1.2.6　Metabolic function

Compared with other tissues and organs，the metabolic function of skin has its particularity and mainly encompasses the following metabolism.

（1）Glycometabolism：The sugars in the skin are mainly glycogen，glucose，and mucopolysaccharides.The dermis is rich in mucopolysaccharides，mainly including hyaluronic acid and chondroitin sulfate.The synthesis and degradation of mucopolysaccharides are mainly accomplished through enzymatic reactions，but certain nonenzymatic substances can also degrade hyaluronic acid.In addition，endocrine factors can also affect the metabolism of mucopolysaccharides.

（2）Protein metabolism：Skin proteins include fibrous proteins and non-fibrous proteins.The former includes keratin，collagen，and elastin，and the latter includes nuclear proteins in cells and various enzymes that regulate cell metabolism.The degradation of proteins in the skin is accomplished by catalyzing the hydrolysis of polypeptide chains under the action of proteolytic enzymes.Proteolytic enzymes are divided into two types：endopeptidase and exonuclease.The enzyme action lacks strict substrate specificity.The proteolytic enzymes in the skin not only participate in the metabolism of extracellular structural substances under normal circumstances，but also participate in the inflammatory process of the skin and the regulation of cell functions.(https://www.daowen.com)

（3）Lipid metabolism：Lipids in the skin include fats and lipoids.Epidermal cells differ significantly in their lipoid composition at various stages of differentiation.From the basal layer to the stratum corneum，the contents of cholesterol，fatty acids，and ceramide gradually increase，while the content of phospholipids gradually decreases.The abundant essential fatty acids in the epidermis are linoleic acid and arachidonic acid，the latter can synthesize vitamin D under the influence of sunlight.

（4）Water and electrolyte metabolism：The water in the skin is mainly distributed in the dermis.When the body is dehydrated，the skin can provide 5％-7％of water content to maintain the stability of the circulating blood volume.The skin contains various electrolytes，mainly stored in the subcutaneous tissue.They play important roles in maintaining the crystal osmotic pressure between cells and the acid-base balance inside and outside the cells.