**Basic Composition 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 and has selective permeability, controlling 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 sap) The substance located between the cell membrane and the nucleus. It contains nutrients required for cell growth, reproduction, and self-repair, making it an important site for cellular metabolism and material synthesis. The cytoplasm consists of matrix and organelles.
(Organelles: Mitochondria, Golgi apparatus, centrioles, inclusions)
(3) Nucleus: The dense material at the center of the cell, which determines the nature of the cell.
(Nucleus: Nuclear envelope, nucleolus, nucleoplasm, chromatin)
Nuclear envelope: A semi-transparent membrane surrounding the nucleus, maintaining its integrity with selective permeability.
Nucleolus: A dense area associated with protein synthesis.
Nucleoplasm: A colorless, transparent gel-like substance inside the nucleus.
Chromatin: Small particles within the nucleus related to heredity. Chromatin aggregates into chromosomes.
2. Functions of cells:
(1) Cell reproduction – leading to growth, development, and wound repair in the human body.
(2) Cellular metabolism – absorbing nutrients for life growth while excreting waste.
(3) Growth conditions of cells – appropriate activity, sufficient nutrition, oxygen, water, and synthetic nutrients intake. Drinking more water, combining massage techniques, and supplementing nutrients benefits skin health.
II. Structure of the skin
1. The skin is located on the outermost layer of the human body and is the largest organ. An adult’s total skin area is approximately 1.5–2 square meters, accounting for about 15% of body weight. The surface of the skin is soft and smooth, appearing under a magnifying glass as many raised papillae and recessed furrows. Many small recesses are called sweat pores.
2. The thickness of the skin ranges from 0.5–4 mm, with the thinnest being the eyelid skin and the thickest being the skin on the palms and soles. Skin not only varies in thickness but also in softness, hardness, and color. Differences in gender affect the fineness of the skin. Skin also changes and varies based on region, race, occupation, and season.
3. The structure of the skin can be divided into: epidermis, dermis, and subcutaneous tissue.
(1) Epidermis: There are no blood vessels in the epidermis, but there are nerve endings that help us perceive external objects.
The epidermis can be further divided into: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.
Basal cells in the stratum basale are the biochemical source of epidermal cells. They continuously produce new cells, pushing them toward the epidermis where they transform into various layers of cells, eventually becoming dead cells. From the production of one basal cell to its shedding as a keratinocyte, it takes about 28 days. The average thickness of the epidermis is 0.07–2 mm.
① Stratum corneum: The outermost layer of the epidermis, consisting of 4–8 layers of flat, non-nucleated cells. These cells are tightly packed and serve a protective function. Metabolism occurs every 7–14 days, forming a non-living protective layer. The stratum corneum represents the final stage of cell differentiation.
The thickness of the stratum corneum affects skin tone and absorption capacity:
a. Too thick: The skin appears dull, lacks luster, is rough, and may even affect normal physiological functions and the absorption of skincare products.
b. Too thin: Resistance decreases, leading to dryness, dehydration, and susceptibility to spots and sensitivity.
Function of the stratum corneum: Resists friction, prevents evaporation of internal moisture, stops external substances from invading and harming the skin, and blocks ultraviolet rays.
Water content: Accounts for 10%–20% of the epidermis. If less than 10%, the epidermis will lack water, appear dark, yellow, and lose luster, leading to tightness. Severe cases may result in peeling.
② Stratum lucidum: Located below the stratum corneum, present only on the hands and soles. It consists of 2–3 layers of flat, non-nucleated, transparent dead cells, appearing colorless and transparent. Light can pass through this layer, preventing evaporation of internal and external moisture and providing protection via electrolytes.
③ Stratum granulosum: Composed of 2–4 layers of rhomboidal cells. These cells are nearly dead and are about to transform into keratinocytes. They contain fine granular substances that refract light, reducing the penetration of ultraviolet rays into the body.
④ Stratum spinosum: Located below the stratum granulosum, consisting of 4–8 layers of spiny polygonal cells, making it the thickest layer of the epidermis. The spines connect the cells, and the intercellular spaces contain tissue fluid that provides nutrition. The stratum spinosum has many sensory nerve endings that perceive external objects.
⑤ Stratum basale: Located below the stratum spinosum, it is the deepest layer of the epidermis, connecting wavelike to the dermis. It is 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 leaving scars.
b. Melanocytes are tree-shaped, scattered sparsely among the basal cells, secreting melanin granules. Melanin granules absorb ultraviolet rays from sunlight, preventing their penetration into deeper tissues. The stronger the external ultraviolet rays, the more melanin granules are secreted by melanocytes, determining the depth of skin tone.
(2) Dermis: Located beneath the epidermis, it is firmly connected in a wavy pattern, approximately ten times thicker than the epidermis. It is composed of a large amount of fibrous connective tissue, cells, and matrix, containing rich blood vessels, lymphatic vessels, nerves, and glands.
The dermis contains three types of fibrous connective tissue: collagen fibers, elastic fibers, and reticular fibers. These give the skin good flexibility and elasticity. Collagen fibers have a certain degree of stretchability, resisting pulling forces. Elastic fibers have excellent 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, making wrinkles more likely to form. When the water content of the dermis accounts for 60% of all skin tissues, the skin may show signs of dryness and wrinkle formation due to dehydration.
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. The fiber bundles are fine, with an undefined arrangement direction, containing abundant capillaries and sensory nerve endings.
Reticular layer: Also located in the deep dermis, primarily composed of collagen fibers and elastic fibers. The fiber bundles are coarse, arranged parallel to the skin surface, forming a network. This layer contains abundant blood vessels, lymphatic vessels, nerves, muscles, sebaceous glands, sweat glands, and hair follicles.
When the skin is cut deeply into the dermis, pain and bleeding occur. During the healing process, fibrous tissue proliferates, and after recovery, scars remain.
(3) Subcutaneous tissue: Located at the deepest layer of the skin, approximately five times thicker than 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 complement each other. If the function of one type of tissue cell declines, the others will also decline.
When the dermis is injured, more or less scarring occurs, and the healing process is directly related to age and the vitality of metabolism.
4. Skin appendages
Sweat glands, hair, sebaceous glands, nails
(1) Sweat glands: Depending on the secretion, they are divided into apocrine sweat glands and eccrine sweat glands.
a. Eccrine sweat glands
Distribution: Except for the lips and nails, eccrine sweat glands are distributed throughout the body, most densely on the palms, soles, forehead, and armpits.
Structure: The eccrine sweat gland consists of a gland and duct. The gland is located in the reticular layer of the dermis and subcutaneous tissue, with the duct opening directly onto the skin surface to form sweat pores.
Function: The eccrine sweat gland secretes sweat, whose main components are water, inorganic salts, uric acid, urea, and other metabolic wastes. It moisturizes the skin, regulates body temperature, and excretes waste, playing a role in metabolism.
b. Apocrine sweat glands
Distribution: Found in the armpits, areola, anus, external genitalia, and ear canal.
Structure: Similar to eccrine sweat glands, with the duct opening into the hair follicle.
Function: Activates during puberty, secreting a thick milky liquid containing proteins, sugars, and fats.
(2) Sebaceous glands
Distribution: Except for the palms and soles, sebaceous glands are distributed throughout the body, most densely on the head and face, followed by the chest and back.
Structure: The sebaceous gland consists of a gland and duct. The gland is located in the shallow layer of the dermis, shaped like a pear-shaped lobule, with the duct opening into the hair follicle.
Function: The sebaceous gland secretes sebum, which enters the hair follicle through the duct and is discharged to the skin surface through the pores. Sebum is an oily semi-fluid mixture containing various lipids. When mixed with sweat on the skin surface, it forms emulsified sebum, which moisturizes the skin and hair and prevents water evaporation. Excessive sebum secretion makes the skin oily, resulting in oily skin.
If the pores are blocked, sebum accumulates in the hair follicle and cannot be discharged smoothly, causing 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 light in color, distributed throughout the body except for the palms, soles, fingertips, and toes.
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 expanded end of the hair root is the hair bulb. The lower end of the hair bulb is concave, with dermal tissue extending into it to form the hair papilla. The hair papilla contains abundant blood vessels and nerve endings, providing nutrients for hair growth.
Below the hair bulb and connected to the hair papilla is the hair matrix, the growth zone of the hair, containing 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: anagen (growth phase), telogen (resting phase), and catagen (shedding phase).
(4) Nails
Consisting 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 nails.
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