13.2: Introduction to the Integumentary System

Art for All Eras

This is Maud Stevens Wagner, a tattoo artist who is pictured in Figure \(\PageIndex{1}\). Maud was pictured in 1907. Clearly, tattoos are not just a late 20th and early 21st-century trend. They have been popular in many eras and cultures. Tattoos literally illustrate the biggest organ of the human body: the skin. The skin is very thin, but it covers a large area — about 2 m² in adults. The skin is the major organ in the integumentary system.

Figure \(\PageIndex{1}\): Maud Stevens Wagner

What Is the Integumentary System?

In addition to the skin, the integumentary system includes the hair and nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense the environment, and help the body maintain homeostasis.

The Skin

The skin is remarkable not only because it is the body’s largest organ. It is remarkable for other reasons as well. The average square inch of skin has 20 blood vessels, 650 sweat glands, and more than a thousand nerve endings. It also has an incredible 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick, or about as thick as the cover of a book. Although the skin is thin, it consists of two distinct layers, the epidermis and dermis, as shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): The epidermis is the thinner outer layer of skin which is composed of tightly packed epithelial cells. The dermis is the thicker inner layer of skin that contains structures such as blood vessels, hair follicles, and sweat glands.

Outer Layer of Skin

The outer layer of skin is the epidermis. This layer is thinner than the inner layer, the dermis. The epidermis consists mainly of epithelial cells, called keratinocytes, which produce the tough, fibrous protein keratin. The innermost cells of the epidermis are stem cells that divide continuously to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing more and more keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer. As the dead cells are shed from the surface of the skin, they are replaced by other cells that move up from below. The epidermis also contains melanocytes, the cells that produce the brown pigment melanin, which gives skin most of its color. Although the epidermis contains some sensory receptor cells, called Merkel cells, it contains no nerves, blood vessels, or other structures.

Inner Layer of Skin

The dermis is the inner and thicker layer of skin. It consists mainly of tough connective tissue and is attached to the epidermis by collagen fibers. The dermis contains many structures, as shown in the figure above, including blood vessels, sweat glands, and hair follicles, which are structures where hairs originate. In addition, the dermis contains many sensory receptors, nerves, and oil glands.

Functions of the Skin

The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions include preventing water loss from the body and serving as a barrier to the entry of microorganisms. Another function of the skin is synthesizing vitamin D, which occurs when the skin is exposed to ultraviolet (UV) light. Melanin in the epidermis blocks some of the UV light and protects the dermis from its damaging effects.

Another important function of the skin is helping to regulate body temperature. For example, when the body is too warm, the skin lowers body temperature by producing sweat, which cools the body when it evaporates. The skin also increases the amount of blood flowing near the body surface through vasodilation (widening of blood vessels), bringing heat from the body core to radiate out into the environment.

Hair

Hair is a fiber that is found only in mammals. It consists mainly of keratin-producing keratinocytes. Each hair grows out of a follicle in the dermis. By the time the hair reaches the surface, it consists mainly of dead cells filled with keratin. Hair serves several homeostatic functions. Head hair is important in preventing heat loss from the head and protecting its skin from UV radiation. Hairs in the nose trap dust particles and microorganisms in the air and prevent them from reaching the lungs. Hair all over the body provides sensory input when objects brush against it or it sways in moving air. Eyelashes and eyebrows protect the eyes from water, dirt, and other irritants.

Nails

Fingernails and toenails consist of dead keratinocytes that are filled with keratin. The keratin makes them hard but flexible, which is important for the functions they serve. Nails prevent injury by forming protective plates over the ends of the fingers and toes. They also enhance sensation by acting as a counterforce to the sensitive fingertips when objects are handled. In addition, fingernails can be used as tools.

Interactions with Other Organ Systems

The skin and other parts of the integumentary system work with other organ systems to maintain homeostasis.

• The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.
• Vitamin D is needed by the digestive system to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.
• Most immune cells, such as B and T cells have Vitamin D receptors. Vitamin D levels in the body are associated with autoimmune diseases and immune deficiencies.
• To control body temperature, the skin works with the cardiovascular system to either lose body heat or conserve it through vasodilation or vasoconstriction.
• To detect certain sensations from the outside world, the nervous system depends on nerve receptors in the skin.

Review

1. Name the organs of the integumentary system.
2. Compare and contrast the epidermis and dermis.
3. Identify the functions of the skin.
4. What is the composition of hair?
5. Describe three physiological roles played by the hair.
6. What do nails consist of?
7. List two functions of nails.
8. What do the outermost surface of the skin, the nails, and hair have in common, in terms of their composition?
9. The innermost layer of the epidermis consists of _________ cells than the outermost layer of the epidermis.

   A. older

   B. younger

   C. more sweat glands

   D. more blood vessel

10. Identify two types of cells found in the epidermis of the skin and describe their functions.
11. True or False. Keratin-producing cells in the epidermis are a type of epithelial cell.
12. True or False. Vasodilation is used to warm the body.
13. Which structure and layer of skin do hair grow out of?
14. Identify three main functions of the integumentary system and give an example of each.
15. What are two ways in which the integumentary system protects the body against UV radiation?

Explore More

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Attributions

1. Maud Stevens Wagner by The Plaza Gallery, public domain via Wikimedia Commons
2. Skin epidermis and dermis by National Cancer Institute, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0