Integumentary System
The integumentary system is an organ system consisting of the skin, hair, nails, and exocrine glands. The skin is only a few millimeters thick yet is by far the largest organ in the body. The average person’s skin weighs 10 pounds and has a surface area of almost 20 square feet. Skin forms the body’s outer covering and forms a barrier to protect the body from chemicals, disease, UV light, and physical damage. Hair and nails extend from the skin to reinforce the skin and protect it from environmental damage. The exocrine glands of the integumentary system produce sweat, oil, and wax to cool, protect, and moisturize the skin’s surface.
The integumentary system of the head and neck includes the skin and hair of the face, head and neck. The skin of the face is the part of the body which is most often exposed to sunlight, the elements, and environmental pollutants; thus, it is more prone to acne and to becoming dry or otherwise showing accumulated damage from these influences than the skin on the rest of the body.
The integumentary system of the upper torso is made up of the skin and its appendages (such as the hair); its primary function is to protect the body from damage; it is also involved in temperature regulation and in the sense of touch. Upon exposure to sunlight, the integumentary system is also the primary vehicle for synthesis of vitamin D.
The integumentary system of the lower abdomen and pelvis includes the skin and hair of this region. The skin of this system protects against damage to the organs contained within the abdomen. Additionally, the skin over the genitals is the first line of defense against sexually transmitted diseases and other diseases that affect these reproductive organs. Both men and women have hair on the lower abdomen that extends from the lower edge of the chest to the pubis. Hair in the genital region is known as pubic hair.
The integumentary system of the upper extremities (including the shoulders, arms, hands and fingers) includes the skin and hair, and nails of the upper extremities. The skin of the hands and arms (especially the hands) is more frequently exposed to sunlight and to environmental toxins than other parts of the body. Therefore, the hands are likely to show evidence of accumulated damage from these influences as a person ages.
The integumentary system of the lower extremities includes the skin, hair and nails of the legs, feet, and toes. The skin of this system is the first barrier against damage to the muscles of the legs, feet, and toes. Both men and women have hair on their legs and the tops of their feet, but this hair is somewhat thicker and coarser in men than in women. The toenails protect the toes from injury; they also enhance the sensitivity of the tips of the toes, although the nails themselves have no nerve endings.
The fingernails are a sort of envelope composed of keratin, a tough protein, which cover the top of the terminal phalanges of a person\'s fingers. Nails are similar to the claws that can be found in other animals. Each nail is made up of a nail plate—this is the top of the nail. It is also made up of the nail matrix, a thin layer of tissue upon which the nail plate rests and the site of the cells that eventually become the nail plate. Each nail also has a nail bed—a layer of skin that connects the nail matrix to the hand. And lastly, each nail has the grooves that surround the nail matrix.
Hair is present on all skin surfaces except the palms, soles, lips, nipples, and various parts of the external reproductive organs; however, it is not always well developed. For example, it is very fine on the forehead and the inside surface of the arm. Each hair develops from a group of epidermal cells at the base of a tube-like depression called a hair follicle. This follicle extends from the surface into the dermis and may pass into the subcutaneous layer. The cells at its base receive nourishment from dermal blood vessels that occur in a projection of connective tissue, called the derma papilla, at the base of the follicle. As these epidermal cells divide and grow, older cells are pushed toward the surface. The cells that move upward and away from the nutrient supply then die. Their remains constitute the shaft of a developing hair. In other words, a hair is composed of dead epidermal cells.
The skin is by far the largest organ of the human body, weighing about 10 pounds (4.5 kg) and measuring about 20 square feet (2 square meters) in surface area. It forms the outer covering for the entire body and protects the internal tissues from the external environment.
The skin consists of two distinct layers: the epidermis and the dermis. Each layer is made of distinct tissues and performs distinct functions to support the body. A third layer of tissue under the skin, known as the hypodermis or subcutaneous layer, is not truly part of the skin itself but connects the skin loosely to the underlying muscles and bones that make up the deeper tissues of the body. The epidermis is made of four to five layers of epithelial tissue that constantly grows from the inside out and replaces most of its cells every few weeks.