Functions of the Integumentary System

Protection

The skin provides an overlaying protective barrier from the environment and pathogens while contributing to the adaptive immune system.

Learning Objective

Describe the ways in which the integumentary system protects the body

Key Takeaways

Key Points

  • The skin provides a protective barrier from the external environment and prevents dehydration.
  • Langerhans cells in the skin also contribute to protection as they are part of the adaptive immune system.
  • The integumentary system protects the body’s internal living tissues and organs, protects against invasion by infectious organism, and protects the body from dehydration.

Key Terms

  • vitamin D: An important vitamin synthesized thanks to the skin.
  • melanocytes: Cells that help protect our body from radiological damage.
  • Langerhans cells: Langerhans cells are dendritic cells (antigen-presenting immune cells) of the skin and mucosa that contain large granules.

The skin helps protect our body’s internal structures from physical, chemical, biological, radiological, and thermal damage as well as damage from starvation and malnutrition.

Physical and Chemical Damage

 

This is a cross-section of human skin with a hair shaft growing from it.

Human skin: A diagram of human skin.

 

The skin is composed of tough skin cells as well as a tough protein called keratin that guard tissues, organs, and structures underneath the skin against physical damage from minor cuts, scratches, and abrasions. Because our skin is tough and largely waterproof, it helps protect internal structures from chemical irritants such as man-made detergents or even natural irritants like poison ivy.

Otherwise, these dangerous chemicals would seep into our sensitive internal environment. The waterproof nature of our skin also ensures that important molecules stay within our body.

Biological Damage

The skin also contains important cells called Langerhans cells. These cells help our immune system fight off infectious biological agents, like bacteria that try to get further into our body through skin that may have been compromised by physical damage.

Sebaceous glands associated with the skin secrete substances that help fight off potentially dangerous microorganisms as well. These glands also secrete substances that help keep our skin hydrated, and thus more resistant to bacterial invasion.

Radiological Damage

Our skin also contains melanocytes that produce a pigment called melanin. This protects the body from radiological damage via the sun’s UV radiation (or that from tanning beds).

Other Protective Roles

Part of our skin is made up of fat. This fat serves three large purposes:

  1. It helps cushion internal structures against any physical blows.
  2. It acts as a food source, protecting our body from the effects of starvation.
  3. It helps insulate us against cold temperatures.

Our skin is also closely associated with sweat glands that help protect us from high temperatures by cooling us off through the process of evaporation. These glands also help to excrete potentially dangerous substances, like urea, out of the body.

All sorts of sensory receptors are found within the skin as well. These help move our body parts away from potential sources of damage, like hot stoves, when they sense danger, thereby protecting our body from great harm.

Finally, the skin is also important for the synthesis of vitamin D, which is an important vitamin for the building of strong and healthy bones. Ergo, the skin protects the body from fractures if we do not otherwise get enough of this vitamin from food-based sources.

Thermoregulation

The integumentary system keeps body temperature within limits even when environmental temperature varies; this is called thermoregulation.

Learning Objective

Explain the skin’s role in thermoregulation

Key Takeaways

Key Points

  • The skin’s immense blood supply helps regulate temperature: dilated vessels allow for heat loss, while constricted vessels retain heat.
  • The skin regulates body temperature with its blood supply.
  • The skin assists in homeostasis.
  • Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.

Key Terms

  • Evaporation: What happens when water crosses the skin via sweat glands and then dissipates into the air; this process cools body temperature to within the body’s tolerance range.
  • homeostasis: The ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium; such as the ability of warm-blooded animals to maintain a constant temperature.
  • vasoconstriction: The constriction (narrowing) of a blood vessel.
  • arrector pili: 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.

This illustrates a cross-section of human skin. This image details the parts of the integumentary system, such as the sweat glands.

Human skin: This image details the parts of the integumentary system.

 

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’s Role in Cooling the Body

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.

Radiation

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.

Convection

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.

Conduction

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 hotter surface.

Perspiration

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 sweating.

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

 

This image details the anatomy of skin.The skin is the largest organ of the integumentary system, made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments, and internal organs.

Anatomy of the skin: The skin is the largest organ of the integumentary system, made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments, and internal organs.

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.

Since the 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 loss.

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.

 

Cutaneous Sensation

The somatosensory system is composed of the receptors and processing centers to produce the sensory modalities, such as touch and pain.

Learning Objective

Differentiate among the types of cutaneous mechanoreceptors

Key Takeaways

Key Points

  • The somatosensory  is the system of nerve cells that responds to changes to the external or internal state of the body.
  • Receptors are spread throughout the body, with large numbers found in the skin.
  • Several distinct receptor types form the somatosensory system including thermoreceptors (heat), nociceptors (pain), and mechanoreceptors (pressure).
  • There are four types of mechanoreceptors that respond to different pressure stimui and provide a wide range of mechanical sensitivity—they are the keys for fine motor control.

Key Terms

  • somatosensory system: A diverse sensory system composed of the receptors and processing centers to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain).
  • sensory receptor: A sensory nerve ending that recognizes a stimulus in the internal or external environment of an organism.

The Somatosensory System

The somatosensory is the system of nerve cells that responds to changes to the external or internal state of the body, predominately through the sense of touch, but also by the senses of body position and movement.

Spread through all major parts of the body, it consists of sensory receptors and sensory neurons in the periphery (for example, skin, muscle, and organs), along with deeper neurons within the central nervous system.

While touch is considered one of the five traditional senses, the impression of touch is actually formed from several diverse stimuli using different receptors:

  • Thermoreceptors (temperature)
  • Nociceptors (pain)
  • Mechanoreceptors (pressure)

Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.

Thermoreceptors

Mammals have at least two types of sensors: those that detect heat and those that detect cold.

Upon deviation from the norm ,sensory receptors trigger an action potential that can provide feedback or lead to alterations in behavior in order to maintain homoeostasis. Two receptors that exhibit the ability to detect changes in temperature include Krause end bulbs (cold) and Ruffini endings (heat).

Nociceptors

A nociceptor is a sensory nerve cell that responds to damaging or potentially damaging stimuli by sending signals to the spinal cord and brain. Nociceptors can respond to excessive thermal, mechanical, or chemical stimulation and often result the generation of an involuntary motor respons—for example, pulling  a hand away from a hot surface.

Mechanoreceptors

Mechanoreceptors are sensory receptors that respond to pressure and vibration. Four key types of mechanoreceptor have been described based on their response to stimulation and receptive field.

Receptors can either induce a slow response to stimulation, whereby a constant activation is initiated, or a fast response, whereby activation is only initiated at the beginning and end of stimulation. The receptive field—the region in which a receptor can sense an effect—can vary from small to large.

  • The Merkel receptor is a disk-shaped receptor located near the border between the epidermis and dermis. It demonstrates a slow response and has a small receptive field; it is useful for detecting steady pressure from small objects, such as when gripping something with the hand.
  • The Meissner corpuscle is a stack of flattened cells located in the dermis, near the epidermis. It demonstrates a rapid response and has a small receptive field; it is useful for detecting texture or movement of objects against the skin.
  • The Ruffini cylinder is located in the dermis and has many branched fibers inside a cylindrical capsule. It demonstrates a slow response and has a large receptive field; it is good for detecting steady pressure or stretching, such as during the movement of a joint.
  • The Pacinian corpuscle is a layered, onion-like capsule surrounding a nerve fiber. It is located deep in the dermis, in the subcutaneous fat. It demonstrates a fast response and has a large receptive field; it is useful for detecting large changes in the environment, such as vibrations.

Together they provide a wide range of mechanical sensitivity that enables fine motor control.

Metabolic Functions

One of the metabolic functions of the skin is the production of vitamin D3 when ultraviolet light reacts with 7-dehydrocholesterol.

Learning Objective

Describe the integumentary system’s role in producing vitamin D

Key Takeaways

Key Points

  • Vitamin D refers to a group of fat-soluble steroids responsible for increasing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc.
  • Foods rich in vitamin D are relatively scarce and so the body synthesises the majority of vitamin D itself, in the skin.
  • Vitamin D deficiency is associated with poor development of bones in children and a softening of bones in adults.
  • Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet light. Vitamin D is produced in the two innermost strata of the epidermis. Cholecalciferol (D3) is produced photochemically in the skin from 7-dehydrocholesterol.
  • Vitamin D is produced in the two innermost strata of the epidermis, the stratum basale and stratum spinosum.

Key Term

  • 7-dehydrocholesterol: 7-dehydrocholesterol is a cholesterol precursor that is converted to vitamin D3 in the skin, therefore functioning as provitamin-D3.

The integumentary system is the largest of the body’s organ systems, made up of the skin and its associated appendages. The integumentary system distinguishes, separates, and protects the organism from its surroundings, but also plays a key metabolic function, as the major region for vitamin D production.

What is Vitamin D?

Vitamin D refers to a group of fat-soluble steroids responsible for increasing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc. In humans, the most important compounds in this group are vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol). Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements, however very few foods are rich in vitamin D; and so synthesis within the skin is a key source.

Vitamin D deficiency is associated with impaired bone development in children, which leads to the development of rickets and a softening of bones in adults. Deficiency in vitamin D has been termed a modern disorder associated with both a poorer diet and reduced time spent outside.

Vitamin D Synthesis

 

This illustration shows a molecular model of the chemical structure of Vitamin D.

Vitamin D: The chemical structure of vitamin D.

 

The human skin consists of three major layers: the epidermis, dermis, and hypodermis. The epidermis forms the outermost layer, providing the initial barrier to the external environment. Beneath this, the dermis comprises two sections, the papillary and reticular layers, and contains connective tissues, vessels, glands, follicles, hair roots, sensory nerve endings, and muscular tissue. The deepest layer is the hypodermis, which is primarily made up of adipose tissue.

Vitamin D is produced in the two innermost strata of the epidermis, the stratum basale and stratum spinosum.

Vitamin D3 is made in the skin when the 7-dehydrocholesterol reacts with ultraviolet light of UVB type at wavelengths between 280 and 315 nm, with peak synthesis occurring between 295 and 297 nm.

Depending on the intensity of UVB rays and the minutes of exposure, an equilibrium can develop in the skin, and vitamin D degrades as fast as it is generated.
Vitamin D from the diet or that is synthesized by the body is biologically inactive; activation requires enzymatic conversion in the liver and kidney.

This is a diagram of how vitamin D is metabolized. It shows the pathway map for vitamin D from the skin, to a hepatocyte (a liver cell), to the kidney.

Metabolism and pathway map for vitamin D: Vitamin D synthesis pathway

 

Blood Supply to the Epidermis

The blood vessels in the dermis provide nourishment and remove waste from its own cells and from the stratum basale of the epidermis.

Learning Objective

Identify the source of the blood supply for the integumentary system

Key Takeaways

Key Points

  • The epidermis contains no blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries present in the upper layers of the dermis.
  • The papillary region of the dermis is composed of loose areolar connective tissue. This is named for its fingerlike projections called papillae, that extend toward the epidermis and contain terminal networks of blood capillaries.
  • The control of blood vessels within the dermis forms a key part of the body’s thermoregulatory capacity.

Key Terms

  • papillary region: The uppermost region of the dermis that is adjacent to the epidermis.
  • reticular region: The lower region of the dermis.

The epidermis does not contain blood vessels; instead, cells in the deepest layers are nourished by diffusion from blood capillaries that are present in the upper layers of the dermis. Diffusion provides nourishment and waste removal from the cells of the dermis, as well as for the cells of the epidermis.

This is an image of the distribution of the blood vessels in the skin of the sole of the foot. The blood vessels that supply the capillaries of the papillary region are seen running through the reticular layer.

The dermis: The distribution of the blood vessels in the skin of the sole of the foot. Corium—labeled at upper right—is an alternate term for dermis. Blood vessels that supply the capillaries of the papillary region are seen running through the reticular layer.

 

The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane.

The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep, thicker area known as the reticular region.

The papillary region is composed of loose areolar connective tissue. This is named for its fingerlike projections called papillae, that extend toward the epidermis and contain terminal networks of blood capillaries.

The reticular region lies under the papillary region and is usually much thicker. It is composed of dense, irregular connective tissue. The reticular region receives its name from the dense concentration of collagenous, elastic, and reticular fibers that weave throughout it.

These protein fibers give the dermis its typical properties of strength, extensibility, and elasticity. Blood vessels that supply the capillaries of the papillary region run through the reticular region.

Control of the blood supply to the dermis forms part of the body’s thermoregulatory capacity. Increasing blood flow, which makes the skin appear redder, will increase the loss of radiant heat through the skin, whereas constricting blood flow, making the skin appear paler, reduces heat loss.

Excretion and Absorption

The integumentary system functions in absorption (oxygen and some medications) and excretion (e.g., perspiration via the eccrine glands).

Learning Objective

Describe the role of glands in excretion and absorption

Key Takeaways

Key Points

  • Eccrine glands, the major sweat glands of the human body, produce a clear, odorless substance, consisting primarily of water and NaCl. NaCl is reabsorbed in the duct to reduce salt loss.
  • Apocrine sweat glands are found only in certain locations of the body: the axillae (armpits), areola and nipples of the breast, ear canal, perianal region, and some parts of the external genitalia.
  • The sebaceous glands secrete an oily/waxy matter called sebum to lubricate and waterproof the skin and hair of mammals. In humans, they are found in greatest abundance on the face and scalp, though they are distributed throughout all skin sites except the palms and soles.

A major function of the integumentary system is absorption and excretion.

Excretion

There are numerous secretory glands present in the skin which secrete a large range of distinct fluids.

Perspiration, or sweating,  is the production of fluids secreted by the sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and apocrine glands.

Eccrine glands are the major sweat glands of the human body, found in virtually all skin. They produce a clear, odorless substance consisting primarily of water and NaCl (note that the odor from sweat is due to bacterial activity on the secretions of the apocrine glands).

NaCl is reabsorbed in the duct to reduce salt loss. Eccrine glands are active in thermoregulation and are stimulated by the sympathetic nervous system.

This is an image of a sweat gland. It is a magnified, sectional view of the skin, with the eccrine glands highlighted.

Sweat gland: A sectional view of the skin (magnified), with the eccrine glands highlighted.

 

Apocrine sweat glands are inactive until they are stimulated by hormonal changes in puberty. Apocrine sweat glands are mainly thought to function as olfactory pheromones, chemicals important in attracting a potential mate. The stimulus for the secretion of apocrine sweat glands is adrenaline, which is a hormone carried in the blood.

The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals. In humans, they are found in greatest abundance on the face and scalp, though they are distributed throughout all skin sites except the palms and soles. In the eyelids, meibomian sebaceous glands secrete a special type of sebum into tears.

Absorption

Due to the absorptive capabilities of skin, the cells comprising the outermost 0.25–0.40 mm of the skin can be supplied by external oxygen rather than via the underlying capillary network. Additionally certain medications can be administered through the skin.

The most common mechanism of administration through the skin is the use of ointments or an adhesive patch, such as the nicotine patch or iontophoresis. Iontophoresis, also called electromotive drug administration, is a technique that uses a small electric charge to deliver a medicine or other chemical through the skin.