In terms of weight and area, skin is the largest organ of human body, accounting for 14% ~ 16% of body weight. An adult weighing 60kg weighs about 8.5kg, a newborn weighing 3kg weighs about 0.5kg, the skin area of an adult is about 1.5 ~ 2.2 m2, and that of a newborn is about 0.
The skin thickness varies with people, sex, age and occupation, and is generally 0.5 ~ 4.0 mm (excluding subcutaneous adipose tissue). Children's skin is much thinner than that of adults, women of the same age are slightly thinner than men, and mental workers are slightly thinner than manual workers.
First, the structure of the skin
Skin is divided into epidermis and dermis.
Epidermis is the surface structure of skin, which is composed of multiple layers of epithelium. From basal layer to surface layer, it can be divided into five layers, namely basal layer, spinous layer, granular layer, transparent layer and stratum corneum.
The dermis is located in the deep layer of epidermis and consists of dense connective tissue. It is divided into papillary layer and reticular layer.
Subcutaneous tissue does not belong to skin, but its fibers are connected with dermis.
Second, the appendages of the skin.
Skin appendages include hair, sebaceous glands, sweat glands and nails.
Hair is widely distributed in the human body, almost all over the body. Hair has many functions, which can help regulate body temperature and is also a tactile organ.
Sebaceous glands are located between hair follicles and hairy muscles. The released secretion is called sebum, which has the function of moistening and protecting the skin.
Sweat secreted by sweat glands is excreted to the skin surface through the catheter, which can moisturize the skin, expel some water and ions, and help regulate body temperature and water-salt balance. In addition, it also secretes a small amount of nitrogen-containing metabolites.
Fingernails are located on the dorsal surfaces of the distal ends of fingers and toes, and are plate-like structures formed by the thickening of epidermal stratum corneum cells.
Human skin has six different colors: red, yellow, brown, blue, black and white, which is mainly due to the different quantity and distribution of melanin in skin. Melanin is a protein derivative, brown or black, produced by melanocytes. Because the quantity, size, type and distribution of melanin are different, different skin colors are determined. Yellow skin melanin is mainly distributed in the basal layer of epidermis, with less spinous layer; There are a lot of melanin in the basal layer, spinous layer and granular layer of blacks; The distribution of melanin in white skin is the same as that in yellow skin, but the amount of melanin is less than that in yellow skin.
In different parts of human skin, the depth of color is also different. The color of neck, back of hand, groin, umbilical fossa, articular surface, nipple, areola, perianal region and perineum is darker, and the color of palmar and plantar skin is lightest. This is because the number of melanocytes in different parts is different. For example, 1mm2 has about 2000 melanocytes in scalp and pudendum, and there are about 1000 melanocytes in other parts. In addition, melanocytes also exist in mucosa.
References:
Is skin temperature as constant as body temperature?
Because human beings are warm-blooded animals, their body temperature is constant between 36℃ and 37℃, so some people think that the skin temperature is as constant as the body temperature, which is actually wrong. Human tissues, organs and skin vary greatly in different parts.
The temperature of human liver is the highest, followed by blood vessels and muscles, and finally the skin. The skin temperature is generally lower than 37℃, which is influenced by blood circulation in the skin and external temperature. The general rule is that the trunk is higher than the limbs, and the proximal end of the limbs is higher than the distal end. The skin temperature of the head, face and palm with rich blood circulation is also higher, and the lowest is the ear shell, nose tip and fingertips. The change of outside temperature also has obvious influence on skin temperature. For example, in summer, the chest skin temperature is 36.6℃, and the thigh skin temperature is 33.6℃. In winter, the chest temperature is 36.4℃, and the hand temperature is only about 10℃, which is very significant. If the heat preservation is improper, frostbite is easy to occur.
Microcirculation of skin plays an important role in regulating body temperature. When the temperature rises inside and outside the body, the blood flow of skin microcirculation increases, the flow velocity accelerates, the blood vessels dilate, sweating increases and heat dissipates; On the contrary, when the body produces less heat and the outside temperature is low, the skin blood flow decreases, the flow rate slows down, and the blood vessels contract to prevent the skin from losing heat. Many people think that drinking can drive away the cold, which is not entirely correct. After drinking, it can dilate blood vessels in skin microcirculation, increase blood flow, temporarily warm skin and dispel cold, but at the same time it will also accelerate the loss of skin heat. Once alcohol disappears, it will feel colder. Moreover, if you drink too much and get drunk, the regulation function of blood vessels in skin microcirculation will be paralyzed, and the resistance to cold will be greatly weakened, which will easily lead to chilblain and frostbite. Therefore, it is not recommended to drink to keep warm in winter, and it is forbidden to go out when you are as drunk as a fiddler.
What physiological function does the skin have?
As the first line of defense and the largest organ of the human body, skin participates in the functional activities of the whole body and maintains the unity of opposites between the body and the external natural environment. At the same time, the abnormal situation of the body will also be reflected on the skin. Skin can accept all kinds of external stimuli, and make the body better adapt to all kinds of changes in the external environment through reflex regulation, so it has very important physiological functions, which are embodied in the following aspects:
(1) barrier skin protects various organs and tissues in the body from harmful factors such as machinery, physics, chemistry and biology in the external environment on the one hand, and prevents the loss of various nutrients, electrolytes and water in the tissues on the other hand.
(2) Sensory nerves and motor nerves are distributed in the skin, and their nerve endings and special receptors are widely distributed in epidermis, dermis and subcutaneous tissue, so as to sense various senses such as touch, cold, warm, pain, pressure and itching, and cause corresponding nerve reflexes to maintain the health of the body.
(3) Regulating body temperature is the expression of heat generated in the process of substance metabolism in the body, and it is also one of the necessary conditions for various biochemical reactions and physiological activities of body cells. Skin is an important part of body heat dissipation. The body temperature can be adjusted by vasoconstriction, hair bristling, perspiration reduction and other forms, and heat can also be dissipated by physical means such as radiation, convection, conduction and evaporation.
(4) Absorbing the skin has the ability to absorb foreign substances through the stratum corneum, hair follicles, sebaceous glands and sweat ducts, which is called percutaneous absorption, penetration or infiltration. It is indispensable for maintaining health, and it is also the theoretical basis for the treatment of dermatoses with external drugs in modern dermatology.
(5) Secreting and excreting skin has certain functions of secretion and excretion, mainly through sweat secretion by sweat glands and sebum excretion by sebaceous glands. Sweating has the functions of cooling, caring skin, excreting drugs and replacing some functions of the kidney. Sebum has the functions of forming lipid membrane on epidermis and moistening hair and skin.
Can the skin breathe?
Perhaps influenced by martial arts novels, many people think that only people with special functions and high martial arts can breathe through the skin and even live in water like fish. Actually, this is a misunderstanding. Human skin can breathe, but the amount of breathing is very small. The amount of oxygen absorbed by skin is only1160 of that of lung, which is not enough for normal metabolism, so people can't live in water.
Skin mainly absorbs external substances through three ways, namely, stratum corneum, sebaceous glands in hair follicles and sweat pores, among which stratum corneum is the most important way for skin to absorb gas. The physical properties of stratum corneum are quite stable. It forms a complete semipermeable membrane on the skin surface. Under certain conditions, gas enters the cell through the cell membrane in the form of combining with water molecules. Both living and dead keratinocytes are semi-permeable, which follows Fick's law, that is, at low concentration, the permeability of substances per unit time and unit area is directly proportional to its concentration.
The oxygen absorbed through the skin is only1160 of the lung intake, which is of little significance.
What types of skin are there?
Human skin can be roughly divided into three categories, namely, neutral skin, oily skin and dry skin.
Neutral skin: It is an ideal healthy skin. This kind of skin is not greasy, dry, dense, moderate in thickness, soft, moist and elastic. This type of skin has a wider choice of skin care products.
Oily skin: The pores of these people are bigger and more obvious than others, and the oil secretion is exuberant. Especially in youth, wiping your forehead with paper will also get a layer of oil. This kind of people are prone to acne, which is caused by excessive sebum secretion and blockage of sebaceous glands. However, such people often have shiny skin and look young. They should pay attention to cleanliness, remove dirt, eat less greasy food and choose skin care products carefully.
Dry skin: dry and rough skin, prone to wrinkles and tiny cracks, and often dandruff. This kind of people should not use more soap, especially soap with strong alkalinity, otherwise the sebum will be lost too much, and the skin will be chapped, itchy or chapped more easily. You should choose oily skin care products.
Which skin care products are most easily absorbed?
People often see or hear the propaganda of how some skin care products are easily absorbed by the skin in advertisements, but most people don't know much about how and to what extent these skin care products are absorbed by the skin. Skin mainly absorbs external substances through three ways, namely cuticle, hair follicle sebaceous glands and sweat pores. Skin care products on the market can be divided into three categories according to different accessories: water-made, oil-made, and water-oil mixed (that is, cream), and their absorption routes are also different. Water-based skin care products are absorbed through the stratum corneum, oily skin care products penetrate through the sebaceous glands of hair follicles, and cream skin care products are absorbed through the above two ways. The absorption degree of skin care products after being applied to the skin surface depends on the physical and chemical interaction between skin care substances, auxiliary materials and stratum corneum, including the following four aspects:
(1) Solubility of skin care substances in excipients The easier it is for drugs to dissolve in excipients, the easier it is to remain in excipients, and the easier it is to enter the epidermis with the molecules of excipients.
(2) Solubility of skin care substances in the stratum corneum The solubility of fat-soluble substances in the stratum corneum is obviously higher than that of pure water-soluble substances, and their absorption degree is also better, and the substances soluble in oil and water are the best.
(3) Absorption of Excipients Generally speaking, both water and oil are easily absorbed by the skin, and the law is lanolin > vaseline > vegetable oil > liquid paraffin.
(4) Generally speaking, the higher the concentration, the easier it is to be absorbed, but it is also more expensive and easy to cause waste.
Therefore, from the above aspects, we can know that all skin care substances that can be dissolved in water and oil in high concentration are easily absorbed by adding mixed excipients of water and lanolin to make oil-in-water or water-in-oil cream skin care products.
Is it a good habit to take a bath with a towel?
Many people like to scrub their skin with a towel when taking a bath, thinking that only in this way can the dirt on their skin be washed away, and regard it as a good habit of hygiene. In fact, from the physiological point of view of the skin, this is unsanitary and harmful to the skin.
The skin surface is a stratum corneum composed of completely keratinized keratinocytes. Keratinocytes are constantly differentiated from basal cells and gradually migrate to the stratum corneum through the spinous layer and granular layer. They are flat and seedless, and constantly disintegrate and fall off, forming so-called "dander". The time of this evolutionary process is called epidermal growth cycle, which takes about 3 ~ 4 weeks, and is also called epidermal renewal rate. The stratum corneum is isomorphic to the lipid film on the epidermis formed by emulsification of sebum and water on its surface, forming the outermost barrier protection layer of the skin, which plays a very important role. On the one hand, it protects all kinds of tissues and organs in the body from all kinds of damage from the external environment, such as mechanical damage such as friction, pulling, squeezing and collision, physical damage such as electric shock, light and magnetic field, and chemical damage such as acid and alkali corrosion, especially the dense stratum corneum can effectively prevent bacteria and viruses with a diameter exceeding 200nm from entering the skin. In addition, the free fatty acids in the lipid membrane on the skin surface make the pH value of the skin surface acidic, which is harmful to the growth of many bacteria. On the other hand, the semi-permeable membrane of stratum corneum can well prevent the loss of nutrients in the body. Adults lose about 240 ~ 480 ml of water through the skin every day, but if the cuticle is removed, the water loss will increase by more than 10 times.
It can be seen that when we take a bath, scrubbing the skin with a washcloth will often make the keratinocytes that have not been completely keratinized fall off prematurely, or even completely peel off the cuticle, exposing the bright red granular layer or spinous layer skin, which greatly weakens the barrier protection of the skin, and the body is vulnerable to various damages from the external environment, which will easily accelerate the aging of the skin for a long time, so it is not recommended to take a bath with a washcloth.
Why does the skin turn black after sun exposure?
In summer, many people like to sunbathe in the beach by the sea and tan their skin, which has become a fashion symbol of summer leisure and vacation in western countries. So why does the skin color darken after sun exposure?
Normal skin has the ability to absorb light to protect internal organs and tissues from light damage. The ability of light to penetrate human tissues is closely related to its wavelength and skin tissue structure. Generally speaking, the shorter the wavelength, the shallower the penetration into the skin. Skin tissue also selectively absorbs light, for example, keratinocytes in the stratum corneum absorb long-wave ultraviolet rays. Light is a continuous electromagnetic radiation wave, in which long-wave ultraviolet rays are the most destructive to living cells, so it is particularly important for the skin to absorb and defend ultraviolet rays, which can make the skin receive certain ultraviolet rays without any response.
Melanin is the best defense against ultraviolet radiation for normal skin. It is produced by melanocytes in basal layer and transported to keratinocytes adjacent to basal layer and spinous layer through dendrites. Melanin plays a major role in photoprotection by absorbing and scattering ultraviolet rays and stabilizing its free radicals. Skin melanin metabolism is generally divided into two parts, one of which is determined by heredity and is not affected by light; The other part is functional, which is influenced by many factors inside and outside the receptor, and ultraviolet light is one of the most important factors. After the sun shines on the skin, because melanin absorbs more ultraviolet rays than visible light, the absorbed energy escapes in the form of heat, which acts on the oxidation of melanin and makes melanin increase obviously. Therefore, tanning after sunbathing in summer is a manifestation of the skin's protective response to ultraviolet damage.
Skin regeneration
After skin damage, the regeneration process and repair time vary greatly due to the different damaged area and depth. Small and shallow wounds can heal in a few days without forming scars due to the migration and proliferation of epidermal cells. The bigger and deeper the damage, the longer the regeneration process. After trauma, the first thing is coagulation and hemostasis, as well as inflammatory reaction. Many neutrophils enter the local area to remove bacteria. Subsequently, many macrophages appeared, which cleared the damaged tissue and released several bioactive substances, which promoted the proliferation of fibroblasts and the growth of capillaries and produced granulation tissue. Granulation tissue is a delicate connective tissue, in which there are more fibroblasts and macrophages, less fibers and abundant capillaries. Shortly after trauma, epidermal cells around the wound proliferated and migrated to the wound surface. Sweat glands and hair follicle epithelium remaining on the wound can also proliferate to form epithelial islands covering the wound and participate in epidermal regeneration. Finally, the wound was completely covered by new epidermis and gradually formed a normal epidermis. Granulation tissue is gradually replaced by fibrous connective tissue. If the wound area is large, skin transplantation is often needed to help repair it.