Any of the small muscles attached to hair follicles in mammals; when the muscles contract they cause the hairs to stand on end.
The integumentary system functions in thermoregulation—the ability of an organism to keep its body temperature within certain boundaries—even when the surrounding temperature is very different. This process is one aspect of homeostasis: a dynamic state of stability between an animal's internal and external environment.
The skin assists in homeostasis (keeping different aspects of the body constant, e.g., temperature). It does this by reacting differently to hot and cold conditions so that the inner body temperature remains more or less constant.
The skin is an incredibly large organ. It is about 2 meters squared (depending on
the size of the individual). Owing to its location at the barrier of the environment and our internal
selves, and its relatively very large surface area, it is plays an
incredibly important role in thermoregulation.
This is because in a healthy individual, when
all else is held equal, their body is constantly generating heat as a result of
its various metabolic and physical processes.
At rest, such an individual is expected to increase their body
temperature by 1 C every 5 minutes as a result of these processes. Left unregulated, this would kill a person quite quickly.
The process of skin-based thermoregulation occurs through several means. The first way involves
the abundance of blood vessels found in
the dermis, the middle layer of the skin. If the body must cool down, the body vasodilates these blood vessels.
Vasodilation refers to the process of
expanding (-dilation) the size of the blood vessels (vaso-). The now enlarged peripheral vessels of the
skin allow for greater amounts of blood to flow near the surface of the skin. This allows for our body to release a lot of body
heat through radiation. Radiation, in
this case, refers to thermal radiation, which is the process of transferring
heat through space via electromagnetic waves.
At the same time, if a fluid such as circulating air or
water in a pool comes into contact with the skin when we are very hot, this
will allow for heat loss through the process of convection. The higher the amount of our body surface
exposed to this (usually) circulating air (e.g. as little clothing as
possible), the higher the speed of the circulating air (e.g. it’s really
windy), and the smaller the distance between the skin surface and the blood
vessels, the greater the loss of heat from our body via convection.
If our skin touches a cold object (like a cold drink), then
we will lose heat via the process of conduction, which is the direct heat
transfer of heat from a hotter surface, to a colder surface touching that
The body also thermoregulates via the
process of sweating (perspiration). Roughly
speaking, sweating begins when the body temperature rises above 37 C. Sweat
production can be increased or lowered as necessary.
For instance, if we must cool down, sweat production
increases. As drops of sweat form on and
then evaporate from our skin surface, they take body heat away with them. All else held equal, the greater the skin
surface area and the higher the sweat rate, the greater the rate of cooling via
With respect to body heat loss, the processes of radiation
and convection are most effective when the environmental temperature is below
20 C, while evaporative cooling accounts for the most heat loss when the environmental
temperature is above 20 C, and especially when it’s hotter than 35 C.
Increased humidity, however, limits the
ability of our body to dissipate heat via perspiration.
Arrector Pili Muscles
The hairs on the skin lie flat and prevent heat from being trapped by the layer of still air between the hairs. This is caused by tiny muscles under the surface of the skin, called arrector pili muscles.
When these muscles relax their attached hair follicles are not erect. These flat hairs increase the flow of air next to the skin and increase heat loss by convection. The exact extent to which this process help keep us cool is debated (read below).
The Skin's Role in Keeping Us Warm
On the other hand, if the body needs to prevent the loss of
excess heat, such as on a cool day, it will end up constricting the blood
vessels of our skin. This process is
known as vasoconstriction.
blood vessels are narrower than they were before, less blood flows through the
skin and thus less heat can escape into the environment via radiation,
convection, and conduction. The body
will also limit or stop the process of sweating to minimize any evaporative heat
In addition, our body thermoregulates
using our hair.
The arrector pili muscles contract (piloerection) and lift the hair follicles upright. This makes the hairs stand on end, which acts as an insulating layer, trapping heat. This is also how goose bumps are caused, since humans don't have very much hair and the contracted muscles can easily be seen.
While this hair-based method of thermoregulation is effective
in many mammals and birds owing to their large and thick amounts of fur and feathers
(respectively), the relative effectiveness of this method of thermoregulation
in humans is in question since we have little to no body hair in comparison.
Finally, while technically not a thermoregulatory mechanism,
the fat associated with our skin does help insulate our body and therefore
increases body temperature as a result.