By Dr. Shahrzad Alimohammadi, Edited by Christopher M. Cirino, DO MPH

Introduction

As the largest organ of the body with an area of approximately 20 square feet, human skin forms a barrier between the body organs and the environment (Proksch and Jensen 2012). The function of skin is complex, such as being a key factor in the immune system as the main barrier, regulating body temperature, moisture, and sensations, including pressure, pain, heat, cold, and touch (Hoffman 2021). The skin consists of 3 main layers: hypodermis, dermis, and epidermis, from the innermost to outermost layer. Each layer is responsible for a specific function based on the cells present. Most pathological skin conditions are due to disorders of cells present in the skin or the mechanism they are involved in and pathogens (Hoffman 2021). However, systemic diseases, including infections, may also show up on the skin.

The article gives a more in-depth summary of the functions of the skin.

Function of Skin #1: Maintain Bodily Hydration

The human body needs to be moist enough to function correctly, and moisture loss affects one’s health. For that, our skin avoids water loss and the entry of harmful substances from the environment (Proksch and Jensen 2012). It plays a pivotal role in maintaining the hydration of the entire body and its organs.

Several factors help skin moisture from inside and out. Drinking enough water daily can prevent dry and dull skin. Moisturizing creams containing hyaluronic acid, glycolic acid, lactic acid, and aloe vera help replenish skin moisture. Besides topical treatments, water intake and hydrating foods with water content such as cucumber, bell peppers, celery, berries, and watermelon are highly recommended.

Limiting baths and soaps reduces the risk of skin dryness and moisture loss. Additionally, avoiding sunburn protects the skin’s barrier to moisture. Moreover, removing dead skin cells via exfoliation scrubs and agents helps skin hydration. Another magical ingredient to keep the skin moist enough is honey, which is antibacterial and helps to smooth patchy skin and reduces dryness (ReNue Pharmacy 2018) (Images Below Photo Credit Pexels).

Function of Skin #2: Regulate Body Temperature 

The skin plays a significant role in thermoregulation (Johnson et al., 2014). Our bodies rely on various reflexes to preserve internal (core) temperature in the setting of thermal stress from endogenous or exogenous triggers, including ambient temperatures, burns, and fevers. There is a thermal gradient between the skin and core body temperature. When the skin temperature is lower than the internal temperature, heat dissipates from the body. When the skin temperature is higher than the core temperature, the body gains heat. Our skin’s blood vessels respond to a significant thermogradient by either constricting or dilating, leading to heat conservation or loss. Sweat glands are also responsible for evaporative heat loss and cooling.

Thermoregulatory application of skin is fundamental in maintaining physiological body temperature and homeostasis. The sympathetic nervous system controls the muscles that line the skin’s blood vessels. In the setting of heat stress, it signals vessels to expand or vasodilate. The body adjusts to excessive core body temperatures (hyperthermia) by extensive skin vessel vasodilation. 

The skin’s vasomotor system reflexively adjusts with dilation or constriction. The system equilibrates pressure changes and is crucial in heat stress as a significant fraction of the cardiac output (8 liters/minute) reaches the skin. Thermal control of local blood vessels is essential in maximal vasodilation of the skin caused by local skin warming, which is responsible for local sensory nerves and nitric oxide production. Cooling of local skin reduces the blood flow in the skin. Many conditions, both healthy and pathologic, may alter skin blood flow. Examples include menopause, diabetes mellitus type 2, erythromelalgia, and Raynaud disorder (Charkoudian 2003).

Function of Skin #3: the Largest Sensory Organ

In addition to the thermoregulation and moisture barrier, the skin is considered the largest sensory organ of the human body. The sensory nerve receptors can detect several stimuli such as pressure, stretching, and thermal in terms of hot and cold sensations. The sensation of these stimuli delivers information from the environment to the brain for interpretation and prevention of damage. The nerve endings are present in the dermal layer of the skin, and their sensitivities vary depending on their sites. There are specialized sensory cells within the skin, which contribute to specific sensations, including touch, vibration, and stretch (see Table). The lips and fingertips are among the most sensitive structures of the skin (Richardson 2003).

Function of Skin #4: A Semi-permeable Barrier

The skin is a semi-permeable or selectively permeable membrane. It maintains a balance between the conservation of water and allowing specific molecules to enter or exit. The dermal layer of the skin contains sebaceous glands, which are outgrowths of hair follicles. These glands are capable of secreting sebum (bactericidal) and lipid to keep the skin water-resistant.

The skin permits low amounts of carbon dioxide to diffuse and releases numerous other waste products like sodium chloride and urea. On the other hand, the skin can absorb some medications, including hormones and glyceryl trinitrate (for treatment of angina) and topical preparations when applied to treat several skin diseases (Richardson 2003). The nerve gas Sarin is a concentrated organophosphate that can cross intact skin and be lethal in minimal doses. 

Function of Skin #5: Role in the Immune System

The skin is the interface of the body with the environment. Consequently, it forms a barrier, albeit semi-permeable, to harmful substances, including microbes, in the environment. The skin barrier is both structural and cellular and contributes to both innate and acquired immunity. The skin and specialized structures like hair, nails, sebaceous and sweat glands, provide a relative resistance to bacterial and fungal infection.

Within the epidermis, there are five layers, or strata, each with differing functions. As epidermal cells are formed, they are pushed outward to the outermost layer of the skin, or stratum corneum. This layer is packed with keratinocytes, dead cells which contain keratin. It is a substance that contributes to protecting and waterproofing the skin. The stratum lucidin layer contributes to thicker skin in certain areas of the body, including the palms and the soles. The statum basale has specialized cells, including basal cells, melanocytes (pigment-forming cells) and Langerhans cells.

The Langerhans cells, a special type of phagocyte, function in antigen presentation, and are sometimes referred to as Sentinel cells, given their role in the immune surveillance at the periphery of the body.

Shahrzad Alimohammadi is a Pharm.D. and currently a junior research fellow at the Department
of Immunology, University of Debrecen. She is a Ph.D. candidate, and her research mainly
focuses on the skin’s immune cells, including Langerhans cells and dendritic cells and their
possible roles in the skin!

Pharmaceutical technology, as well as dermatology, are her favorite topics when it comes to
research. In the dermatological field, Shahrzad is interested in inflammatory and immune-related
skin diseases and cosmetic dermatology.

References

Caterina MJ, Pang Z. TRP Channels in Skin Biology and Pathophysiology. Pharmaceuticals (Basel). 2016;9(4):77 Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5198052/

Charkoudian N. Skin blood flow in adult human thermoregulation: how it works, when it does not, and why. Mayo Clin Proc. 2003;78(5):603–12 

Hoffman M. The Skin (Human Anatomy): Picture, Definition, Function, and Skin Conditions [Internet]. 2021 [cited 2021 Aug 13]. Available from: https://www.webmd.com/skin-problems-and-treatments/picture-of-the-skin

Johnson JM, Minson CT, Kellogg DL. Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation. Compr Physiol. 2014;4(1):33–89 

Proksch E, Jensen J-M. Chapter 47. Skin as an Organ of Protection. In: Goldsmith LA, Katz SI, Gilchrest BA, Paller AS, Leffell DJ, Wolff K, editors. Fitzpatrick’s Dermatology in General Medicine. 8th ed. New York, NY: The McGraw-Hill Companies; 2012 [cited 2021 Aug 13]. Available from: accessmedicine.mhmedical.com/content.aspx?aid=56034571

ReNue Pharmacy. 8 of the Best Ways to Put Moisture Back Into Your Body [Internet]. ReNue Rx. 2018 [cited 2021 Aug 13]. Available from: https://renuerx.com/8-of-the-best-ways-to-put-moisture-back-into-your-body/

Richardson M. Understanding the structure and function of the skin [Internet]. Nursing Times. 2003 [cited 2021 Aug 13]. Available from: https://www.nursingtimes.net/clinical-archive/dermatology/understanding-the-structure-and-function-of-the-skin-05-08-2003/