Basic Skin Physiology
Adapted from Orsted HL, Keast D, Lalande FL, Kuhnke J, O’Sullivan-Drombolis D, Jin S, Haley J, Evans R. Skin: Anatomy, Physiology and Wound Healing. Wound Care Canada. 2016.
What is skin?
Skin is the largest organ in the body. It has two main layers: the epidermis and the dermis. These layers are supported by several other underlying structures. Skin’s most important job is to create a barrier between the environment and your internal organs. Skin also regulates and controls various other functions. Any breach of the skin is considered a wound.
What is skin made of?
1. Epidermis: outermost layer of the skin.
The epidermis creates a waterproof barrier to hold moisture in and keep moisture out. The thickness of the epidermis varies depending on where it is located on the body. For example, it is thinner on the tympanic membrane of your eardrum and thicker on the sole of your foot.
2. Dermis: inner layer of the skin, which lies just beneath the epidermis.
The dermis provides structure to the skin and is necessary for wound closure. The dermis contains sweat glands, which help regulate body temperature, and sebum, which keeps the skin from drying out.
3. Blood: the bodily fluid that supplies nutrients to body tissue and collects waste.
Blood is mostly water (about 90%), but also contains proteins, glucose, mineral ions, hormones, carbon dioxide, platelets and blood cells. Blood distributes the nutrients and collects the waste necessary for tissue to heal.
4. Lymph: a clear fluid that consists of water, lipids, waste products, white blood cells, and several other materials.
Lymph does several things, including: returning proteins to the bloodstream, picking up bacterial and transporting them to damaged lymph nodes, and transporting fats from the digestive system.
What other structures underlie skin?
1. Subcutaneous tissue (hypodermis): fat-filled cells under the dermis.
The hypodermis regulates skin and body temperature, insulates the body and absorbs trauma. The size of this layer varies throughout the body, and from person to person.
2. Fascia: web of strong connective tissue between the skin and muscles.
Fascia maintains the skin’s structural integrity, provides support and protection, and absorbs shock. Fascia is the first line of defence against infections.
3. Muscle: specialized tissue that is able to contract and to conduct electrical impulses.
4. Tendons: bands of connective tissue that connect muscle to bone.
5. Ligaments: short bands of connective tissue that connect bones to other bones to form a joint.
6. Bones: hard, white, connective tissue that provides protection and rigid strength and support.
7. Joints: the locations at which two or more bones make contact.
Joints allow movement and provide mechanical support.
8. Synovium: thin layer of tissue lining joints and tendons.
Synovium acts as a lubricant to reduce friction in the joint during movement.
9. Cartilage: dense connective tissue that intermediates between bones and tendons.
What does skin do?
1. Skin is a defence barrier.
Skin protects against trauma from water, chemicals, microorganisms, mechanical stress and irradiation.
2. Skin regulates temperature.
Skin regulates temperature by interacting with a brain structure called the hypothalamus. The hypothalamus has temperature sensors that cause sweating when the body overheats and increase heat production when body temperature drops too low.
3. Skin connects the mind, the body and the external world.
4. Skin manufactures hormones and neurotransmitters.
Skin makes and uses hormones like vitamin D, steroids and thyroid hormones. Skin also makes neurotransmitters and hormones used in the brain.
5. Skin assists the immune system.
When a foreign material is present, special skin cells (called mast cells) trigger an immune reaction, which signals other components of the immune system to become active.
6. Skin is a sensory organ.
Skin helps us detect temperature, touch and vibration.
7. Skin helps us communicate.
Skin acts a signal for social and sexual communication.
8. Skin is self-sustaining.
Millions of skin cells are shed every day, and new skin cells are regenerated to take their place. Humans are left with a brand new epidermis ever 35-45 days!
How does skin change with age?
1. The skin’s regrowth process slows.
The thinning of the outer epidermal layer can decrease a protein called collagen, which gives skin its ability to stretch and then return to shape. This means that older skin is more likely to wrinkle.
2. Skin’s pH becomes more neutral, making it more susceptible to infection.
As we age, our skin becomes less acidic. Reduced acidity results in skin killing fewer bacteria than before.
3. Skin becomes less firm and less elastic.
Skin cells are replaced more slowly as we age. The aging process causes biochemical changes in the protein collagen, which gives skin its structure and its firmness, and the connective tissue elastin, which gives skin its elasticity. The rate of this change differs from individual to individual, depending on genetics, overall health, sun exposure, and skin care.
How does skin heal?
When skin is damaged, it attends to heal itself to continue to protect the body. This regeneration process occurs naturally in healthy individuals.
There are four phases of wound healing:
1. Hemostasis: begins immediately following injury to the skin.
Special blood cells called platelets block off damaged blood vessels. Platelets also secrete proteins called growth factors, which help initiate later healing steps.
2. Inflammation: swelling and warmth associated with the pain of the injury.
Inflammation usually lasts up to 4 days following the injury. Inflammation causes blood vessels to leak plasma and microorganisms into surrounding tissue, which helps prevent infection. During this stage, growth factors help the cell divide and produce more cells. Specialized cells called macrophages can consume bacteria, providing another line of defence against infection. These cells also help resolve the inflammation at the end of this stage, causing transition into stage 3.
3. Proliferation (also known as granulation and contraction): involves replacement of dermal tissue (and subdermal tissue in deeper wounds).
The proliferation stage begins about 4 days after injury to the skin, and usually continues until day 21. Special cells called fibroblasts secrete collagen, which assists with the regeneration of dermal tissue. This healing stage is also characterized by contraction, during which the edges of the wound are pulled together.
4. Remodeling (also known as maturation): realignment of collagen.
Remodeling can take up to 2 years after injury to the skin. After remodeling occurs, skin’s ability to stretch and return to shape will be only 70-80% as strong as before the injury. Scar tissue forms and becomes thicker over time.
What is the difference between an acute wound and a chronic wound?
An acute wound heals normally, following the four steps outlined above. This occurs when the cause of the wound is removed and the environment is ideal for healing. Acute wounds differ in the time it takes to heal depending on the size of the wound.
Chronic wounds are acute wounds that do not progress through the four stages of healing normally. Failure to follow these stages in the correct order and within the given timeframe is caused by unresolved factors that interfere with healing.
Some chronic wounds eventually progress through these stages, but proper anatomy and function are not restored. This occurs when causes of the wound are not corrected or the environment for healing is not ideal.
Some chronic wounds do not progress through these stages. This occurs when the cause of the wound, or other factors affecting the wound, cannot be resolved.