Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

and thermal stimulus promotes secretions of eccrine glands. Eccrine glands are found throughout the skin’s surface area and their main function is to help the body sweat or dissipate heat.

      Eccrine glands look like a coiled bundle of string that originate in the secretory portion of the sweat gland. One end rises toward the surface of the skin in a relatively straight line, then forms a number of spirals that make their way upward through the corium (dermis). This ‘tube’ extends from the sweat gland up to the surface of the skin, where the ‘outlet’ is found in a skin pore.

It is estimated that sweat glands, under certain conditions, produce and secrete up to two quarts of sweat daily.15

      The sebaceous glands secrete sebum or oily secretions. Sebaceous glands are found just about everywhere on the body except for the hands and feet.

Most of us are more familiar with these glands because of their association with hair follicles and development of acne. These glands are quite densely found on the scalp, the chest, the face, and the back. They’re influenced by androgenic hormones and produce sebum, a substance that serves as a lubricant and is also partly responsible for the manufacture and perpetuation of vitamin D in the skin. In fact, acne is defined as a sebaceous gland disorder or malfunction.16

      Figure 3-5 Eccrine gland.

       ■Hair and Fingernails

      Hair and fingernails are considered accessory structures of the integumentary system. In medical scenarios, pattern of hair growth, hair growth itself, and distribution of hair on the scalp or body are often perceived as general indicators of health and wellness.

      Earlier in the chapter, two conditions affecting the hair were mentioned: hypertrichosis and alopecia.

      Another example involves lack of hair growth on the toes. While many people shave the hair from their toes for aesthetic reasons, ‘natural’ hair loss on the toes can indicate a lack of adequate circulation. Hair thickness, volume, and growth changes over time, affected by nutrition, hormones, and general aging patterns.

      Figure 3-6 Hair follicles.

      In mammals, hair is intended to help keep animals warm. In human beings, hair is more functional as a protectant rather than for warmth. For example, our eyelashes protect our eyes. Nose hairs (which many people pluck) are designed to filter air and restrict particles or insects from entering nasal passages. The hair on our head protects our scalp from direct sunlight and reduces loss of heat.

      Structure of hair

      Hair in medical terminology is known as pili. Hair is manufactured in hair follicles, producing individual and flexible strands that contain mainly keratinized or dead skin. However, there is a difference between keratin cells: one is the soft keratin of the epidermal cells of the skin, while the other is termed hard keratin that is mainly found in the fingernails and hair. Hard keratin is extremely durable and – as its name implies – hard, and resistant to flaking.

      The most prominent part of hair is the hair shaft. At the base of the hair shaft is the root; this is where the shaft of hair embeds in the skin of the scalp. The shape of the hair shaft defines the general appearance of hair; a flat shaft produces curly or kinky hair. An oval shaft produces wavy and soft hair. A round shaft typically produces coarser hair.

      Hair contains three layers of keratinized cells. These layers are concentric and include the:

      •Medulla – this layer is known as the inner layer and is typically the only portion of hair that contains soft keratin. A microscopic view of the medulla displays large air spaces and cells.

      •Cortex – the middle layer surrounds the medulla and is composed of layers of flattened cells.

      •Cuticle – the cuticle is the outermost layer of a hair shaft, and like that of dermis, is composed of a single-cell layers that overlap one another. The overlapping of cells separates one hair from another, and resists clumping. The cuticle also contains high levels of keratin and is responsible for mainly providing structure for the inner layers. The cuticle takes a good deal of abuse from the environment and vigorous hair brushing, and is the portion of hair that has a greater tendency to damage. This damage is commonly known as a ‘split end’.

      Figure 3-7 Anatomy of hair.

      A number of melanocytes are found at the base of the hair follicle. Different amounts of melanin (which around found in numerous colors such as yellowish, rusty, brown, or black) combine together to produce a certain hair color. These melanocytes are absorbed into the cortical cells and are what define hair pigmentation or color. As production of melanin decreases due to aging and diminished hormone production, air bubbles inside the hair shaft replace the melanin, turning hair gray.

      Hair follicle structure

      The structure of the hair follicle is precise and does not change between gender or ethnicity. As mentioned, the main structure of hair is the hair follicle. At the base of the hair follicle, just below the surface of the skin, is the hair bulb.

      Around each hair bulb is a bundle of sensory nerve endings. Depending on source, these nerve endings are called the root hair plexus or the hair follicle receptor. When you pluck out a shaft of hair, you may notice a tiny spot of dermal tissue. This is known as the hair bulb or root ball, which contains capillaries that supply nutrients to the hair.

      The actual wall of a follicle of hair is composed of two sheaths; the outermost sheath is known as the peripheral connective tissue sheath or fibrous sheath. The inner sheath is known as the epithelial root sheath. The peripheral or connective tissue fibers sheath is manufactured from the dermis while the epithelial root sheath is manufactured from the epidermis.

      Figure 3-8 Hair bulb.

      Immediately above the hair “bulb” are cells that compose the matrix that actually manufactures the hair, called the hair bulge. This bulge is situated just above the hair bulb. Chemical signals trigger cells to migrate upward toward the papilla, where they produce hair cells. This is how hair grows; as the matrix manufactures new hair cells, the older parts of the hair shaft are pushed in an upward direction. Eventually, these cells are keratinized.

      Another interesting aspect of hair follicles is the origination of that sensation a person gets when hair follicles are stimulated (goosebumps). How do goosebumps appear? Each hair follicle is linked to arrector pili, a small muscle constructed of smooth muscle cells literally translated “raiser of hair”.

      Figure 3-9 Fingernail construction.

      As hair protrudes past the scalp, it does so at a small angle. Arrector pili muscles attach to hair follicles in such a way that when they contract, the hair stands straighter and causes raised areas to appear on the surface of the skin. This reaction is typically caused either by cool temperatures or the adrenaline-triggered fear response.

       ■Fingernails and Toenails

      What’s the purpose of fingernails and toenails – other than aesthetics, of course? Actually, they’re quite effective tools, both in humans and animals. While animals have claws, humans have fingernails and toenails.

      Fingernails and toenails are composed of and contain hard keratin. The main components of fingernails and toenails are mentioned here:

      •Edge – also called the “free edge” - the part of the finger or toenail that produces the familiar white bridge along the tip