Blood circulation and neurological structures in the skin — expanded notes
Vascular organisation and microcirculation
The cutaneous circulation is arranged as a network of interconnected vessels rather than a simple terminal supply. Perforating arteries from deeper tissues enter the subcutis and form a deep cutaneous plexus close to the dermis–hypodermis interface. Ascending arterioles supply a superficial plexus in the papillary dermis. From this plexus, capillary loops enter dermal papillae. Venules drain the capillary beds and join superficial and deep venous networks.
The epidermis contains no blood vessels. Oxygen and nutrients diffuse from the capillary loops through the basement membrane and viable epidermis. This explains why epidermal viability depends on dermal perfusion and why severe vascular compromise delays re-epithelialisation and wound healing.
Regulation of skin blood flow
Sympathetic adrenergic vasoconstriction: reduces flow during cold exposure, haemorrhage or stress and limits heat loss.
Active vasodilatation: increases blood flow during heat exposure and exercise; local endothelial mediators, nitric oxide and neurovascular mechanisms contribute.
Local control: temperature, tissue metabolites, inflammatory mediators and endothelial function modify vascular tone.
Arteriovenous anastomoses: particularly abundant in glabrous acral skin; they allow rapid shunting of blood and are important in heat exchange.
Cutaneous innervation in more detail
Sensory axons have cell bodies in dorsal root ganglia. Their peripheral branches terminate as free endings or specialised receptors in the skin. Central branches enter the spinal cord and transmit information through pathways for pain/temperature and discriminative touch/vibration. The somatosensory cortex allows localisation and interpretation of sensation.
A-delta fibres are thinly myelinated and mediate relatively rapid, sharp pain and cold. C fibres are unmyelinated and mediate slow burning/aching pain, warmth and itch. A-beta fibres are larger and myelinated and convey touch, pressure and vibration.
Autonomic innervation
Postganglionic sympathetic fibres regulate cutaneous vessels, eccrine sweat glands and arrector pili muscles. Eccrine sweating is sympathetic but predominantly cholinergic, an important physiological exception. Apocrine gland activity is also under autonomic influence.
Clinical examination and interpretation
- Assess colour, temperature, capillary refill, oedema, pulses, hair distribution, ulcers and trophic changes.
- Compare both sides and test sensation in a mapped distribution. Monofilament testing assesses protective sensation; vibration testing can detect large-fibre dysfunction.
- A lesion that blanches with pressure is usually due to intravascular erythema; non-blanching purpura indicates extravasated blood.
- Neuropathic skin may be dry because autonomic denervation reduces sweating. Loss of protective sensation increases risk of unnoticed trauma and ulceration.