1.2.4　Secretory and excretory functions

Human skin mainly secretes and excretes sweat and sebum through rich sweat glands and sebum to complete skin secretion and excretion functions.

（1）Small glands：The secretion and excretion functions of the small glands are mainly affected by internal and external temperature，mental factors，and diet.In normal settings，the sweat secreted by the small sweat glands is colorless，transparent，and acidic，and the sweat becomes more alkaline when sweating a lot.The moisture in sweat accounts for 99％，and other components only account for 1％.The latter includes inorganic ions，lactic acid，and urea.The secretion of small sweat glands is very important to maintain electrolyte balance in the body.

（2）Apocrine sweat glands：The secretion of apocrine sweat glands during adolescence is strong，and when the emotion is agitated and the ambient temperature increases，its secretion also increases.The newly secreted sweat from the apocrine sweat glands is an odorless liquid，which can emit a foul smell after bacterial fermentation.Some people's apocrine sweat glands can secrete some colored substances，causing local skin or clothing to change color，which is called chromidrosis.(https://www.daowen.com)

（3）Sebaceous glands：Sebum is a mixture of various lipids.Sebaceous gland secretion is mainly affected by various hormones（e.g.，androgens，progesterone，estrogen，glucocorticoids，pituitary hormones），among which androgens can accelerate the division of sebaceous gland cells，making their volume increase and cortical synthesis increase.Estrogen can inhibit endogenous androgen production or directly induce sebaceous glands to reduce sebum secretion.