In this article we will discuss about the anatomy of cutaneous circulation in humans with the help of suitable diagram.
Vascular architecture of the skin has the general pattern of the capillary circulation. For the most part, papillae contain capillary blood vessels and nerve endings. The arteries that supply the skin, originate from richly anastomosing irregular plexus (first plexus) of the deepest part of the corium (dermis).
From this cutaneous arterial plexus, the single arteriole arises and ascends through the corium and forms the second plexus just below the dermis. Capillaries arising from this plexus supply the hair follicles and papillae of the dermis. The arterioles also ascend towards the superficial layer and form the third plexus in the sub-papillary region of the dermis. Every papilla gets capillary network from this plexus.
The arterial limb of the capillary-loop ascends in the papillae and turns back to form the venous limb. The venous limb then reaches the base of the papillae and joins with the venous limb of the neighbouring loops to form the collecting venule.
The collecting venules then anastomose with one another and form the so-called sub-papillary venous plexus. This sub-papillary venous plexus runs horizontally at the base of the papillae and – drains in the deeper veins (Fig. 7.110).
Rate of Blood Flow:
Rate of blood flow alters under different conditions like haemorrhage, temperature of the surroundings, metabolic activities of the body, emotional and physical stress of the body. Under ordinary cooled atmosphere, the blood flow is about 0.30 litre/sq. metre of the body surface or total about 400-450 ml per minute. This value is increased during exercise or decreased during fall of surrounding temperature.
Regulation of Blood Flow —Nervous Control:
Considering the temperature control as the main function of the skin, the blood flow through the skin is mainly maintained through the nervous mechanism rather than through auto-regulatory mechanism.
i. Role of Hypothalamus:
Hypothalamus, being the autonomic centres for the sympathetic and the parasympathetic, plays an important role in controlling blood supply of the skin. Besides this, the temperature-regulatory centre is situated at the pre-optic region of the anterior hypothalamus which modifies the skin blood flow in altered body temperature, through the activation of either the sympathetic vasoconstrictor or sympathetic vasodilator pathway.
If the above temperature-regulatory centre is heated, then vasodilatation along with sweating occurs and on the contrary if the same centre is cooled, then vasoconstriction along with cessation of sweating occurs.
ii. Sympathetic Adrenergic Vasoconstrictor Pathways:
These fibres secrete noradrenaline at their nerve endings and when excited, on exposure to cold, profound vasoconstriction occurs. These vasoconstrictor effects are most powerful in the hands, lips nose, ears, etc.
iii. Sympathetic Cholinergic Vasodilator Pathways:
These fibres produce vasodilatation by liberating acetylcholine at their endings. When the body temperature is increased, these fibres are activated causing vasodilatation and sweating. It has been claimed that the sympathetic nerve supply to human fingers exerts a tonic dilator influence on vessels through the release of acetylcholine.
But the basic mechanisms by which the vasodilatation occurs are not clearly known. Recently it has been claimed that the factors which excite the sweat glands may cause formation of a substance bradykinins. This bradykinin is a potent vasodilator. On excitation, sweat glands release a proteolytic enzyme— kallikrein which acts on the decapeptide, bradykinogen forming the nonapeptide, bradykinin.
Vascular Response of Skin:
i. White Line:
In a normal subject, if a light stroke is applied on the skin, a white line appears exactly on the area stroked, after a brief latent period. It persists for a minute or two and then gradually fades away. It occurs even when nerves are cut and allowed to degenerate. Hence, it is due to local constriction of capillaries as a response to the mechanical stimulus.
ii. Triple Response:
In subjects with hypersensitive skin, a moderate stroke elicits the following triple response:
a. Red Line:
A red line appears on the line of stroke after a short latent period, caused by vasodilatation independent of nerves, because it occurs even after section and degeneration of nerves.
b. Flare:
After 15-30 seconds a red flush or flare appears and spreads beyond the area. It is not obtained when peripheral nerves degenerate. It is caused by arteriolar dilatation through antidromic axon reflex.
c. Wheal:
A local oedema develops in the area of stroke and spreads a little beyond, reaching its maximum in about 5 minutes. It is caused by escape of fluid from blood vessels resulting from increased permeability of the latter.
All evidences suggest that triple response is due to the action of histamine, which is liberated in the area as a result of stroke. The red line is due to dilatation of capillaries and venules caused by the direct action of histamine on them. The flare is due to vasodilatation brought about by the action of histamine on the nerves, thus eliciting axon reflex.
The wheal is due to increased permeability caused:
(1) By stagnation of blood flow, and
(2) By the action of histamine on the capillaries.
iii. Direct Effect of Cold on the Skin:
If cold is directly applied on the skin then vasoconstriction occurs. If a finger is immersed in cold water then vasoconstriction occurs very promptly but fluctuation in blood flow occurs in most cases due to alternate vasoconstriction and vasodilatation. This vasodilatation is presumably the cause of local axon reflex and occurs as a protection against cold injury.
Auto-Regulation of Skin Blood Flow:
Regarding the auto-regulation of skin blood flow, evidence for the presence of auto-regulation in skin blood flow is lacking. Barcroft (1968) did not observe any such auto-regulation in the skin of cats and dogs. But the vessels that supply the nutrition to the skin may have some basal tone and auto-regulation. Thus local auto-regulation mechanisms in the skin like other tissues are linked with the nutritional aspects during emergency.
Normal Colour of the Skin:
Colour of the skin is mostly dependent upon the blood flow through the capillary loops and sub capillary plexus. Diameter and degree of engorgement of superficial blood vessels actually determine the depth or intensity of the skin colour.
Light and pale colour of the skin is due to the constriction of blood vessels. Intense scarlet colour is due to increased blood flow and dilated vessels. Deep blue colour is due to decreased blood flow and dilated vessels.