The integumentary system is the organ system that comprises the skin and its associated structures. It is the largest system in the human body and plays a crucial role in protecting and maintaining homeostasis. The integumentary system serves several functions including protection, regulation, sensation, and secretion.

The skin, which is the primary organ of the integumentary system, consists of three main layers: the epidermis, dermis, and hypodermis (subcutaneous tissue). The epidermis is the outermost layer and provides a protective barrier against environmental factors such as bacteria, UV radiation, and chemicals. It also contains melanocytes, which produce the pigment melanin responsible for skin color.

The dermis might be considered the "core" of the integumentary system [derma = "skin"], as distinct from the epidermis [epi = "upon" or "over"] and hypodermis [hypo = "below"]. It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers produced by fibroblasts.

The hypodermis (the subcutaneous layer or superficial fascia) is a layer directly below the dermis. It connects the skin to the underlying fascia (fibrous tissue) of the bones and muscles. Although it is not strictly a part of the skin, the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.

The integumentary system performs various essential functions. One of its primary functions is protection, as it acts as a physical barrier against pathogens, harmful chemicals, and ultraviolet (UV) radiation. The skin's waterproof nature prevents excessive water loss from the body and helps maintain internal hydration.

Regulation of body temperature is another vital function of the integumentary system. Through blood vessels and sweat glands, the skin helps dissipate excess heat from the body when it is hot and conserve heat when it is cold.

The integumentary system also plays a role in sensation. Nerve endings in the skin allow us to perceive touch, pressure, temperature, and pain. The sense of touch helps us interact with our environment and detect potential dangers.

The integumentary system is involved in secretion. Sweat glands secrete sweat, which helps regulate body temperature and excrete waste products. Sebaceous glands secrete sebum, an oily substance that moisturizes the skin and hair.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.1: Layers of the Skin