Introduction

To understand the concepts of a wound and wound healing, we must examine the skin and its pathophysiology, as well as its unique structures and functions. Skin care and wound management must be grounded in a comprehensive knowledge base of the structure and functions of the skin.¹The skin is the largest organ of the body, covering approximately 18 square feet and weighing about 12 pounds, or up to 15% of total adult body weight. It requires one-third of an individual’s circulating blood volume to sustain it. Normal surface skin temperature is 92 degrees, compared with a core body temperature average of 98.6 degrees.²

Functions of Skin

Our skin has multiple functions integral to our well-being and protection from the caustic environment we all inhabit.¹ These include:

• Protection against microbes, toxins, and mechanical injury
• Slightly acidic, with a pH of 4.5 to 5.5
• Sensation via nerve receptors
• Requiring close monitoring, especially in patients with neuropathy
• Water retention and maintaining needed fluids and nutrients within the skin
• Skin’s ability to utilize and maintain moisture balance affected by many disease processes and medications
• Thermoregulation via circulation and sweating
• Controlled by the dilation or constriction of the blood vessels and capillaries within the skin
• Synthesis of vitamin D in the presence of sunlight
• Expression of emotion

Let’s look a little deeper into how the skin is structured that aids in these functions. The stratum corneum is the outermost layer of the skin and is made up of dead cells called keratinocytes or corneocytes, held together by lipids. The skin’s structure can be compared to that of a brick wall. The corneocytes represent the bricks, and the intercellular lipid matrix represents the mortar or cement that holds the bricks together.¹ Knowledge of skin and wound care techniques is paramount to attain and maintain healthy skin. Health care professionals are called on to care for individuals who are at risk for alterations in skin integrity or those who present with persistent and/or recurring wounds.

Our sense of touch allows us to discern differences in pain, temperature, touch, and pressure.³ Skin temperature is 92 degrees and contributes to the homeostatic state of the body. For example, when we are exposed to excessive heat and our core body temperature rises, the blood vessels dilate in an attempt to reduce the core body temperature. The reverse is true as well. When we are exposed to excessive cold, the skin’s blood vessels constrict and help to conserve heat and energy for the core body and vital organs. Sweating is also an example of how the skin helps maintain homeostasis. By allowing fluid to be lost through the skin, the evaporation creates a cooling effect. The skin has the unique feature of being the only place in the body where the synthesis of vitamin D occurs. This process depends on the presence of sunlight. And of course, the skin contributes to our expression of our emotions.

The normal flora of the skin may include Staphylococcus epidermidis and some forms of Streptococcus. The acidic pH of the skin, or its acid mantle, along with sweat and sebum, aids in preventing these organisms from becoming pathogenic. This makes it very important to carefully select skin care products that match this pH range and reinforce rather than strip the stratum corneum of its lipids and proteins.⁴

Darker Pigmented Skin

Darker pigmented skin is impacted by pH, which may vary relative to disease processes. Diseases affecting the skin manifest with unique characteristics in darker pigmented skin. Various shades of brown skin occur across Black, African, African American, Hispanic, Asian, White, and Native American peoples. The shades of color, or pigmentation, are influenced by the distribution of melanosomes. In darkly pigmented skin, melanosomes can be seen more readily, whereas light skin has no melanosomes in the stratum corneum. Intensely pigmented skin can mask the detection of skin inflammatory reactions, which normally are seen as reddened areas on light-skinned individuals.

Inflammation may appear as darker areas in black skin, violet-black areas in intensely black skin, or black areas in brown skin.⁵ Other signs of inflammation may include the detection of heat or warmth to touch or induration, which is detected as skin tightening or hardening over areas where there is skin damage. Studies of skin biology across the range of skin color spectrum lack universal agreement because of multiple climates, customs, lifestyles, and so on. However, darker pigmented skin has a higher lipid content in the stratum corneum, increased junctional integrity, less abundant distribution of elastic fibers, and reduced levels of vitamin D.⁶

Anatomy of Skin Layers

Human skin is divided into two primary layers: the epidermis and the dermis. The epidermis is the outermost layer of the skin. The basal layer is composed of keratinocytes that lie at the base of the epidermis and migrate upward, where they eventually slough off at the outermost layer of the epidermis, known as the stratum corneum. The stratum corneum is enriched with a lipid matrix that enhances the barrier properties of the skin. It takes approximately 30 days for a layer of epidermis to be replaced for individuals under the age of 60. For those over the age of 60, this process may take as long as 45 days. The epidermis varies in thickness over the body. The thickest area can be up to 1.5 mm and is typically found on the palms of the hands and the soles of the feet. There is essentially no blood flow to the epidermis, a state known as avascularity, and is a unified layering of dead skin cells.⁷

The basement membrane zone or dermal-epidermal junction is that area found below the epidermis that separates and joins the epidermis to the dermis. Rete ridges, also called rete pegs, are epidermal “appendages” that anchor the epidermis to the dermis and are partly responsible for skin integrity. These finger-like projections help maintain the continuity of skin layers. This area is often impacted by aging because, with advancing age, this area thins out and decreases in height and strength. Older adult patients are more prone to separation or tearing of the epidermal-dermal junction, as we see in skin tears. Closer examination of the basement membrane zone in the past decade has revealed it to be more complex than previously acknowledged. Proteins, collagen, and growth factors are found in the basement membrane, and they play an important role in advanced wound care technology of biomaterials and regenerated tissue, or skin substitutes.⁸

The dermis is the inner most layer of skin, found underneath the basement membrane zone. It is primarily a connective tissue matrix made up of approximately 90% collagen along with elastin fibers that provide bulk, strength, support, and elasticity. It is where we find important structures such as blood vessels, lymph vessels, sweat, and sebaceous glands, as well as where the hair follicles reside. The epidermis and dermal layers are important to understand in discussing wound healing and help us recognize and differentiate partial-thickness wounds from full-thickness wounds.

The subcutaneous layer is a layer of fat or adipose tissue that provides insulation, cushioning, and support for other internal tissues. Serving as a storage depot for excess calories, the subcutaneous layer is poorly vascularized. It is important to note that when staging full-thickness or stage 3 and 4 pressure injuries on some anatomical locations such as the heel, malleolus, occiput, and bridge of the nose, these areas have very little to no observable subcutaneous tissue.

Conclusion

To review, the layers and structures of the skin include the epidermis, the basement membrane zone, the dermis with all of its structures, and the subcutaneous or fatty tissue underneath. This anatomy supports the skin’s overall physiology—maintenance of homeostasis, protection from external threats, vitamin D absorption, sensation, excess energy storage, and water retention. A working knowledge of the anatomy and physiology of skin helps clinicians expand their expertise beyond routine wound care practices and provides a deeper understanding of wound etiology.

References

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2. Nunez K. What are the largest organs of the body? Healthline. 2020. Accessed March 16, 2022. https://www.healthline.com/health/largest-organs-in-the-body
3. Kim JY, Dao H. Physiology, integument. In: StatPearls. StatPearls; 2021. Accessed March xx, 2022. https://www.ncbi.nln.nih.gov/books/NBK554386/
4. Yousef H, Alhaij M, Sharma S. Anatomy, skin (integument), epidermis. In: StatPearls. StatPearls; 2021. Accessed March 16, 2022. https://www.ncbi.nln.nih.gov/books/NKB470464/
5. Bino SD, Duval C, Bernard F. Clinical and biological characterization of skin pigmentation diversity and its consequences on UV impact. Int J Mol Sci. 2018;19(9):2668. doi:10.3390/ijms19092668. Accessed March 16, 2022. https://www.mdpi.com/1422-0067/19/9/2668
6. Black J, Simende A. Top ten tips: assessing darkly pigmented skin. Wounds Int. 2021;11(3):8-11. Accessed March 16, 2022. https://www.woundsme.com/uploads/resources/d34b6a6f02b2659b00636c49b453…
7. Lawson S. Skin 1: the structure and functions of the skin. Nursing Times. 2019;115(12):30-33. Accessed March 16, 2022. https://www.nursingtimes.net/clinical-archive/dermatology/skin-1-the-st…
8. Rousselle P, Gentilhomme E, Neveux Y. Epidermal physiology. In: Humbert P, Fanian F, Maibach H. Agache P, eds. Agaches’s Measuring the Skin.2nd ed. Springer; 2017:397-405. doi.org/10.1007/978-3-319-32383-1_36