Cosmetics are any parts of the surface of the human body, such as skin, hair, nails, and lips, which are applied by spraying and other methods. It is a chemical industrial product for the purpose of cleaning, maintaining, grooming, modifying and changing the appearance, or correcting the human body odor and maintaining good condition.
Obviously, cosmetics solve the apparent problem of the skin. Healthy skin is determined by the healthy structure and function of the skin. In order to maintain the beauty of the skin and slow down the aging process, the structure and function of the skin must be supplemented and protected. Therefore, we need to fully understand the composition of my skin and their function.
The skin consists of the epidermis, dermis, and subcutaneous tissue and is attached to the underlying tissue. The skin is in direct contact with the external environment, its structure is complex and the cell function in the skin is highly specific. The skin contains various types of cells, such as fibroblasts, keratinocytes, melanocytes, and the like. They each perform a corresponding function, making the skin a relatively independent tissue system, participating in the metabolic activities of the body together with other organ tissues.
The skin thickness is usually 0.5 to 4 mm. Among them, the thickness of the epidermis is from 0.07 mm to 1.6 mm (0.04 mm in the eye and 1.6 mm in the ankle); the thickness of the dermis is 15 to 40 times that of the epidermis, from 0.4 to 2.4 mm.
If the skin is too thick, especially if the stratum corneum and granular layer are too thick, poor light transmission will cause the skin to become yellow. If skin is too thin, the resistance to the external environment is weakened, and the sensitivity of the skin is increased.
Factors affecting pore size include endogenous and exogenous factors. Endogenous factors include heredity, exuberant secretion of hair follicle sebaceous glands, hormone levels, vitamin A deficiency and natural skin aging. Exogenous factors include chemical substances, chronic ultraviolet radiation and chronic radiation exposure.
Physiological function of the skin
- Barrier function
The normal skin of the human body has two barrier functions: on the one hand, it protects various organs and tissues in the body from mechanical, physical, chemical and biological harmful factors in the external environment. On the other hand, the loss of various nutrients, moisture, electrolytes and other substances in the tissues is prevented. Therefore, the skin plays an important role in maintaining the stability of the environment inside the body.
- Absorption function
The skin absorbs foreign substances mainly through four channels, namely the stratum corneum, hair follicles, sebaceous glands and sweat veins. The stratum corneum is the most important route of skin absorption. The physical properties of the stratum corneum are fairly stable, forming a complete semi-permeable membrane on the surface of the skin. Under certain conditions, water can pass freely through the cell membrane and enter the cell.
- Respiratory function
Except for certain special parts with a thickness of more than 1mm, the epidermis can be diffused through the surface of the skin to obtain all the oxygen it needs. The carbon dioxide that is excreted through the skin comes partly from the skin itself, and partly from the blood of superficial blood vessels. Skin respiration accounts for the proportion of the body’s total respiration: oxygen absorption is less than 1% to 1.9%, and carbon dioxide is 2.7%.
- Excretion function
After the sweat and sebum are excreted on the surface of the skin, they are mixed with the dandruff, air pollutants, etc. from which the epidermis is shed. Under the action of skin microorganisms, a sebum film that protects the skin is formed. Sebum membranes often have undesirable odors due to individual differences and environmental changes. Under certain circumstances, it produces a special odor that causes people to be unpleasant or even pungent.
- Temperature regulation function
Cosmetics are used for body surface treatments, and they are often described in terms of ease of application, heavyness, and stickiness. On the one hand, these descriptions are related to the touch of the skin. On the other hand, it is also closely related to the body temperature regulation function. Cosmetics with different dosage forms and different rheological parameters seriously affect the feeling of product use. The main factor is that the product is not closed to the skin. The closed state affects the evaporation heat or radiant heat of the skin, and the skin feels changed, causing consumers to have different preferences for certain products.
- Sensory function
Sensory nerves and motor nerves are distributed in normal skin, and their nerve endings and special receptors are widely distributed in the epidermis, dermis and subcutaneous tissues. Perceives various stimuli in the body and outside, produces various sensations, and causes corresponding nerve reflexes to maintain the health of the body. They can transmit six basic sensations: tactile, sensation, cold, warm, painful, and itchy.