THE SKIN

The skin is the body's largest and primary protective organ, covering its entire external surface and serving as a first-order physical barrier against the environment. The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.

The Skin Anatomy  

Eight Key Functions of the Skin

  1. Protection: Skin acts as a physical barrier to the external environment and provides protection for internal organs against external threats.
  2. Immune function: Our skin contains a protective acidic barrier called the acid mantle. This acidic layer has a pH between 4.2 and 6.0, which creates a hostile environment for harmful invading organisms while maintaining a favourable environment for beneficial microbes. The acid nature of skin is a key requisite for healthy skin. Skin pH can affect the synthesis and maintenance of a competent skin barrier, play a role in skin pigmentation, and ion homeostasis. Skin pH tends to be lower in people with darker skin because melanin by-products are acidic. Immune function of skin and its pHSkin pH also tends to increase slightly as we age, which can contribute to an increased risk of infection and alter our healing ability. Specific immune cells and proteins contained in the dermis activate the immune response and attack invading microbes. These cells include: Langerhans cells, memory T cells, and lymphoid cells. The surface of our skin houses millions of bacteria, fungi and viruses that compose the skin microbiome and serves as a physical barrier to prevent the invasion of pathogens. As in our gut, the skin microbiome plays essential roles in the protection against invading pathogens, the education of the immune system, and contributing back to the pH of the acid mantle.
  1. Thermoregulation: Humans, and all mammals, can maintain a stable core body temperature via thermoregulatory responses. By increasing blood flow to the skin through vessel dilation, body temperates are lowered by evaporative cooling of moist/sweaty surfaces to release body heat. However, if water lost to evaporative cooling is not replaced, body fluid homeostasis will be challenged. Sin Thermoregulation
  2. Prevention of fluid loss: In addition to the physical barrier provided by skin, it also contains lipids, proteins, amino acids, and salts that work to maintain internal body homeostasis by attracting and holding onto water. Due to this mechanism, under normal circumstances, the outer layer of our skin is about 30% water.
  1. Synthesis of vitamin D: Vitamin D is recognised as a pro-hormone, also known as calciferol. There are two major forms of vitamin D: D2 which is human-made and fortified into foods (such as milk, cheese, yogurt, cereals, and juices) and D3 which is synthesised by the skin and from eating animal-based foods (fatty fish, fish liver oil, and egg yolk). While vitamin D can be ingested through food or supplements, the skin and exposure to sunlight is the body's primary source of vitamin D. Vitamin D is essential for calcium and phosphate absorption, bone formation, renal function, and our immune function.
  2. Protection from ultraviolet radiation: Ultraviolet radiation (UVR) can cause DNA photodamage, sunburn, and both local and systemic immunosuppressive properties. Melanin and carotene give skin its colour and serve to reflect UVR as a protective mechanism to radiation damage. Melanin also has antioxidant and radical scavenging properties. Melanin is produced by melanocytes in response to increased sunlight, which is why populations that evolved in areas with more sun exposure tend to have darker skin.
  3. Interaction with the environment: Our skin gathers sensory information through free nerve endings, hairs, receptors to touch, temperature, and pain. In addition, through physiological processes like sweating or blushing, information is shared about our internal state to the outside world.
  4. Healing: Tissue restoration in response to injury.

 Skin Anatomy and Physiology

It is important to understand the layers of our skin so that we can understand how healing occurs differently based on depth. The skin has two principal layers, the epidermis and the dermis. The hypodermis is considered an extension of the skin by some sources, but not by others.

The Epidermis

  • Composed of five layers (Stratum corneum, Stratum lucidum, Stratum granulosum, Stratum spinosum, Stratum basale)
  • It is avascular
  • Its thickness varies based on location. For example, it is thickest on the heels and thinnest on the eyelids. Areas that have increased use from friction or weight bearing can build up thicker layers of skin (e.g., where a pencil rubs your writing finger or shoe rubs against your foot).
  • It has no nerves, but free nerve endings from the dermis do extend into the mid layers of the epidermis.

 

The Dermis

  • Located deep to the epidermis
  • Contains blood vessels and nerves which supply the epidermis via capillary loops and free nerve endings
  • Composed of two layers (Papillary layer, Reticular layer),

 

The Hypodermis

  • Located below the dermis and contains subcutaneous tissue.
  • It is made up of loose connective tissue, adipose tissue. It is well vascularised and well innervated.
  • It helps to attach the skin to the muscles and bones through superficial fascia, and provides insulation and cushioning through fat storage.

 

Common Skin Conditions

  • Rash: Nearly any change in the skin’s appearance can be called a rash. Most rashes are from simple skin irritation; others result from medical conditions.
  • Dermatitis: A general term for inflammation of the skin. Atopic dermatitis (a type of eczema) is the most common form.
  • Eczema: Skin inflammation (dermatitis) causing an itchy rash. Most often, it’s due to an overactive immune system.
  • Psoriasis: An autoimmune condition that can cause a variety of skin rashes. Silver, scaly plaques on the skin are the most common form.
  • Dandruff: A scaly condition of the scalp may be caused by seborrheic dermatitis, psoriasis, or eczema.
  • Acne: The most common skin condition, acne affects over 85% of people at some time in life.
  • Cellulitis: Inflammation of the dermis and subcutaneous tissues, usually due to an infection. A red, warm, often painful skin rash generally results.
  • Skin abscess (boil or furuncle): A localized skin infection creates a collection of pus under the skin. Some abscesses must be opened and drained by a doctor in order to be cured.
  • Warts: A virus infects the skin and causes the skin to grow excessively, creating a wart. Warts may be treated at home with chemicals, duct tape, or freezing, or removed by a physician.
  • Actinic keratosis: A crusty or scaly bump that forms on sun-exposed skin. Actinic keratoses can sometimes progress to cancer.
  • Herpes: The herpes viruses HSV-1 and HSV-2 can cause periodic blisters or skin irritation around the lips or the genitals.
  • Hives: Raised, red, itchy patches on the skin that arise suddenly. Hives usually result from an allergic reaction.
  • Scabies: Tiny mites that burrow into the skin cause scabies. An intensely itchy rash in the webs of fingers, wrists, elbows, and buttocks is typical of scabies.
  • Ringworm: A fungal skin infection (also called tinea). The characteristic rings it creates are not due to worms.
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