Cutaneous Circulation Physiology Notes

Cutaneous Circulation Architecture Of Cutaneous Blood Vessels

1. The arterioles arising from the smaller arteries reach the base of the papillae of the dermis.
2. After taking origin, the arterioles turn horizontally and give rise to meta-arterioles.
3. From meta-arterioles, hairpin-shaped capillary loops arise. The arterial limb of the loop ascends vertically in the papillae and turns to form a venous limb, which descends down.
4. After reaching the base of the papillae, a few venous limbs of neighboring papillae unite to form the collecting venule.
5. The collecting venules anastomose with one another to form the subpapillary venous plexus.
6. The subpapillary plexus runs horizontally beneath the bases of the papilla and drain into deeper veins.

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Functions Of Cutaneous Circulation

Cutaneous blood flow performs two functions:

• The supply of nutrition to the skin
• The loss of heat from the body and regulation of body temperature.

Normal Blood Flow To the Skin

• Under normal conditions, the blood flow to the skin is about 250 mL/square meter/minute.
• When the body temperature increases, cutaneous blood flow increases up to
  2800 mL/sq. meter/minute because of cutaneous vasodilatation.

Regulation Of Cutaneous Blood Flow

• Cutaneous blood flow is regulated mainly by body temperature.
• Hypothalamus plays an important role in regulating cutaneous blood flow.
• When body temperature increases, the hypothalamus is activated. Hypothalamus in turn causes cutaneous vasodilatation by acting through the medullary vasomotor center.
• Now, blood flow increases in the skin. An increase in cutaneous blood flow causes the loss of heat from the body through sweat.
• When body temperature is low, vasoconstriction occurs in the skin. Therefore, the blood flow to the skin decreases and prevents heat loss from the skin.

Vascular Responses Of Skin To Mechanical Stimuli

Vascular responses of skin are the reactions developed in the blood vessels of the skin when some mechanical stimuli are applied over the surface of it.

The vascular responses of skin are of two types:

1. White reaction
2. Lewis triple response.

Vascular Responses White Reaction

• The white reaction is the response of the blood vessels in the skin to a mechanical stimulus.
• When the surface of the skin is stroked lightly with a pointed object, a pale line appears within 20 seconds.
• This line takes the path of the stroke. This response in the skin is known as the white reaction.
• The maximum intensity of the line is obtained in one minute and it fades away after 5 minutes.
• The white reaction is due to the constriction of cutaneous capillaries. The capillaries constrict because of the local stimulation of the capillary wall and exertion of tension upon the capillary wall. No nervous factor is involved in this process.

Lewis Triple Response

It is the vascular response of the skin that includes three reactions of blood vessels of the skin to a mechanical stimulus.

It was discovered by Lewis Sir Thomas in 1927. He noticed that the vascular reactions of the skin to various injuries occur in three stages and named these reactions as a triple response.

The three reactions to this response are:

1. Red reaction
2. Flare
3. Wheal.

1. Red Reaction

• The red reaction is the appearance of a red line when a pointed instrument is drawn firmly over the surface of the skin.
• This reaction occurs over the line of the stroke The red reaction appears within 15 seconds after the stroke.
• It obtains the maximum intensity at the end of one minute and disappears later gradually.
• The red reaction is because of the dilatation of capillaries due to mechanical stimulus.
• This reaction is purely a local response. It occurs due to the release of histamine-like substances from the tissues damaged by the stimulus. Lewis called it an H substance.
• Red reaction does not depend upon nervous factors. It occurs even after the sectioning or degeneration of the nerves of the skin.

2. Flare

• If the stroke is applied with little more force or if the stroke is repeated on the same line, the red reaction spreads around the line of stroke.
• It spreads for about 10 cm from the line of stroke, depending upon the force applied.
• This is called a flare or spreading flush. Flare appears within 30 seconds after the appearance of the red line.
• It also disappears later. Flare is due to dilatation of arterioles. It depends upon the nervous mechanism and is due to the axon reflex.

Lewis Triple Response Axon Reflex

• Axon reflex or antidromic reflex is the process by which the impulses are conducted in a direction opposite to the normal direction.
• Normally, the impulses produced by a cutaneous pain receptor pass through sensory nerve fiber towards the nerve cell body in the posterior nerve root ganglion.
• Some of these impulses pass through the other branches of the same fiber in the opposite direction and reach the blood vessels supplied by these branches.
• The Impulses now dilate the blood vessels. This is called the axon reflex.
• The nerve fibers the impulses in the opposite direction are edited antidromic vasodilator fibers.
• Flare occurs if the main trunk of nerves is cut. It does not occur when the nerves degenerate.

3. Wheal

• When the intensity of the stimulus is severe, the surface of the skin on the line of stroke is interrupted.
• A small elevation or swelling is seen in the surrounding area up to a height of 2 mm. It is called wheal or local edema.
• Wheal appears within 3 minutes after the stimulus and it replaces the red line.
• The maximum height is obtained within 5 minutes and it disappears after several hours.
• Wheal appears due to the leakage of fluid from capillaries. The permeability of the capillary membrane is increased. Wheal does not depend upon the nervous mechanism.

Lewis Triple Response Dermographism

• This process of embossing signs over the skin is called dermographism. It is also called writing on the skin.
• Some letters or designs can be embossed upon the skin over the back or in the forearm in the same manner by which the wheel is produced.