7.3 Functions of the Skin

NIC Scientific Concepts I.E closes histology with functions of the skin. Illinois 1175.710 again filters the list: anatomy and physiology as they relate to applicable services. Memorize six functions, then attach a treatment-room consequence to each. A seventh textbook topic — cutaneous vitamin D synthesis under UV — shows up in some outlines; even then, you still finish with sunscreen and you still do not diagnose a deficiency.

The six functions you must be able to name, in any order the stem uses:

1. Protection
2. Sensation
3. Heat regulation (thermoregulation)
4. Excretion
5. Secretion
6. Absorption

Protection: Acid Mantle and Melanin

Protection is physical, chemical, and pigment-based.

• Physical: stacked corneocytes and keratin in the stratum corneum resist abrasion and slow water loss.
• Chemical: the acid mantle — sebum, sweat, and barrier lipids — sits at about pH 4.5–5.5. That slight acidity discourages many pathogens and keeps enzymes that process desquamation in range. Alkaline soaps and over-stripped barriers shove pH up and leave the surface vulnerable.
• Melanin: produced by basal melanocytes, packaged into keratinocytes, and used as a UV barrier. It is not a substitute for sunscreen. Darker constitutive pigment is more UV-protective and still burns; lighter skin has less melanin and more sunburn risk. Post-inflammatory hyperpigmentation is melanin responding to injury — including injury from a peel that went too far.

Protection is why Illinois sanitary rules refuse service on inflamed, infected, or erupting skin, and why a living-layer wound is not a facial: you would be breaking the organ whose job is to keep the outside out.

Sensation

Cutaneous nerve endings report touch, pressure, temperature, and pain. Papillary Meissner’s corpuscles read light touch; deeper Pacinian receptors read pressure; free nerve endings read pain and temperature. Sensation is the client’s emergency brake. A peel, wax, or machine that is “within the recipe” is still too much if this client feels burning. Stop, remove, cool as protocol allows, and reassess. Numb skin from a medical anesthetic is not an Illinois esthetics workaround; if the client cannot feel, you have lost the warning system 3A-1 already refused to let you replace with muscular or nervous “treatment.”

Heat Regulation

Skin dumps or conserves heat three ways you must name:

If a stem asks how the skin cools the body, start with eccrine sweat and vasodilation. Do not credit apocrine odor glands as the primary cooling system.

Excretion and Secretion

Candidates mix these two because both involve glands.

• Excretion is waste leaving in sweat: water, salt, small amounts of urea and other solutes. It is not a liver cleanse. A body wrap does not “excrete toxins from organs.”
• Secretion is sebum from sebaceous glands (plus the hydrolipid mix that becomes the acid mantle). Secretion lubricates and waterproofs. Excess sebum is an oily-skin and comedone conversation, not a diagnosis of PCOS.

If the stem says waste in sweat, the function is excretion. If it says oil onto the surface, the function is secretion.

Absorption and Product Chemistry

The stratum corneum is a lipid-rich brick-and-mortar wall. Lipid-soluble (oil-soluble, lipophilic) substances move through that mortar more readily than large water-soluble (hydrophilic) molecules. That is the product-chemistry hook 1175.710 already said the exam would test:

• Small, oil-soluble cosmetics can enter the corneum more easily.
• Water-based gels hydrate the surface and may sit in the horny layer.
• Occlusives (petrolatum, heavy oils) reduce TEWL by sitting on top.
• Humectants attract water to the surface; without occlusion on dry air, they can pull water from deeper tissue and leave the client tighter.

Illinois absorption is still cosmetic and corneum-limited. You are not delivering a systemic drug through living dermis. You do not claim a serum “replaces a prescription.” You choose texture and solubility so the product behaves on the surface you are licensed to beautify.

Barrier disruption after over-exfoliation, and TEWL

Transepidermal water loss (TEWL) is water evaporating through the stratum corneum. A healthy acid mantle and intact lipids keep TEWL low. Over-exfoliation — stacked acids, daily scrubs, aggressive microdermabrasion, dermaplaning on already-compromised skin — strips corneocytes and lipids. TEWL rises. The client reports tightness, sting, flakes, and sometimes delayed post-inflammatory hyperpigmentation. Living keratinocytes below are now under-protected. That is a barrier-repair problem (gentle cleanse, occlusive/moisturizer, SPF, pause acids), not a reason to “go deeper.”

Over-exfoliation is also how a superficial service starts to function like a living-layer injury. Stay inside the intended corneum target. If the barrier is already disrupted, you do not add another mechanical pass because the textbook said 28-day turnover “should” be sped up.

Why SPF Is the Finish of a Facial

Finishing, moisturizer, and home care sit in a later Skin Care and Services block (NIC II.I territory). Histology is why that finish is not optional. After you cleanse and exfoliate, the corneum is thinner, more UV-permeable, and more likely to pigment or burn. Melanin is a UV barrier, not a complete one. SPF (broad-spectrum sunscreen as a cosmetic last step) is how you protect the organ you just worked on. Skipping SPF after a peel or dermaplaning is how a legal surface service produces an illegal-looking injury. Preview the rule now: the facial is not done when the mask comes off. It is done when the barrier is covered and the client is told to reapply sun protection.

Beautify; Do Not Give Medical-Treatment Advice

225 ILCS 410/3A-1 is cosmetic or beautifying purposes, not treatment of disease. You may:

• Improve the look of the surface.
• Advise what is cosmetically appealing (a gentler cleanser, an SPF, a pause on acids).
• Refuse service and refer suspicious lesions, infection, or disease.

You may not:

• Diagnose eczema, psoriasis, or cancer.
• Tell the client which drug, laser, or prescription peel to use.
• Hold out a living-layer medical procedure as esthetics.

Protection, sensation, heat, excretion, secretion, and absorption are the functions. The Illinois sentence that sits on top of all six is the same one that opened this chapter: work the stratum corneum, know the living layers, and leave medical treatment to medicine.