Parts and Functions of the Human Body

• Describe the skin of the human body
• Discuss glands of the skin
• Explain the structure and function of hair
• Discuss about nails

What you should note

• Dermis
• Epidermis
• Eponychium
• Hypodermis
• Hyponychium
• Lunula
• Sebaceous glands
• Sudoriferous glands

The integumentary system consists of the skin and its derivatives. These include hair, nails, and several types of glands.

The system functions in protection, in the regulation of body temperature, in the excretion of waste materials, in the synthesis of vitamin D3 with the help of sunrays, and in the reception of various stimuli perceived as pain, pressure, and temperature.

The Skin

Skin is the largest organ in the human body, occupying almost 2m² of surface area and with a thickness of 2mm. The human skin has 3 main parts which include the epidermis, dermis, and hypodermis.

• Epidermis: This is the outer layer of the skin, and the outer layer of the skin is made of stratified squamous epithelium. It lacks fluid supply and contains basically 4-5 strata: stratum corneum, lucidum, granulosum, spinosum, and basal.
• Stratum corneum: the outer, dead, flat, keratinized, and thicker layer.
• Stratum lucidum: consists of flat, translucent layers of cells, found only in thick skin.
• Stratum granulosum: cells in this layer are in the process of keratinization.
• Stratum spinosum: cells in this stratum have a polyhedral shape and are in the process of protein synthesis.
• Stratum basale: the last layer of the epidermis, resting on the basement membrane. Together with the stratum spinosum, it constitutes the stratum germinativum.
• Dermis: A strong, flexible, connective tissue meshwork of collagen, reticular, and elastic fibers. Most of the skin is composed of the dermis, containing papillary and reticular layers.
• Papillary layer: next to the stratum basale of the epidermis, containing loose connective tissue with bundles of collagenous fibers. It also contains loose capillaries that nourish the epidermis and special nerve endings that serve as touch receptors (Meissner’s corpuscles).
• Reticular layer: made of dense connective tissue with coarse collagenous fiber bundles forming a stroma of elastic network. It contains fluid and lymphatic vessels, nerves, fat cells, sebaceous (oil) glands, and hair roots.
• Hypodermis: Found beneath the dermis, it is a subcutaneous layer composed of loose, fibrous connective tissue, richly supplied with lymphatic and fluid vessels and nerves. It is thicker than the dermis and contains coils of ducts of sudoriferous (sweat) glands and the base of hair follicles.

Functions of Skin

1. Protection: against harmful microorganisms, foreign material, and excessive loss of body fluids.
2. Temperature regulation: with sweat, heat leaves the body.
3. Excretion: small amounts of waste products such as urea.
4. Synthesis: by the action of UV. Vitamin D is synthesized in the skin, necessary for absorbing calcium from the intestine.
5. Sensory reception: contains sensory receptors for heat, cold, touch, pressure, and pain.

Color of the Skin

Skin’s color is determined by three factors:

1. The presence of melanin, a dark pigment produced by specialized cells called melanocytes.
2. The accumulation of the yellow pigment carotene.
3. The color of fluid reflected through the epidermis.

• The main function of melanin is to screen out excessive ultraviolet rays.
• All races have some melanin in their skins, although darker races have slightly more melanocytes. A person genetically unable to produce any melanin is an albino.

Glands of the Skin

The glands of the skin are the sudoriferous and sebaceous glands.

Sudoriferous (Sweat) Glands

Types: Eccrine and Apocrine glands.

• Eccrine glands: small, simple coiled tubular glands distributed over nearly the entire body, absent over nail beds, margins of lips of vulva, tips of the penis. Eccrine glands are numerous over the palms and soles. They secrete a colorless, aqueous fluid containing neutral fats, albumin, urea, lactic acid, and sodium chloride, helping to regulate body temperature.
• Apocrine glands: odiferous, found at the armpits, in the dark region around nipples, the outer lips of the vulva, and the anal and genital regions. They are larger and deeper than eccrine sweat glands and become active at puberty. Responding to stress, including sexual activity, they secrete a thicker sweat. The female breasts are apocrine glands that have become adapted to secret and release milk instead of sweat.

Sebaceous (Oil) Glands

Simple branched alveolar glands found in the dermis, sebaceous glands secrete oily substance called sebum. Sebum functions as a permeability barrier, skin softener, and protective agent against bacteria and fungi. Sebaceous glands are connected to hair follicles and are found all over the body except in the palms and soles.

Hair

Hair is composed of keratinized threads of cells, developing from the epidermis. It covers the entire body except the palms, soles, lips, tip of the penis, inner lips of the vulva, and nipples.

Function of Hair

• Insulation against cold in the scalp.
• Protection against glare in eyebrows.
• Screening against foreign particles (eyelashes).
• Trapping dust particles in the inhaled air in the nostrils.
• Protecting openings from foreign particles.

Structure of Hair

Hair has two parts: the shaft (above the skin) and the root (embedded in the skin). It consists of epithelial cells arranged in three layers: medulla, cortex, and cuticle.

Nails

Nails, like hair, are modifications of the epidermis, made of hard keratin. Composed of flat, cornified plates on the dorsal surface of the distal segment of the fingers and toes, nails have a body and a root. The thicker layer of skin beneath the nail root is the matrix, where new cells are generated. Nails grow 0.5mm a week. Our nails protect our fingers and toes, and they allow picking up and grasping objects as well as scratching.