Even at its thickest point, our skin is only a few millimeters thick. But it is still our heaviest and largest organ, making up about one seventh of our body weight: Depending on your height and body mass, it weighs between 3.5 and 10 kilograms (7.5 and 22 pounds) and has a surface area of 1.5 to 2 square meters. This goes to show how important skin is for your body and metabolism.
What does skin do?
▪️ Skin has a lot of different functions. It is a stable but flexible outer covering that acts as barrier, protecting your body from harmful things in the outside world such as moisture, the cold and sun rays, as well as germs and toxic substances.
▪️ Just looking at someone’s skin can already tell you a lot – for instance, about their age and health. Changes in skin color or structure can be a sign of a medical condition. For example, people with too few red blood cells in their blood may look pale, and people who have hepatitis have yellowish skin.
▪️ Skin also plays an important role in regulating your body temperature. It helps prevent dehydration and protects you from the negative effects of too much heat or cold. And it allows your body to feel sensations such as warmth, cold, pressure, itching and pain. Some of these sensations trigger a reflex, like automatically pulling your hand back if you accidentally touch a hot stove.
▪️ Skin also functions as a large storeroom for the body: The deepest layer of skin can store water, fat and metabolic products. And it produces hormones that are important for the whole body.
▪️ If skin is injured, the blood supply to the skin increases in order to deliver various substances to the wound so it is better protected from infections and can heal faster. Later on, new cells are produced to form new skin and blood vessels. Depending on how deep the wound is, it heals with or without a scar.
To be able to do all of these things, skin consists of three different layers:
1. the outer layer (epidermis).
2. the middle layer (dermis).
3. and the deepest layer (subcutis).
Depending on where it is on your body and the demands made on it, your skin varies in thickness. The thickness of your skin depends on your age and sex too: Older people generally have thinner skin than younger people do, and men generally have thicker skin than women do.
The outermost layer of the skin, composed of epithelial tissue, is known as the epidermis. It contains squamous cells, or keratinocytes, which synthesize a tough protein called keratin. Keratin is a major component of skin, hair, and nails. Keratinocytes on the surface of the epidermis are dead and are continually shed and replaced by cells from beneath. This layer also contains specialized cells called Langerhans cells that signal to the immune system when there is an infection. This aids in the development of antigen immunity.
The innermost layer of the epidermis contains keratinocytes called basal cells. These cells constantly divide to produce new cells that are pushed upward to the layers above. Basal cells become new keratinocytes, which replace the older ones that die and are shed. Within the basal layer are melanin-producing cells known as melanocytes. Melanin is a pigment that helps protect the skin from harmful ultraviolet solar radiation by giving it a brown hue. Also found in the basal layer of the skin are touch receptor cells called Merkel cells.
The epidermis is composed of five sublayers:
▪️ Stratum corneum: The top layer of dead, extremely flat cells. Cell nuclei are not visible.
▪️ Stratum lucidum: A thin, flattened layer of dead cells. Not visible in thin skin It is found in the thick skin of the palms and soles of the feet.
▪️ Stratum granulosum: A layer of rectangular cells that become increasingly flattened as they move to the surface of the epidermis It contains diamond-shaped cells with granules, and the keratinocytes are pushed from the squamous layer to this layer, as they enlarge, flatten and stick together.
▪️ Stratum spinosum: A layer of polyhedral-shaped cells that flatten as they get closer to the stratum granulosum They contain pale-pigmented nuclei but are active producing fibrous proteins called cytokeratin that assemble inside the cell to form tensor fibrils that in turn form binding particles. These bonding particles provide a strong bond between adjacent keratinocytes.
▪️ Stratum basale: The innermost layer of elongated column-shaped cells. Known as the germ layer, it contains small, circular cells called basal cells that divide continuously and push new cells to push older cells toward the surface of the skin.
The layer beneath the epidermis is the dermis, the thickest layer of the skin. The main cells in the dermis are fibroblasts, which generate connective tissue as well as the extracellular matrix that exists between the epidermis and the dermis. The dermis also contains specialized cells that help regulate temperature, fight infection, store water, and supply blood and nutrients to the skin. Other specialized cells of the dermis help in the detection of sensations and give strength and flexibility to the skin.
Components of the dermis include:
▪️ Blood vessels: Transport oxygen and nutrients to the skin and remove waste products. These vessels also transport vitamin D from the skin to the body.
▪️ Lymph vessels: Supply lymph (milky fluid containing white blood cells of the immune system) to skin tissue to fight microbes.
▪️ Sweat glands: Regulate body temperature by transporting water to the skin’s surface where it can evaporate to cool down the skin.
▪️ Sebaceous (oil) glands: Secrete oil that helps waterproof the skin and protect against microbe build-up. These glands are attached to hair follicles.
▪️ Hair follicles: Tube-shaped cavities that enclose the hair root and provide nourishment to the hair.
▪️ Sensory receptors: Nerve endings that transmit sensations such as touch, pain, and heat intensity to the brain.
▪️ Collagen: Generated from dermal fibroblasts, this tough structural protein holds muscles and organs in place and gives strength and form to body tissues.
▪️ Elastin: Generated from dermal fibroblasts, this rubbery protein provides elasticity and helps make the skin stretchable. It is also found in ligaments, organs, muscles, and artery walls.
The innermost layer of the skin is the hypodermis or subcutis. Composed of fat and loose connective tissue, this layer of the skin insulates the body and cushions and protects internal organs and bones from injury. The hypodermis also connects the skin to underlying tissues through collagen, elastin, and reticular fibers that extend from the dermis.
A major component of the hypodermis is a type of specialized connective tissue called adipose tissue that stores excess energy as fat. Adipose tissue consists primarily of cells called adipocytes that are capable of storing fat droplets. Adipocytes swell when fat is being stored and shrink when fat is being used. The storage of fat helps insulate the body and the burning of fat helps generate heat. Areas of the body in which the hypodermis is thick include the buttocks, palms, and soles of the feet.
Other components of the hypodermis include blood vessels, lymph vessels, nerves, hair follicles, and white blood cells known as mast cells. Mast cells protect the body against pathogens, heal wounds, and aid in blood vessel formation.
“How does skin work?”, www.ncbi.nlm.nih.gov,
Regina Bailey (8-11-2017), “Integumentary System”، www.thoughtco.com
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