Onycholysis (ŏn-ĭ-KOL-ĭ-sĭs) refers to the separation of a fingernail or toenail from its nail bed. Onychophagia (ŏn-ĭ-KOF-ă-jē-ă) refers to the habit of biting one’s nails.

Sweat Glands

When the body becomes warm, sweat glands produce sweat to cool the body. There are two types of sweat glands called eccrine sweat glands and apocrine sweat glands, each secreting slightly different products.

An eccrine sweat gland (ĔK-rĭn swĕt glănd) is type of gland that produces sweat for thermoregulation, as previously described in this section. These glands are found all over the skin’s surface but are especially abundant on the palms of the hand, the soles of the feet, and the forehead. This type of sweat is composed mostly of water and some salt.^[13] See Figure 3.4^[14] for an illustration of an eccrine sweat gland.

Apocrine sweat glands (AP-ŏ-krĕn swĕt glănds) are found in hair follicles in densely hairy areas, such as the armpits and genital regions. In addition to water and salt, apocrine sweat glands release organic compounds that make the sweat thicker and subject to bacterial decomposition, resulting in odor. The release of apocrine sweat is controlled by hormones and the nervous system. Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop sweat. When the antiperspirant enters the sweat gland duct, the aluminum-based compounds physically block the duct and prevent sweat from coming out of the pore.^[15]

Anhidrosis (ăn-hī-DRŌ-sĭs) is the medical term used for the absence of sweat production. Hyperhidrosis (hī-pĕr-hī-DRŌ-sĭs) is the medical term used for excessive sweating.

A sebaceous gland (sē-BĀ-shŭs glănd) is a type of oil gland that lubricates and waterproofs the skin and hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells. The fatty acids of sebum also have antibacterial properties and prevent water loss from the skin in low humidity environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus, sebaceous glands are relatively inactive during childhood.^[16]

Physiology of the Integumentary System

The skin and accessory structures perform a variety of these essential functions^[17]:

• Protecting the body from invasion by microorganisms, chemicals, and other environmental factors
• Acting as a sensory organ
• Modulating body temperature and electrolyte balance
• Synthesizing vitamin D

Protection

The skin protects the body from wind, water, and ultraviolet (UV) light. It acts as a protective barrier against excessive water loss referred to as dehydration (dē-hī-DRĀ-shŏn). It is also the first line of defense against abrasive activity such as grit, microbes, or harmful chemicals. Sweat excreted from sweat glands also deters microbes from overcolonizing the skin surface.^[18]

Sensory Function

The epidermis, dermis, and the hypodermis contain specialized sensory nerve structures that detect touch, surface temperature, and pain. These receptors are concentrated on the tips of the fingers, which are very sensitive to touch.^[19]

Thermoregulation

The integumentary system helps regulate body temperature in association with the sympathetic nervous system. The sympathetic nervous system is continuously monitoring body temperature and initiating appropriate motor responses.^[20]

When the body becomes excessively warm, sweat glands secrete water, salt, and other substances to cool the body. Even when the body does not appear to be noticeably sweating, approximately 500 mL of sweat are secreted a day. This amount can greatly vary depending on the person’s metabolic rate, physical activity, heat exposure, and medical condition. When the sweat evaporates from the skin surface, the body is cooled as body heat is dissipated. In addition to sweating, small blood vessels called arterioles in the dermis dilate so that excess heat carried by the blood can dissipate through the skin and into the surrounding environment.^[21]

When body temperatures drop, the arterioles constrict to minimize heat loss, particularly in the ends of the fingers and toes and the tip of the nose. This constriction reduces blood flow to the skin to prevent the loss of heat and keep the internal organs warm. It can cause the skin to take on a whitish hue. If the temperature of the skin drops too much (such as in environmental temperatures below freezing), the conservation of body core heat can result in frostbite (FROST-bīt).^[22] See Figure 3.5^[23] for illustrations demonstrating thermoregulation.

The epidermal layer synthesizes vitamin D when exposed to UV light. Vitamin D is essential for normal absorption of calcium and phosphorous, which are required for healthy bones. In addition to its essential role in bone health, vitamin D is essential for general immunity against bacterial, viral, and fungal infections. Significant absence of sun exposure can lead to a lack of vitamin D in the body called rickets (RIK-ĕts). For this reason, vitamin D is added as a supplement to many foods, including milk and orange juice.^[24]

Changes in the Integumentary System Due to Aging

All body systems change as a person ages. In the skin, older adults develop a thinner epidermis. Additionally, the dermis, responsible for the elasticity and resilience of the skin, exhibits a reduced ability to regenerate, which leads to slower wound healing. The hypodermis, with its fat stores, loses structure due to the reduction and redistribution of fat, which, in turn, contributes to the thinning and sagging of skin.^[25] See Figure 3.6^[26] for an illustration of these signs of aging skin.

The accessory structures also have lowered activity, generating thinner hair and nails and reduced amounts of sebum and sweat. A reduced sweating ability creates a risk for older adults in extreme heat. Other cells in the skin, such as melanocytes and dendritic cells, also become less active, leading to a paler skin tone and lowered immunity.^[27]