Dermis The dermis lies beneath the epidermis and contains the protein collagen, blood vessels, nerve endings, glands, sensory receptors, smooth muscles, and hair follicles. Structures in the dermis interact with other body systems to maintain homeostasis by helping to regulate body temperature. When the body needs to conserve heat on a cold day, the blood vessels in the dermis narrow. This brings blood closer to the body's core and prevents heat from escaping through the skin. On hot days, the blood vessels widen, bringing heat from the body's core to the skin.

Sweat glands in the dermis also aid temperature regulation. Excess heat is released when sweat glands produce perspiration, or sweat. When sweat evaporates, it takes heat away from your body.

The skin also contains sebaceous (suh BAY shus) glands, which secrete an oily substance called sebum that is released at the surface of the skin. Sebum helps to keep the keratin-rich epidermis flexible and waterproof. Because it is acidic, it can kill bacteria on the surface of the skin.

In Your Notebook Explain whether the epidermis, the dermis, or both layers are involved in protection and temperature regulation.

Hair The basic component of human hair and nails is keratin. In other animals, keratin forms a variety of structures, including bull horns, reptile scales, bird feathers, and porcupine quills.

Hair covers almost every exposed surface of the human body and has some important functions. Hair on the head protects the scalp from ultraviolet light from the sun and provides insulation from the cold. Hairs in the nostrils, external ear canals, and around the eyes (in the form of eyelashes) prevent dirt and other particles from entering the body.

Hair is produced by cells at the base of structures called hair follicles. Hair follicles are tubelike pockets of epidermal cells that extend into the dermis. New research has shown that hair follicles contain stem cells that help to renew the skin and heal wounds. The hairs shown in Figure 32–13 are actually large columns of cells that have filled with keratin and then died. Rapid cell growth at the base of the hair follicle causes the hair to grow longer. Hair follicles are in close contact with sebaceous glands. The oily secretions of these glands help hairs stay soft and flexible.

FIGURE 32–13 Hair As a new hair grows, it pushes the old hair out of the follicle. The micrograph shows individual hairs in their follicles.

d

Nails Nails grow from an area of rapidly dividing cells known as the nail root. The nail roots are located near the tips of the fingers and toes. During cell division, the cells of the nail root fill with keratin and produce a tough, platelike nail that covers and protects the tips of the fingers and toes. Nails grow at an average rate of 3 millimeters per month, with fingernails growing about three times faster than toenails.

Table of Contents