Your Skin: The Largest Organ of the Human Body
It provides protection, thermoregulation, sensation, and allows secretion. The skin is made up of two main layers called the epidermis and the dermis. It also has a number of appendages like hair, nails and sweat glands.
As the protector of the body, the skin is the first defensive element against foreign substances entering the body as well as protecting the body from trauma. The melanin in our skin protects us from ultraviolet light. It protects our bodies from chemicals and most bacteria and other microorganisms. Our skin is slightly acidic to kill those bacteria and keep them from entering the body.
The skin also helps regulate the body’s temperature. Temperature regulation is important to the survival of the body. Our bodies contain enzymes that are important for normal chemical reactions. Excessive heat can kill these enzymes and allow the bodies cellular machinery to break down.
When external body temperatures increase blood vessels dilate and bring more blood to the surface causing body heat to decrease by radiation, convection, conduction or evaporation. Sweating is a way that the body regulates its temperature through evaporation.
When external temperatures decrease the blood vessels in the dermis dilate to bring heat to the surface of the body and protect our extremities. But excessive exposure to frigid temperatures cannot be maintained and the blood vessels constrict to bring the heat further inside the body to protect the internal organs. Frostbite occurs when the extremities of the body are no longer receiving a supply of blood due to the body’s defense of keeping the organs protected during extreme cold temperatures.
Our skin is sensitive to touch and contain receptors to tell the body what sensation is being felt. Temperature receptors produce sensations of hot or cold. Pressure receptors produce sensations of pressure against the body like when we get pinched. Combinations of these receptors allow us to feel sensations of burning, itching, and tickling. All of these receptors allow us to respond to the sensations that the body feels from the outside world. Without these receptors we would never know if we grabbed something hot or cold which could result in great damage to the skins surface.
Our skin allows for the secretion of sebum and sweat. The sebaceous glands secrete sebum, which is important to our skin by maintaining the moisture content in our bodies and also has antifungal and antibacterial properties. Sweat is produced by sweat glands and is important to the cooling process of the body. Sweat also contains waste products such as urea, ammonia, and uric acid.
The skin is also beneficial in the production of Vitamin D by exposure to ultraviolet rays. Vitamin D is important for calcium and phosphate absorption.
The first layer of the skin or the epidermis is made up of epithelial tissue and can be divided into sub layers. The outermost layer of the epidermis is made up of stratified, squamous, keratinized epithelial cells. These cells are held together by desmosomes that are highly convoluted, interlocking cellular links. The desmosomes are responsible for the whole continuous structure of the skin.
The epidermis layer will be thickest at points where it receives the most abrasion and weight like the palms of the hands and soles of the feet. The epidermis is broken down into five sublayers: the stratum corneum, the stratum lucidum, the stratum granulosum, the stratum spinosum, and the stratum germinativum.
The stratum corneum is the outermost layer and is often referred to as the horny or leathery layer. This layer contains the dead skin cells that have been converted to protein. These cells are considered keratinized because of their lack of moisture with only about 20% moisture content whereas the lower layers contain about 70% moisture. These cells are shaped like scales and can be up to 25 cell layers thick. This layer also functions as a protector and physical barrier to light, heat and microorganisms. The stratum corneum is also where callus’s develop due to abrasion and friction on this layer.
The stratum lucidum is directly beneath the stratum corneum and is only one or two cell layers thick. These cells are flat and transparent. The stratum granulosum contains two or three layers of flattened cells. This layer of the epidermis is very active in the keratinization process. The stratum spinosum is made up of seven to eight layers of spiny-shaped cells.
The stratum germinativum is the deepest and most important layer in the epidermis. This layer contains the only cells that are capable of mitosis or dividing. The melanocytes that are responsible for skin coloring are in this layer. All races have the same number of melanocytes but different races have specific genes that determine the amount of melanin that is produced by the melanocytes.
The other layer of skin is known as the dermis or the corium. It lies directly beneath the epidermis and has been called the true skin. It is composed of dense connective tissue. The dermis is also where the blood vessels, lymph vessels, smooth muscles, sweat glands, hair follicles and sebaceous glands are embedded.
The dermis layer can be divided into two portions: the papillary portion and the reticular portion. The papillary portion is adjacent to the epidermis while the reticular portion is between the papillary portion and the fatty subcutaneous tissue. The appendages of the skin are hair, nails, sebaceous glands, ceruminous glands (wax glands in the ear canal) and the sweat glands.
The function of the hair is for protection and can be found on most parts of the body. The areas of the body that aren’t covered by hair are the palms of the hands, soles of the feet and certain portions of the genitalia. In some areas the hair is so small and fine that they appear invisible, but in other parts it is highly obvious like the head, armpits and around the genitalia.
Nails appear at the end of the fingers and toes. Nails are a modification of the horny epidermal cells and are composed of very hard keratin.
The sebaceous glands develop along the walls of hair follicles and produce an oily substance called sebum. The sebum is responsible for lubricating the surface of the skin.
Sweat glands are tubular glands that are found in most areas of the body except the margins of the lips and the head of the penis. Each sweat gland contains a secretory portion and an excretory duct. The secretory portion is located below the dermis in the subcutaneous tissue and produces sweat. The excretory duct opens on the surface of the skin to allow the sweat to escape. The skin can also have disorders such as skin cancer, ringworm, psoriasis, warts, cold sores, impetigo, shingles and vitiligo.
Skin cancer is usually caused by excessive exposure to ultraviolet rays and there are several types of skin cancer. The most common is basal cell carcinoma and can be easily treated and rarely spreads. The second type is squamous cell carcinoma and is found in the epidermis layer of the skin. This cancer can spread to the dermis, metastasize and cause death if left untreated. The worst and most dangerous type of skin cancer is malignant melanoma. This type is rare and is associated with a mole on the skin.
Ringworm is an itchy, patchy, scale like lesion called by fungus. It can be known by different names depending on the area of the body that it’s located. On the feet it is called athletes foot, in the groin area it is called jock itch, and on the scalp it is called tinea capitus.
Psoriasis can be a genetic disorder and is characterized by red patchy skin. Warts are caused by the human papillomavirus and is an uncontrolled growth of epidermal tissue. This virus can be transmitted by direct contact. Cold sores are also known as fever blisters and are caused by the herpes simplex virus and seem to be activated by stress. Impetigo is a highly contagious skin disease and is characterized by small blisters that contain pus.
Shingles are caused by the chickenpox virus or the herpes zoster virus and usually remains dormant unless triggered by stress or trauma. Vitiligo is an acquired skin disease and is often characterized by patches of white depigmented skin. The skin is one of the most important organs of the body. Without the functions of the skin the human body would never survive.
