarily shunting vessels out of the blood flow. This is followed by the structural phase—irreversible degeneration and reduction of redundant capillaries.
- Response to microgravity: In space, hydrostatic pressure disappears, causing blood to redistribute toward the upper body and creating a risk of elevated intracranial pressure. In response, the microcirculatory bed of the head and neck proliferates, increasing the number of capillaries and arteriovenous anastomoses. Their purpose is to pool and vent excess blood volume.
Angiogenesis
Adaptation also manifests as the active sprouting of new vessels (angiogenesis). This process is essential for tissue repair and growth. Intensive angiogenesis accompanies wound healing, where it supports inflammation and subsequent tissue reparation. Furthermore, capillary sprouting is critical during endochondral ossification, where the cartilage model of a future bone is replaced by osseous tissue, requiring an active blood supply.
Topography of *Rete Mirabile*
A rete mirabile is a specialized capillary network positioned between two vessels of the same type (e.g., between two arterioles or two venules). There are three primary localizations of such networks in the human body:
- Hepatic portal system: Blood from GI tract capillaries collects into the portal vein, which then breaks up into a second capillary network inside the liver before draining into the hepatic veins. This is a venous network ensuring the function of the hepatic blood filter.
- Hypophyseal portal system: Hypothalamic capillaries converge into portal veins running to the adenohypophysis, where they branch again into capillaries. This is a venous network for the direct transport of releasing hormones.
- Renal circulation: The renal artery transitions into the afferent arteriole, which forms the glomerular capillaries that converge into the efferent arteriole before branching once more into peritubular capillaries. This is an arterial network. The first network filters urine; the second provides tubular reabsorption.
Key Rule: If the supplying vessel is arterial, the first capillary network is considered the "miraculous" (rete) network. If the supplying vessels are venous, the second network is considered the "miraculous" one.
Geometric Characteristics of the Vascular Bed
Vessel diameters range from 1.5–2 cm (aorta) down to 6–7 µm (capillaries), which is comparable to the size of blood cells. There is a strict inverse relationship between the diameter of an individual vessel and the total cross-sectional area of all vessels of that type.
The narrow aorta has a cross-sectional area of only 2–3 cm², whereas the total surface area of all capillaries reaches ~2,000 cm². Hemodynamic principles dictate that the exact same volume of blood per unit time must flow through both the aorta and this giant capillary "lake." This difference in cross-sectional area produces the necessary deceleration of blood flow velocity within the microcirculation to allow for efficient metabolic exchange.
Interestingly, the metabolic exchange segment within a capillary represents a tiny fraction of the total blood pathway, which averages about 240 cm through the systemic circulation.