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3.2 Functions of the Skin

Thermoregulation

  • The skin's thermoregulatory function helps maintain a stable internal body temperature through two main mechanisms when temperature changes occur.

    • When you're hot:

    • Eccrine sweat glands increase sweat production.

    • Evaporation of sweat from the skin surface carries heat away, cooling the body.

    • Dermal blood vessels dilate (vasodilation), bringing more blood to the surface to increase heat loss.

    • When you're cold:

    • Eccrine sweat glands decrease sweat production.

    • Blood vessels constrict (vasoconstriction), reducing heat loss by limiting blood flow to the skin.

    • Skeletal muscles may contract continuously when you shiver to generate heat.

    • Erector pili muscles attached to hair follicles contract, causing goosebumps and trapping a thin layer of warm air near the skin.

Protection and barrier functions

  • The skin acts as a physical barrier defending against:

    • Microbes, ultraviolet radiation, physical injury, chemicals, and dehydration.

  • Stratum corneum (outermost layer):

    • Packed with tough, dead, keratinized cells that form a protective waterproof shield.

    • Rich in keratin and surrounded by lipids to form a water-resistant seal that minimizes transepidermal water loss (TEWL).

  • Water balance and entry prevention:

    • The skin helps keep the body hydrated overall and limits water entry into the skin during showers and swims.

  • Chemical and microbial defense on the surface:

    • The acidic pH of sweat slows bacterial growth on the skin.

    • Bactericidal chemicals from sebaceous secretions help kill microbes.

  • Immune defense:

    • Dendritic cells in the epidermis and macrophages in the dermis help the immune system remove foreign invaders if microbes breach the skin.

  • Pigment and UV protection:

    • Melanin, produced by melanocytes, absorbs and disperses ultraviolet radiation to protect deeper skin layers from DNA damage.

Sensory reception

  • The skin is loaded with sensory receptors that detect:

    • Pressure, vibration, temperature, pain, and light touch.

  • Signals are sent to the brain so fast you can react in milliseconds (e.g., jerking your hand away from a hot stove).

  • The skin helps you stay connected to your environment and respond to it instantly, whether it's the feel of fabric, a paper cut, or the warmth of sunlight.

Excretion and absorption

  • Excretion:

    • The stratum corneum is nearly waterproof, and about 400\,\mathrm{mL} of water evaporates through it daily.

    • Sweat also eliminates small amounts of urea, ammonia, salts, and carbon dioxide (\mathrm{CO_2}).

  • Absorption:

    • The skin can absorb certain lipid-soluble substances such as vitamins A, D, E, and K, some medications, and even small amounts of oxygen and carbon dioxide (\mathrm{O2} and \mathrm{CO2}).

    • Harmful substances can also be absorbed, including acetone, heavy metals (lead, mercury, arsenic), and irritants from poison ivy or poison oak.

  • Topical treatments:

    • Lipid-soluble substances on the skin can reduce inflammation by preventing histamine release from mast cells (e.g., cortisone creams).

    • Drugs can be delivered through the skin using adhesive patches, allowing slow, controlled release into the bloodstream.

Vitamin D synthesis

  • The synthesis of vitamin D begins in the skin when exposed to ultraviolet rays from sunlight.

  • A precursor molecule in the skin is activated by UV, then modified by the liver and kidneys to produce calcitriol, the most active form of vitamin D.

  • Vitamin D plays a role in calcium absorption and bone maintenance; without the skin's initiation of this process, calcium absorption and bone health would be impaired.

Interconnectedness and significance

  • The skin is a multitasking organ that:

    • Maintains homeostasis, defends against pathogens, regulates temperature, and initiates production of vitamin D.

    • Interacts with and supports other body systems; it communicates with the body and responds to environmental stimuli continually.

Real-world relevance and practical implications

  • Everyday skincare and decisions relate to the skin's barrier and thermoregulatory functions (e.g., sunscreen, hydration, winter skin care).

  • Topical medications rely on lipid solubility for efficacy (e.g., anti-inflammatory cortisone creams) and sometimes use adhesives patches for controlled drug delivery.

  • UV exposure has practical implications for vitamin D synthesis and bone health.

  • The skin's ability to absorb certain substances also highlights safety considerations with chemicals and irritants.

Context and closing notes

  • This video is part of Learning Biology with Dr. Vanessa, aiming to make science accessible and meaningful.

  • It references a previous video on the integumentary system (layers, cells, accessory structures) and connects to broader understanding of skin functions.

  • The creator invites viewers to like, share, subscribe, and enable notifications to stay updated with new content.

  • The takeaway: the skin is more than a protective covering; it maintains homeostasis, defends against pathogens, regulates temperature, and initiates essential processes like vitamin D production, illustrating the interconnectedness of body systems.