Anatomy & Physiology - Hair

Hair Structure

The three layers of the hair shaft — and why color, perms, and relaxers all work at the cortex.

~4 min read·3 sections·4 key terms

circle.grid.cross.fill

The Three Layers

Every hair shaft has three concentric layers:

CUTICLE — the outermost layer. Made of overlapping scales like fish skin or roof shingles. Acts as a protective barrier. When chemical services are performed, the cuticle must be opened first so the active ingredients can reach the cortex.

CORTEX — the middle layer. Contains 90% of the hair's weight and ALL its natural melanin pigment. This is where chemical services actually work — color, relaxers, perms, lighteners all act on the cortex. Strength, elasticity, and color all live here.

MEDULLA — the innermost core. A loose layer of cells. Present in coarse and medium hair, often absent in fine hair. The medulla has no impact on chemical services.

drop.triangle.fill

What Each Layer Means in Practice

Cuticle problems → frizz, dullness, breakage. Treatment: closing the cuticle (cool water rinse, acid-balanced products).

Cortex problems → loss of color, loss of strength, breakage during chemical services. Treatment: protein treatments, lower-strength chemicals.

Key rule: ALL permanent chemical services must penetrate the cuticle and act on the cortex. This is why pH matters — alkaline products swell and open the cuticle so the cortex is accessible.

thermometer.medium

Hair pH at a Glance

Healthy hair pH: 4.5 to 5.5 (slightly acidic).

Shampoo: pH 4.5–6.5 (gentle, near hair's natural state).

Permanent color, relaxers, perms: pH 8.5–10 (alkaline — opens the cuticle to penetrate the cortex).

Neutralizing rinse / acidic conditioner: pH 2.5–4 (closes the cuticle after chemical services).

Low pH = closes cuticle = smooths hair. High pH = opens cuticle = penetrates hair.

📖 Key Terms

Cuticle: Outermost layer of overlapping scales. Protects the hair shaft.
Cortex: Middle layer containing pigment and most of the hair's weight. Where chemical services act.
Medulla: Innermost core, present in coarse hair, absent in fine hair.
Melanin: Natural hair pigment, stored in the cortex. Eumelanin = brown/black; pheomelanin = red/yellow.

💡 Exam Tips

▸If a question asks where color or chemical services work — the answer is almost always the cortex.
▸Hair's natural pH is 4.5–5.5. Anything that 'opens' the cuticle is alkaline (>7); anything that 'closes' is acidic (<7).
▸Medulla rarely matters for the exam unless the question is specifically about fine vs coarse hair.