The vasa vasorum and nervi vasorum are the nutrient and neural supply systems for the walls of large blood vessels. Understanding these structures is essential for comprehending vascular disease and its treatment.
Vasa Vasorum
Vasa vasorum (vessels of the vessels) are the network of small arteries, capillaries, and veins that supply and drain the walls of large blood vessels.
Definition
The vasa vasorum is a microvascular network that nourishes the outer layers (adventitia and outer media) of blood vessels > 1 mm in diameter. The inner layers are nourished by diffusion from the luminal blood.
Distribution
Presence by vessel type:
| Vessel Type | Vasa Vasorum |
|---|---|
| Large elastic arteries | Extensive (aorta, pulmonary artery) |
| Medium muscular arteries | Present (in larger specimens) |
| Small arteries | Absent |
| Arterioles | Absent |
| Capillaries | Absent |
| Large veins | Extensive (more than in arteries) |
| Medium veins | Present |
| Small veins | Absent |
Within the vessel wall:
- Adventitia: Dense network (vasa vasorum externa)
- Outer media: Capillaries penetrate (vasa vasorum interna)
- Inner media: No vasa vasorum (avascular, supplied by luminal diffusion)
- Intima: No vasa vasorum
Origin
Arterial supply:
- Aorta: From aortic branches (intercostal, lumbar, bronchial)
- Large arteries: From the parent artery or nearby branches
- Coronary arteries: From the coronary lumen (proximal) and from cardiac branches
Venous drainage:
- Large veins: Adjacent veins or the parent vein
- Aortic: Azygos, hemiazygos, intercostal, lumbar veins
Anatomy
Vasa vasorum externa:
- Network in the adventitia
- Derived from external vessels
- Form a longitudinal plexus
Vasa vasorum interna:
- Capillaries that penetrate the media
- Arise from the external plexus
- Extend into the outer half of the media
- Anastomose with each other
Microvascular Architecture
Arterial supply:
- Small arteries enter the adventitia
- Branch into arterioles
- Form a capillary network
- Drain into venules and veins
Capillary characteristics:
- Fenestrated (more permeable)
- Continuous basement membrane
- Pericyte coverage
- High surface-to-volume ratio
Functions
| Function | Mechanism |
|---|---|
| Nutrition | Oxygen and nutrient delivery to the outer vessel wall |
| Waste removal | CO2 and metabolite removal |
| Thermoregulation | Heat dissipation from the vessel wall |
| Immune surveillance | Lymphocyte and macrophage trafficking |
| Vascular remodeling | Growth factor delivery, matrix turnover |
| Plaque neovascularization | Vasa vasorum proliferation in atherosclerosis |
Vasa Vasorum in Disease
Atherosclerosis:
- Vasa vasorum density increases in atherosclerotic arteries
- Neovascularization from adventitial vasa
- Intraplaque hemorrhage from fragile new vessels
- Plaque progression and instability
- Plaque rupture associated with vasa density
Aortic dissection:
- Vasa vasorum rupture can initiate intramural hematoma
- Impaired vasa flow contributes to medial degeneration
- Vasa neovascularization in cystic medial degeneration
Vasculitis:
- Vasa vasorum involvement in giant cell arteritis
- Granulomatous inflammation of the vasa
- Medial necrosis from impaired vasa flow
Aortic aneurysm:
- Reduced vasa density in aneurysmal aortic wall
- Medial ischemia and weakening
- Vasa neovascularization in the thrombus
Aging and Vasa Vasorum
- Increased vasa density with age
- Tortuosity and dilation of vasa vessels
- Reduced perfusion efficiency
- Impaired angiogenesis in response to ischemia
Vascular Innervation
Blood vessels receive extensive innervation from the autonomic nervous system and sensory nerves.
Types of Vascular Nerves
| Nerve Type | Function | Location |
|---|---|---|
| Sympathetic (vasoconstrictor) | Norepinephrine release, alpha-1 activation | Most arteries and veins |
| Sympathetic (vasodilator) | Epinephrine, beta-2 activation | Skeletal muscle arteries |
| Parasympathetic | Acetylcholine, NO-mediated vasodilation | Limited (erectile tissue, salivary glands) |
| Sensory | Pain, mechanoreception, chemoreception | All vessels (periadventitial) |
| Peptidergic | Substance P, CGRP, VIP | Plexus in adventitia |
Sympathetic Innervation
Anatomy:
- Preganglionic: Intermediolateral cell column (T1-L2)
- Postganglionic: Sympathetic chain ganglia
- Distribution: Adventitial-medial junction
- Density: Higher in arteries than veins
Neurotransmitters:
- Norepinephrine: Vasoconstriction (alpha-1 receptors)
- Neuropeptide Y: Potentiation of norepinephrine
- ATP: Co-transmitter, vasoconstriction
Receptor distribution:
- Alpha-1: Most vascular smooth muscle
- Alpha-2: Some resistance vessels
- Beta-2: Skeletal muscle arterioles (vasodilation)
Regional variation:
| Bed | Sympathetic Tone | Response |
|---|---|---|
| Skin | High | Strong vasoconstriction |
| Skeletal muscle | Moderate | Vasoconstriction (alpha), vasodilation (beta) |
| Kidney | High | Vasoconstriction, renin release |
| Brain | Low | Weak vasoconstriction |
| Coronary | Moderate | Both constriction and dilation |
| Pulmonary | Low | Weak response |
| Splanchnic | High | Strong vasoconstriction |
Parasympathetic Innervation
Limited distribution:
- Cranial outflow: Facial (VII), glossopharyngeal (IX), vagus (X)
- Sacral outflow: Pelvic splanchnic (S2-S4)
- Mainly to head, pelvic organs, and erectile tissue
Mechanism:
- Acetylcholine -> endothelial NO -> vasodilation
- VIP (vasoactive intestinal peptide) -> direct vasodilation
Sensory Innervation
Sources:
- Dorsal root ganglia
- Trigeminal ganglion
- Vagus nerve (afferent fibers)
Sensory modalities:
| Modality | Receptor | Sensation |
|---|---|---|
| Pain | Nociceptors | Ischemia, inflammation, distension |
| Pressure | Baroreceptors | Blood pressure (carotid sinus) |
| Chemistry | Chemoreceptors | O2, CO2, pH (carotid body) |
| Stretch | Mechanoreceptors | Vessel distension |
Neurovascular Units
The concept of local neurovascular regulation:
- Perivascular nerves interact with endothelial cells, smooth muscle, and adventitial cells
- Tripartite synapse: Nerve terminal, smooth muscle, and endothelial cell
- Local feedback loops regulate blood flow
Nerves in the Vessel Wall
Adventitial plexus:
- Dense network of nerves in the adventitia
- Penetrate to the adventitial-medial junction
- Do not penetrate the inner media
Neuromuscular junction:
- Varicose nerve terminals (no specialized end-plate)
- Neurotransmitter release en passant
- Diffusion to multiple smooth muscle cells
Density of innervation:
- Arteries: Dense innervation
- Veins: Sparse innervation
- Resistance vessels: Moderate innervation
- Capacitance vessels: Sparse innervation
Clinical Significance
Sympathectomy:
- Surgical destruction of sympathetic ganglia
- Used for: Hyperhidrosis, Raynaud disease, complex regional pain syndrome
- Effect: Vasodilation, increased blood flow
Raynaud phenomenon:
- Exaggerated sympathetic vasoconstriction
- Triggered by cold or stress
- Digital artery spasm
- Treatment: Calcium channel blockers, sympathetic blockade
Neurogenic inflammation:
- Substance P, CGRP from sensory nerves
- Vasodilation and plasma extravasation
- Contributes to: Asthma, arthritis, migraine
Baroreflex failure:
- Loss of carotid sinus afferent input
- Labile hypertension
- Importance of carotid body and sinus innervation
Autonomic Control of Specific Vascular Beds
Cutaneous circulation:
- Dense sympathetic innervation
- Thermoregulation: Vasoconstriction (cold), vasodilation (heat)
- Emotional blushing: Vasodilation (facial)
Skeletal muscle circulation:
- Sympathetic vasoconstriction (alpha)
- Sympathetic vasodilation (beta, during exercise)
- Functional sympatholysis: Local metabolites override sympathetic tone
Splanchnic circulation:
- Rich sympathetic innervation
- Major capacitance reservoir
- Venoconstriction mobilizes blood
Cerebral circulation:
- Moderate sympathetic innervation
- Weak vasoconstrictor response
- Autoregulation dominates over neural control
Pulmonary circulation:
- Sparse innervation
- Hypoxic pulmonary vasoconstriction dominates
- Nervous system plays minor role