Basic Composition Structure of the Human Body
I. Basic structure of cells.
1. Human body cells are composed of cell membranes, cytoplasm, and nuclei.
(1) Cell Membrane: A semi-transparent membrane that wraps around the surface of the cell. It maintains the integrity of the cell, has selective permeability, controls the entry and exit of ions and molecules. Nutrients can enter the cell through the cell membrane to supply the needs of cellular life activities. Waste within the cell can also be expelled through the cell membrane. The cell membrane has a certain self-repairing ability; when it is slightly damaged, it can repair itself. If severely damaged, it can lead to cell death.
(2) Cytoplasm: (Cell plasma) located between the cell membrane and nucleus. Contains nutrients required for cell growth, reproduction, and self-repair. It is an important site for cell metabolism and substance synthesis. The cytoplasm is composed of matrix and organelles.
(Organelles: Mitochondria, Golgi apparatus, centrioles, inclusions)
(3) Nucleus: The dense material at the center of the cell, it determines the nature of the cell.
(Cell nucleus: Nuclear membrane, nucleolus, nucleoplasm, chromatin)
Nuclear membrane: A semi-transparent membrane surrounding the cell nucleus, it maintains the integrity of the nucleus and has selective permeability.
Nucleolus: A dense area, it is related to protein synthesis.
Nucleoplasm: A colorless, transparent gel-like substance within the nucleus.
Chromatin: Small particles within the nucleus, related to heredity. Chromatin aggregates from dispersion to form chromosomes.
2. Functions of cells.
(1) Cell proliferation —— Enables human growth, development, and wound healing.
(2) Cellular metabolism —— Absorbs nutrients to promote life growth while excreting waste.
(3) Growth conditions of cells —— Appropriate activity, sufficient nutrients, oxygen, water, synthetic nutrition intake. Drinking more water, combined with massage techniques, supplementing nutrients, benefits skin health.
II. Structure of the skin.
1. The skin is located on the surface of the human body and is the largest organ. The total skin area of an adult is approximately 1.5-2 square meters. Its weight accounts for about 15% of body weight. The surface of the skin is soft and smooth, and under a magnifying glass, it appears with many raised skin ridges and depressed skin grooves. Many small holes on the surface are called sweat pores.
2. The thickness of the skin is 0.5-4mm, where the skin on the eyelids is the thinnest, and the skin on the palms and soles is the thickest. Skin not only varies in thickness but also in softness, hardness, and color. Gender differences affect the fineness of the skin. Skin changes and varies due to region, race, occupation, and season.
3. The structure of the skin can be divided into: epidermis, dermis, subcutaneous tissue.
(1) Epidermis: There are no blood vessels in the epidermis, but there are nerve endings that help us perceive external things.
The epidermis can be divided into: stratum corneum, translucent layer, granular layer, spinous layer, basal layer.
Basal cells in the basal layer are the biochemical source of epidermal cells. They continuously produce new cells, which are pushed towards the epidermis to become various layers of cells, eventually turning into dead cells. From the generation of one basal cell to its shedding as keratin, it takes about 28 days. The average thickness of the epidermis is 0.07-2mm.
① Stratum Corneum: The outermost layer of the epidermis, composed of 4-8 layers of flat, non-nucleated cells. Cells are tightly arranged, providing protection for the body. Metabolism occurs every 7-14 days. This is a non-living protective layer, and the stratum corneum represents the final stage of cell differentiation.
The thickness of the stratum corneum affects skin tone and absorption capabilities:
a. Overly thick stratum corneum: Skin appears dull yellow, lacks luster, roughness, and may even affect normal physiological functions, impacting skincare product absorption.
b. Overly thin stratum corneum: Decreased resistance, prone to dryness, dehydration, lack of protection, leading to spots and sensitivity.
Functions of the stratum corneum: Resistant to friction, prevents evaporation of internal moisture, prevents invasion and harm from external substances, protects against UV rays.
Water content: Accounts for 10%-20% of the epidermis. If less than 10%, the epidermis will lack water, appear dull, yellowish, lack luster, feel tight. Severe cases may result in peeling.
② Translucent Layer: Located below the stratum corneum, found only on hands and soles. Composed of 2-3 layers of flat, non-nucleated, transparent dead cells, appearing colorless and transparent. Light can pass through this layer, preventing the evaporation of internal and external moisture, acting as a protective barrier.
③ Granular Layer: Composed of 2-4 layers of rhomboidal cells. These cells are almost dead and are about to transform into keratinized cells. Contain fine granular substances that refract light, reducing UV penetration into the body.
④ Spinous Layer: Located below the granular layer, composed of 4-8 layers of spiny polygonal cells, making it the thickest layer of the epidermis. Spines connect the cells, with intercellular spaces containing tissue fluid providing nutrients. The spinous layer contains numerous sensory nerve endings that can perceive external stimuli.
⑤ Basal Layer: Located below the spinous layer, it is the lowest layer of the epidermis, connected wavelike to the dermis, composed of basal cells and melanocytes.
a. Basal cells are cylindrical, arranged in a single layer, directly absorbing nutrients from capillaries in the dermal papillary layer. They have the ability to divide and reproduce, serving as the biochemical source of all epidermal layers. When the skin is injured, if the basal cells are not destroyed, the skin can fully recover over time without scarring.
b. Melanocytes are tree-shaped, sparsely distributed among the basal cells, with the function of secreting melanin granules. Melanin granules absorb UV rays from sunlight, preventing them from entering deeper tissues. The stronger the external UV rays, the more melanin granules secreted by melanocytes, determining the depth of one's skin tone.
(2) Dermis: Located beneath the epidermis, firmly connected in a wavy pattern, its thickness is approximately ten times that of the epidermis. Composed of large amounts of fibrous connective tissue, cells, and matrix, it contains abundant blood vessels, lymphatic vessels, nerves, and glands.
There are three types of fibrous connective tissue in the dermis: collagen fibers, elastic fibers, and reticular fibers. They give the skin good flexibility and elasticity. Collagen fibers have a certain degree of stretchability, resisting pulling forces.
Elastic fibers have good elasticity, allowing stretched collagen fibers to return to their original state. If these three types of fibers decrease in the dermis, the skin's elasticity and toughness will decline, easily causing wrinkles. When the skin's water content is 60% of the entire skin tissue, it will present a dry, wrinkled, dehydrated state.
The dermis can be divided into two layers: the upper papillary layer and the lower reticular layer.
Papillary layer located in the deep dermis, mainly composed of collagen fibers and elastic fibers, with fine fiber bundles, irregular arrangement, rich in capillaries and sensory nerve endings.
Reticular layer located in the deep dermis, mainly composed of collagen fibers and elastic fibers. Fiber bundles are coarse, parallel to the skin surface, forming a mesh-like structure. This layer contains abundant blood vessels, lymphatic vessels, nerves, muscles, sebaceous glands, sweat glands, hair follicles, etc.
When the skin is cut deeply into the dermis, pain is felt, and bleeding occurs. During the wound healing process, fibrous tissue proliferates, and after recovery, scars remain.
(3) Subcutaneous tissue: Located in the deepest layer of the skin, its thickness is about five times that of the dermis, mainly composed of a large number of fat cells and loose connective tissue, containing abundant blood vessels, lymphatic vessels, nerves, sweat glands, and deep hair follicles.
Note: The three types of fibrous connective tissue in the dermis are complementary. If one type of tissue cell function decreases, another will also decrease.
When the dermal tissue is injured, more or less scars will remain. The repair process is directly related to age and the vigor of metabolism.
4. Appendages of the skin.
Sweat glands, hair, sebaceous glands, nails
(1) Sweat glands: Depending on the secretion, they are divided into apocrine and eccrine sweat glands.
a. Eccrine sweat glands
Distribution: Except for the lips and nail beds, eccrine sweat glands are distributed throughout the body, most abundantly on the palms, soles, forehead, and armpits.
Structure: Eccrine sweat glands consist of a gland and duct. The gland is located in the reticular layer of the dermis and subcutaneous tissue, and the duct opens directly onto the skin surface forming sweat pores.
Function: Eccrine sweat glands secrete sweat, whose main components are water, inorganic salts, uric acid, urea, and other metabolic wastes. They moisturize the skin, regulate body temperature, and excrete waste. This is part of the metabolic process.
b. Apocrine sweat glands
Distribution: Armpits, areola, anus, and external genitalia, external auditory canal.
Structure: Similar to eccrine sweat glands, the duct opens into the hair follicle.
Function: Begin functioning during puberty, secreting a thick, milky liquid containing proteins, carbohydrates, and fats.
(2) Sebaceous glands
Distribution: Except for the palms and soles, sebaceous glands are distributed throughout the body, most abundantly on the head and face, followed by the chest and back.
Structure: Sebaceous glands consist of a gland and duct. The gland is located in the superficial dermis, pear-shaped lobes, and the duct opens into the hair follicle.
Function: Sebaceous glands secrete sebum, which enters the hair follicle via the duct and is then discharged onto the skin surface through the pore. Sebum is an oily, semi-fluid mixture containing various lipids. Mixed with sweat on the skin surface, it forms emulsified sebum, which can moisturize the skin and hair, preventing water evaporation. Excessive sebum secretion makes the skin oily, resulting in oily skin.
If the pore is blocked, sebum accumulates in the hair follicle and cannot be discharged smoothly, leading to acne.
(3) Hair
Human hair can be divided into long hair, short hair, and vellus hair. Its composition is keratin.
Long hair includes: scalp hair, armpit hair, pubic hair.
Short hair includes: eyebrows, eyelashes, nose hair.
Vellus hair: Soft and pale, except for the palms and soles and the tips of fingers and toes, it is distributed throughout the body.
The part of the hair exposed on the skin surface is the hair shaft, the part inside the skin is the hair root, and the enlarged end of the hair root is the hair bulb. The bottom of the hair bulb is concave, and the dermal tissue extends into it to form the hair papilla. The hair papilla contains abundant blood vessels and nerve endings, providing nutrients for hair growth.
The lower part of the hair bulb connects with the hair papilla, forming the hair matrix, the growth zone of the hair. It contains melanocytes that secrete melanin granules and transport them to hair cells, giving the hair a darker color. Less melanin results in gray hair, and no melanin results in white hair. High iron content in the hair gives it a red or brown color.
Hair growth is divided into three stages: growth phase, resting phase, shedding phase.
(4) Nails
Composed of the nail plate and nail root.
The nail plate is the part exposed outside the skin.
The nail root is the part hidden inside the skin, with the underlying skin being the nail matrix or nail bed. The nail matrix has strong division capabilities, producing new cells to form the nail. Related articles: Learning beauty should involve acquiring professional knowledge!