The skin is the largest organ of all vertebrates.
Its main functions are:
Protective (in terrestrial vertebrates it protects from drying out as well).
Sensitive or sensory: perception of pressure, temperature, vibration…
Excretory: elimination of waste substances.
In some vertebrates it also participates in thermoregulation and water balance through sweating and vasodilatation-vasoconstriction. For example, when it is hot, the vessels dilate so that heat is lost through the blood and vice versa.
In all of vertebrates the skin always consists of 2 tunics:
Epidermis: this is the outermost layer, formed by a lining epithelium (layer of cells), which it is different according to the group of vertebrates.
Dermis: always formed by connective tissue (set of filling, supporting and connecting tissues).
In some body regions of mammals another deeper tunic may also appear > the hypodermis, formed by lax connective tissue (which also provides flexibility) and adipose tissue (fat).
THE FACT
The skin of an adult human measures about 1.5-2m² and weighs about 4 kg.
