Читать книгу Pathy's Principles and Practice of Geriatric Medicine - Группа авторов - Страница 79

The skin is the largest organ of the human body and the most important barrier for maintaining homeostasis. It has endocrine and immune functions, such as vitamin D synthesis and microbial defence. Skin also helps to maintain body temperature in the normal range. As it is exposed to environmental threats, such as ultraviolet lights and chemicals, ageing of the skin can be described as intrinsic and extrinsic: intrinsic ageing is the consequence of genetic heritage, while extrinsic ageing is caused by environmental factors.⁹⁵

The major actor in extrinsic ageing, ultraviolet light, originates from sun irradiation. Two types of ultraviolet light are responsible for skin damage: ultraviolet A (UVA) and B (UVB). UVA penetrates to the dermis layer, whereas UVB penetrates only to the epidermis. UVB is responsible for sunburn and epidermal carcinogenesis. UVA is more responsible for the photoageing of skin than UVB. UVA and UVB, along with other precipitating factors such as smoking, contribute to the skin’s ageing, characterized by wrinkles, shrinking, and fragility. The more the skin is exposed to the sun, the more rapidly ageing occurs.^96‐97