The cutaneous blood flow is primarily regulated by extrinsic factors, i.e. the sympathetic and parasympathetic nervous system. Another extrinsic factor, exclusive to cutaneous blood flow, is the regulation of blood flow due to thermal changes. Therefore, skin is the only organ whose blood supply is regulated with alterations in temperature.
REGULATION OF BLOOD FLOOD FLOW VIA THE AUTONOMIC NERVOUS SYSTEM (ANS)
The blood vessels of skin are innervated by both PANS and SANS. The receptors present are predominantly α-1 adrenergic receptors which are G-q coupled. α-1 adrenergic receptors, when stimulated, cause vasoconstriction of the blood vessels perfusing the skin. This results in reduced blood flow and hence, reduced blood volume supplied to the cutaneous regions. This vasoconstriction is also accompanied with pressure changes within the cutaneous blood flow. Vasoconstriction in arterioles causes a decrease in downstream pressure and an increase in the upstream pressure with respect to the area of constriction. The significance of these changes lies in the fact that lesser amount of blood is now exposed to a large surface area through which heat exchange is carried out.
REGULATION OF BLOOD FLOW DUE TO CHANGES IN TEMPERATURE
The anterior neurons of hypothalamus are modified to detect changes in temperature. This enables the hypothalamus to work as a thermostat of the body. If the temperature of the body undergoes changes, as happens during fever, the hypothalamus makes some adjustments in the body in order to bring the temperature back to normal (homeostasis). During a fever, the default thermostat of the hypothalamus which is normally set at 37’C, is disturbed and is raised to a higher value. Inflammatory mediators, such as Interleukin-1 and PGE2, play a major role in disturbing this set temperature point. The temperature of the body is raised in order to equalize the newly set temperature by the hypothalamus. The following events occur in the body in order to raise the body’s temperature:
- Shivering: This is caused by rapid contraction and relaxation of the muscles. A large amount of heat is generated during the process. This heat is utilized to raise the temperature of the body.
- Cutaneous Vasoconstriction: The arterioles and the venules supplying the superficial parts of the body begin to constrict. This is mediated by increase in SANS outflow to the superficial vessels. As a result, the fractional volume of blood exposed for heat conduction reduces and minimal amount of heat is dissipated via the superficial vessels. This cutaneous vasoconstriction causes a decrease in cutaneous blood pressure, blood flow and blood volume; however, the velocity of the blood in these cutaneous vessels increases. All these changes ensure that less heat is dissipated to allow for conservation of heat within the core of the body.
- Decreased Sweating: Decreased cholinergic sympathetic activity causes the sweat glands to suppress sweat production. Heat is, therefore, prevented from losing through evaporation of sweat.
- Hair Erection: The contraction of pilorector muscle causes the hair on skin to stand on their ends. These hair trap air which acts as an insulator (air is a poor conductor of heat). Hence, heat loss is minimized.
The opposite of the aforementioned sequence happens when fever is broken and the pyretic (fever causing) agents are neutralized. The hypothalamic set point is normalized and the body’s temperature is brought back to 37’C through the reversal of events described earlier:
- Cutaneous Vasodilation: SANS outflow to cutaneous vessels is reduced. This increases the cutaneous blood flow allowing for more heat dissipation from the superficial vessels. . This cutaneous vasodilation causes an increase in cutaneous blood pressure, blood flow and blood volume; however, the velocity of the blood in these cutaneous vessels decreases. All these changes ensure that more heat is dissipated to allow for less conservation of heat within the core of the body.
- Increased Sweating: Increased cholinergic sympathetic outflow to sweat glands increases perspiration. This sweat, upon evaporation, has a cooling effect on the body.
- Hair erection: The pilorector muscles relax and the cutaneous hair are relaxed. The insulating effect of trapped air is reduced. More heat is lost from the body.