appendeges of the skin Flashcards
(9 cards)
skin apendages
The skin appendages include cutaneous glands, hair and hair follicles, and nails (see Figure 4.3). Despite being physically located in the dermis, each of these appendages is produced by cells that originate in the epidermal layer and plays a unique role in maintaining body homeostasis.
cutenous glands
The cutaneous glands are all exocrine glands that release their secretions to the skin surface via ducts. They fall into two groups: sebaceous glands and sweat glands. As these glands are formed by the cells of the stratum basale, they push into the deeper skin regions and ultimately reside almost entirely in the dermis.
sabcous oil
Sebaceous (oil) glands are found all over the skin—except on the palms of the hands and the soles of the feet.
Most of these glands have ducts that empty into hair follicles, releasing oil (called sebum) to keep the hair and skin soft, moisturized, and protected.
However, some sebaceous glands open directly onto the skin surface, especially in hairless areas like the lips or eyelids.
The product of the sebaceous glands, sebum (se′bum; seb = grease), is a mixture of oily substances and fragmented cells. Sebum is a lubricant that keeps the skin soft and moist and prevents the hair from becoming brittle. Sebum also contains chemicals that kill bacteria, so it is important in preventing bacterial infection of the skin. The sebaceous glands become very active when androgens (male sex hormones) are produced in increased amounts (in both sexes) during adolescence. Thus, the skin tends to become oilier during this period of life.
too much sebum
When sebaceous gland ducts are blocked by sebum, acne appears on the skin surface. Acne is an active infection of the sebaceous glands. If the accumulated material oxidizes and dries, it darkens, forming a blackhead. If the material does not dry or darken, a whitehead forms. Acne can be mild or extremely severe, leading to permanent scarring.
Seborrhea (seb″o-re′ah; “fast-flowing sebum”), known as “cradle cap” in infants, is caused by overactivity of the sebaceous glands. It begins on the scalp as pink, raised lesions that gradually form a yellow-to-brown crust that sloughs off oily scales and dandruff. Careful washing to remove the excessive oil often helps cradle cap in a newborn baby.
sweat gland sudoriferous
Sweat glands, also called sudoriferous (su″do-rif′er-us; sudor = sweat) glands, are widely distributed in the skin. Their number is staggering—more than 2.5 million per person. There are two types of sweat glands, eccrine and apocrine.
eccrine
The eccrine (ek′rin) glands are far more numerous and are found all over the body. They produce sweat, a clear secretion that is primarily water plus some salts (sodium chloride), vitamin C, traces of metabolic wastes (ammonia, urea, uric acid), and lactic acid (a chemical that accumulates during vigorous muscle activity). Sweat is acidic (pH from 4 to 6), a characteristic that inhibits the growth of certain bacteria, which are always present on the skin surface. Typically, sweat reaches the skin surface via a duct that opens externally as a funnel-shaped sweat pore (Figure 4.6b). Notice, however, that the facial “pores” commonly referred to when we talk about our complexion are the external outlets of hair follicles where sebaceous ducts empty, not these sweat pores.
The eccrine sweat glands are an important and highly efficient part of the body’s heat-regulating equipment. They are supplied with nerve endings that cause them to secrete sweat when the external temperature or body temperature is too high. When sweat evaporates off the skin surface, it carries large amounts of body heat with it. On a hot day, it is possible to lose up to 7 liters of body water in this way. The heat-regulating functions of the body are important—if internal temperature changes more than a few degrees from the normal 37°C (98.6°F), life-threatening changes occur in the body. (We discuss body temperature regulation in more detail in Chapter
approcine
Apocrine (ap′o-krin) glands are largely confined to the axillary (armpit) and genital areas of the body. They are usually larger than eccrine glands, and their ducts empty into hair follicles. Their secretion contains fatty acids and proteins in addition to all the substances present in eccrine sweat; consequently, it may have a milky or yellowish color. The secretion is odorless, but when bacteria that live on the skin use its proteins and fats as a source of nutrients, it can take on a musky, unpleasant odor.
Apocrine glands begin to function during puberty under the influence of androgens. Although they produce their secretions almost continuously, apocrine glands play a minimal role in thermoregulation. Their precise function is not yet known, but they are activated by nerve fibers during pain and stress and during sexual arousal.
hair and hair follicules
Hair is an important part of our body image. Consider, for example, the spiky hair style of punk rockers and the flowing locks of high-fashion models. Millions of hairs, produced by hair follicles, are found all over the body surface except on the palms of the hands, soles of the feet, nipples, and lips. Humans are born with as many hair follicles as they will ever have, and hairs are among the fastest growing tissues in the body. Hair serves a few minor protective functions, such as guarding the head against bumps, shielding the eyes (via eyelashes), and helping to keep foreign particles out of the respiratory tract (via nose hairs). Hormones account for the development of hairy regions—the scalp and, in the adult, the pubic and axillary areas. Despite these functions, however, our body hair has lost much of its usefulness. Hair served early humans (and still serves hairy animals) by providing insulation in cold weather, but now we have other means of keeping warm.
