Integumentary (skin, hair, nails) system

Structure

The integumentary system includes the skin, hair, and nails, and it serves as a critical protective barrier for the body.

The skin is organized into several layers, each with distinct cell types and function:

• The outermost layer, the epidermis, is primarily composed of keratinocytes and also contains melanocytes for pigmentation, Langerhans cells for immune defense, and Merkel cells for tactile sensation.
• Beneath the epidermis lies the dermis, a layer rich in connective tissue, collagen, and elastin fibers that provides structural support and elasticity. This layer contains the blood and nerve supply as well as fibroblasts, which make fiber and ground substance (glue) for the extracellular matrix that makes up connective tissue
• The innermost layer, the hypodermis, is mainly made up of adipose tissue, which insulates the body and stores energy.

Hair and nails are formed predominantly from keratin, a tough protein that reinforces their protective role. The high concentration of keratin and lipids in the epidermis renders the skin relatively impermeable to water, minimizing water loss and safeguarding the internal environment.

Functions in homeostasis and osmoregulation

The integumentary system helps maintain homeostasis by regulating body temperature, managing water balance, and providing physical defense. In heat homeostasis, when the body is too cold, hair stands up (creating goose bumps) and vasoconstriction reduces blood flow to the skin, minimizing heat loss. Conversely, if the body overheats, sweat evaporates to cool the surface, and vasodilation boosts blood supply at the skin to release more heat.

With respect to water homeostasis, the skin’s keratinized outer layers insulate against water loss, while sweat aids in osmoregulation by excreting salts and nitrogenous wastes such as urea, uric acid, and ammonia.

Other functions include protecting against harmful UV radiation by producing melanin, which absorbs harmful rays; synthesizing vitamin D in response to sunlight, serving as a blood reservoir (through vasoconstriction that can redirect blood to other organs), and sensing pressure, temperature, and enabling the detection of touch, pressure, pain, heat, and cold.

Functions in thermoregulation

Thermoregulation also relies on erectile musculature in the dermis that raises hairs for better insulation (hairs trap air near the body for warmth) and a fat layer (hypodermis) that provides further warmth.

Sweat glands in the dermis release fluid for evaporative cooling, and surface capillaries adjust heat loss through vasoconstriction or vasodilation.

Hormonal control: sweating, vasodilation, and vasoconstriction

• Hormonal regulation of sweating, vasodilation, and vasoconstriction is coordinated between the endocrine and nervous systems. Epinephrine and norepinephrine, catecholamines released by the adrenal medulla, stimulate sweat glands (controlled by sympathetic pathways) to produce sweat for evaporative cooling. In parallel, these hormones modulate blood vessel tone: they induce vasodilation in certain tissues to enhance heat loss, while promoting vasoconstriction in peripheral regions to conserve core temperature and maintain blood pressure.

Additionally, ADH (antidiuretic hormone), secreted by the hypothalamus and stored in the posterior pituitary, intensifies vasoconstriction by increasing vascular smooth muscle responsiveness and limiting water excretion, contributing to blood pressure regulation. The renin-angiotensin-aldosterone system further supports vasoconstriction when blood volume or pressure is low, ensuring vital organs receive adequate blood flow.

Physical protection

The system’s physical protection is reinforced by nails, calluses, and hair—all composed of various forms of hard keratin—while the tight, keratin-rich seal of the skin prevents pathogen entry and offers some protection against abrasion. Chemically, the acidic nature of sweat and the antibacterial properties of sebum hinder microbial growth, and the skin’s natural flora outcompete harmful organisms.