Hair Physiology
The development of hair begins in the mother's womb, which can be considered as fetal origin. The quality of each person's hair is determined in the womb and carries genetic traits. Human hair organs develop between 9 weeks and 4 months after fetal life begins. Hair organs start forming from the head and gradually move downwards, appearing on the skin. The hair matrix cells, like the growth of a fetus, deeply penetrate into the skin to form the hair follicle.
Functions:
1. Keeping warm
2. Protecting the scalp (from UV rays)
3. Aesthetic appeal
Formation Process of Hair:
Firstly, food enters the intestines, where the gastric acid enzyme in the stomach breaks it down into various amino acids. These nutrients are then transported through capillaries to the hair papilla (Xuzhou Beauty and Hairdressing School [http://www.xzainisi.org]). The hair papilla converts these nutrients into proteins, which harden, stack, and align to form hair.
Hair is formed by the hardening and alignment of dead proteins.
Common Knowledge about Hair:
A person can grow approximately one million hairs in their lifetime.
Hair grows about 1-2 cm per month. Growth is faster in spring and summer than in autumn and winter, faster in women than in men, and faster during the day than at night.
Only healthy hair can create beautiful hairstyles.
Dry hair may be caused by genetic factors.
Using conditioners or deep conditioning treatments can improve hair quality, making hairstyles last longer and appear shinier.
Hair Growth Cycle:
1. Initial Stage - Growth cells sprout within the hair papilla, beginning to divide.
2. Growth Stage - Grows at an average rate of 0.4-0.5 mm per day.
3. Degeneration Stage (Transition Stage) - Growth slows and stops.
Approximately 25cm - 30cm
4. Resting Stage - Hair cells die (when hair reaches 30cm, it stops growing for 3-4 months), and some hair begins to naturally fall out.
Life Cycle: From the start of growth in the hair follicle to natural shedding, each strand of hair lasts approximately 2-6 years.
After shedding, a new hair grows from the original follicle.
The peak of hair growth speed occurs between ages 16-24, and after age 65, the growth rate slows.
Hair growth rates vary depending on location, so after some time, they become uneven and require trimming to maintain a hairstyle (Xuzhou Beauty and Hairdressing Training [http://www.xzainisi.org]).
7. Hair that has stopped growing: Once growing hair becomes stagnant, it no longer grows. At this point, the hair follicle gradually loosens, and some hair slowly moves upward until it falls out. This period can range from days, months, even years. External forces such as scalp massage, washing, and combing will accelerate the shedding of stagnant hair. Stagnant hair accounts for 15% of all hair.
On average, hair lost due to natural metabolism is about 75 strands per day. Therefore, with black hair totaling around 120,000 strands, the replacement of new hair is not very noticeable.
Generally speaking, each strand of hair has a lifespan of about 3-7 years. Under normal circumstances, 50-100 strands of hair fall out daily, and the birth rate and death rate of hair are the same, meaning that every hair that falls out is replaced by new hair.
Number of Hairs:
African people have approximately 90,000 hairs.
Caucasian people have approximately 140,000 hairs.
Asian people have approximately 100,000 - 110,000 hairs.
Straight hair: Hair diameter is round.
Wavy hair: Hair diameter is elliptical.
Curly hair: Hair diameter is flattened.
The shape of the hair is due to the different arrangements of hair follicles under the skin.
Structure of Hair:
1. Cuticle Layer - Composed of fish-scale-like keratinocytes arranged in the direction of the hair tip. Generally, the cuticle layer of hair consists of 6-12 layers of cuticles (fine soft hair 6-8, coarse hard hair 8-12). It protects the hair against external damage, such as structural breakdowns. When the hair is wet, the cuticle scales swell and are more easily damaged (alkaline opens at high temperature, acidic closes at low temperature).
2. Cortex Layer - Composed of protein cells and pigment cells, it makes up 80% of the hair, forming the main body of the hair. It contains the following connectives: salt bridges, sulfur bridges, fibrous cortical cells twisted like braids, giving elasticity, tension, and toughness. The natural color of hair (melanin) exists within the cortex layer, composed of two pigments: blue-black melanin and red-yellow melanin, whose proportions determine the color of the hair.
3. Medulla Layer - Located in the innermost layer of the hair, surrounded by cortical cells, arranged as vacuum sponge-like structures (determines the thickness of the hair). When tightly packed, the hair is coarse and hard (some hair lacks this).
Four Key Bonds:
1) Hydrogen Bond: Reorganizes upon high temperature, breaks when exposed to water.
2) Salt Bond: Healthiest when the pH value of the hair is 4.5-5.5.
Hydrogen Bond & Salt Bond both break when exposed to water; using hot air recombines them until the next rinse (affected by water and temperature, control these bonds while blow-drying).
3) Disulfide Bond: A strong bond that can break and recombine (controlled during hair perming), determining whether the hair is straight or curly.
4) Amide Bond: Determines the shine and elasticity of the hair (non-existent in fine soft hair, only present in coarse hard hair).
Properties of Hair:
1 Physical: Elasticity, Tension, Stretchability
2 Chemical: Hygroscopicity, Drainage
PS: Healthy hair has weak hygroscopicity and strong drainage.
Damaged hair has strong hygroscopicity and weak drainage.
Hair Quality Analysis:
Healthy Hair: No permanent waving or dyeing, tightly closed cuticles, good luster, easy to comb.
1. Oily Hair: Excessive sebum secretion.
2. Dry Hair: Lack of oil, dry without luster, difficult to comb.
3. Neutral Hair: Beautiful, shiny, resilient.
Damaged Hair:
1. General Damage: Color level 5-6, dry ends.
2. Severe Damage: Color level 8-10, loss of cuticles (strong hygroscopicity, hair doesn't dry easily if not blow-dried, loses drainage ability).
PS: Cuticles flip up, fall off, lack luster, no elasticity, tangles easily.
pH Value of Hair:
Hair pH is 4.5-5.5
Shampoo 6-6.5
Permanent wave solution 8-10
Conditioner 2.8-3.5 Mask 3.5-3.8
Straightening cream 11-13
Hydrogen peroxide 3 Dye 9
PS: Alkaline opens cuticles, acidic tightens cuticles.
Factors that Damage Hair in Daily Life:
1. Chemical Damage (excessive perming and dyeing)
2. Incorrect or low-quality shampoo (improper washing techniques)
3. Sunlight UV exposure
4. Incorrect cutting and blow-drying
5. Sea water (alkaline)
6. Sulfate copper in tap water
Related Articles: Basic Common Knowledge of Hair