How Skin Works
How Skin Works — the body's largest organ and its first line of defence. It is built from three layers — the epidermis (the outer barrier), the dermis (which holds collagen, elastin and blood vessels) and the hypodermis (the deeper fat layer) — that together protect against water loss, microbes, UV and physical damage while regulating temperature and sensation.
Key points
- The epidermis constantly renews itself, pushing new cells to the surface roughly every 4–6 weeks in a process called cell turnover.
- The outermost stratum corneum, plus the acid mantle and microbiome, form the barrier that keeps water in and irritants out.
- The dermis contains the collagen and elastin that give skin its firmness and bounce; these decline with age and sun exposure.
- Healthy skin sits at a slightly acidic pH of about 4.7–5.5, which supports barrier enzymes and beneficial bacteria.
In this guide
- Skin Anatomy
- The Epidermis
- The Dermis
- The Hypodermis (Subcutaneous Layer)
- The Stratum Corneum
- Skin Layers
- Keratinocytes
- Fibroblasts
- Melanocytes
- Langerhans Cells
- The Skin Microbiome
- The Acid Mantle
- Skin pH
- Transepidermal Water Loss (TEWL)
- Natural Moisturizing Factor (NMF)
- The Skin Renewal Cycle
- Cell Turnover
- Desquamation
- Sebum and the Sebaceous Glands
- Skin Lipids
- Ceramides in the Skin Barrier
- Cholesterol and Fatty Acids in Skin
- Collagen
- Types of Collagen in Skin
- Elastin
- The Extracellular Matrix
- Hyaluronic Acid in the Skin
- Glycosaminoglycans
- Melanin and Skin Pigmentation
- Melanogenesis
- The Fitzpatrick Skin Types
- Skin Tone and Undertone
- Inflammation in Skin
- The Skin Immune System
- Oxidative Stress
- Free Radicals and Reactive Oxygen Species
- Antioxidant Defense in Skin
- Glycation and Skin Aging
- The Endocrine System and Skin
- Hormones and Skin Health
- Cortisol and the Skin
- Androgens and Sebum Production
- Estrogen and Skin
- Blood Supply to the Skin
- Nerve Endings and Skin Sensation
- Sweat Glands and Thermoregulation
- Hair Follicles and the Pilosebaceous Unit
- The Nail Unit
- Skin Hydration and Water Content
- The Lipid Bilayer of the Stratum Corneum
- Skin Elasticity and Firmness
- Skin Density and Thickness
- Intrinsic vs Extrinsic Skin Aging
- The Circadian Rhythm of Skin
Frequently asked
What are the three layers of skin?
From outside in: the epidermis (barrier and pigment), the dermis (collagen, elastin, blood supply, glands) and the hypodermis or subcutaneous fat.
How long does skin take to renew itself?
A full cell-turnover cycle takes roughly 28–40 days in healthy adults and slows with age, which is why exfoliation and retinoids can help maintain a brighter surface.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
