Vascular plasticity: response to loads and gravity
The vascular system responds sensitively to environmental changes. Two main models of functional adaptation are distinguished:
- Response to decreased load (hypofunction): Occurs, for example, during prolonged muscle immobility. Adaptation proceeds in two stages. The first is the functional phase, during which smooth myocytes contract, causing arteriolar spasm and the temporary exclusion of vessels from the blood flow. This is followed by the structural phase—irreversible degeneration and reduction of "excess" capillaries.
- Response to weightlessness: In space, hydrostatic pressure disappears, causing blood to redistribute to the upper half of the body, creating a risk of high intracranial pressure. In response, the microvasculature of the head and neck proliferates: the number of capillaries and arteriovenous anastomoses increases. Their purpose is to pool and offload the excess blood volume.
Angiogenesis
Adaptation also manifests as the intensive sprouting of new vessels (angiogenesis). This process is essential for tissue regeneration and growth. Intensive angiogenesis accompanies wound healing, where it provides for inflammation and subsequent repair. Furthermore, capillary sprouting is critically important in indirect osteogenesis, when the cartilaginous model of a future bone is replaced by bone tissue, requiring an active blood supply.
Topography of "wonderful nets" (Rete mirabile)
A "wonderful net" (rete mirabile) is a specific capillary network located between two vessels of the same type (e.g., between two arterioles or two venules). Three main localizations of such networks are distinguished in the human body:
- Hepatic portal system (liver): Blood from the GI tract capillaries collects into the portal vein, which then breaks up into a second capillary network inside the liver, and only then drains into the hepatic veins. This is a venous network that ensures the function of the hepatic barrier.
- Hypophyseal portal system: Hypothalamic capillaries converge into portal veins running to the pituitary gland, where they branch again into capillaries. This is a venous network for the direct transport of regulatory hormones.
- Renal blood flow system: The renal artery leads into the afferent arteriole, which forms glomerular capillaries that collect into the efferent arteriole, which then breaks up into peritubular capillaries around the tubules. This is an arterial network. The first network filters urine, while the second ensures reabsorption.
Important rule: if the vessel is arterial, the first capillary network is considered "wonderful". If venous, the second network.
Geometrical characteristics of the vascular bed
Vessel diameter varies from 1.5–2 cm (aorta) 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-section of only 2–3 cm², whereas the total area of all capillaries reaches ~2000 cm². The law of hemodynamics states that the exact same volume of blood per unit of time must pass through both the aorta and the giant "lake" of capillaries. It is this difference in cross-sectional area that provides the necessary slowdown of blood flow velocity in the microvasculature for effective metabolic exchange.
Interestingly, the metabolic exchange segment in a capillary is a tiny fraction of the total blood pathway, which averages about 240 cm in the systemic circulation.
How to remember the "wonderful net" rule: In the Arterial system (kidneys), the First network is wonderful (A–F [Arterial–First]). In the Venous system (liver, pituitary), the Second network is considered wonderful (V–S [Venous–Second]).
Class and exam questions
What happens to blood vessels during prolonged lack of muscular activity?
First, the functional phase occurs (arteriolar spasm and exclusion of capillaries from blood flow), followed by the structural phase—complete degeneration of non-functioning vessels.
Why might astronauts experience elevated intracranial pressure in space?
In weightlessness, gravitational hydrostatic pressure disappears, causing blood to redistribute to the upper half of the body and rush to the head.
Where is the arterial "wonderful net" located and what is its purpose?
It is located in the kidneys between the afferent and efferent arterioles. The first capillary network is responsible for blood filtration, while the second handles the reabsorption of substances from the primary urine.
What else to study on this topic
- Morphology of arteriovenous anastomoses
- Histological structure of hepatic plates and sinusoidal capillaries
- Mechanism of hormone transport through the hypophyseal portal system
- Structure of the renal corpuscle filtration barrier
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Blood vessel adaptation and plasticity" and 3,485 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Cardiovascular system: arteries, microvasculature"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.