Hair and nails are formed predominantly from keratin, a tough protein that supports their protective role. The high concentration of keratin and lipids in the epidermis makes the skin relatively impermeable to water, which helps minimize water loss and protect the internal environment.

Functions in homeostasis and osmoregulation

The integumentary system supports homeostasis by helping regulate body temperature, managing water balance, and providing physical defense.

In heat homeostasis, the skin adjusts heat loss and heat conservation:

• When the body is too cold, hair stands up (creating goose bumps), and vasoconstriction reduces blood flow to the skin, minimizing heat loss.
• When the body overheats, sweat evaporates to cool the surface, and vasodilation increases blood flow to the skin to release more heat.

In water homeostasis, the skin’s keratinized outer layers reduce water loss. Sweat also contributes to 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 and temperature, enabling the detection of touch, pressure, pain, heat, and cold.

Functions in thermoregulation

Thermoregulation also depends on structures in and beneath the skin.

Erectile musculature in the dermis raises hairs for better insulation (hairs trap air near the body for warmth), and the fat layer (hypodermis) provides additional insulation.

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. The tight, keratin-rich seal of the skin helps prevent pathogen entry and provides protection against abrasion.

Chemical defenses also play a role: the acidic nature of sweat and the antibacterial properties of sebum hinder microbial growth, and the skin’s natural flora outcompete harmful organisms.