Anatomy and Physiology of the Skin and Body Systems
Layers, outside in
Skin is the body's largest organ. The epidermis is avascular and has five strata: corneum, lucidum (palms and soles only), granulosum, spinosum, and basale. The basal layer is where mitosis happens and where melanocytes (melanin), Merkel cells (touch), and Langerhans cells (immune defense) live. The dermis has a papillary layer (capillaries, nerve endings) and a reticular layer (collagen for strength, elastin for elasticity); it houses follicles, sebaceous glands, sweat glands, and arrector pili muscles. The subcutaneous layer is fat for insulation and shock absorption.
Functions and glands
Protection, sensation, heat regulation, excretion, secretion, absorption, and vitamin D synthesis. Sebaceous glands secrete sebum and are attached to follicles, densest on the face, scalp, chest, and back. Eccrine sweat glands are everywhere; apocrine glands are in the axillae and groin. The acid mantle sits near pH 4.5–5.5 and helps suppress bacteria.
Body systems that show up on the face
Endocrine (hormones drive acne and melasma), circulatory (flushing, bruising), lymphatic (puffiness), nervous (sensation; motor nerves to arrector pili), skeletal (facial bones), and muscular (muscles of expression, which determine massage direction).
Traps: 'the epidermis contains blood vessels' (false - that is why shallow wounds do not bleed), 'melanocytes live in the dermis' (false), 'the dermis is mostly fat' (false).
Rule of thumb: when a question asks how deep something acts, think layer by layer - most cosmetics act in the corneum, acids work through the epidermis, and anything reaching the dermis is a medical-level intervention.