Integumentary System and Thermoregulation

Epidermis and dermis have different jobs

The epidermis is the thin outer layer. It has no blood vessels. Its keratin-filled cells make a tough, water-resistant barrier, and melanocytes make melanin. The deeper dermis contains connective tissue, blood vessels, nerves, sensory receptors, hair follicles, and glands. Below the skin, the hypodermis contains fat and connective tissue that cushion and insulate.

Epidermal cells are made in the deepest epidermal layer and move upward as they mature. Near the surface they flatten, fill with keratin, and die. They are eventually shed. Because epidermis lacks vessels, nutrients diffuse to it from vessels in the dermis.

Barrier, sensation, and synthesis

Skin blocks many microbes and chemicals and slows water loss. Sensory receptors detect touch, pressure, pain, and temperature. Ultraviolet light starts vitamin D production in the skin; the liver and kidneys later activate it. Vitamin D helps the intestine absorb calcium. Skin also repairs small injuries through clotting, inflammation, and new tissue growth.

Keratin strengthens the barrier, while skin lipids help limit water loss. Melanin absorbs some ultraviolet energy and helps protect cell DNA, but it does not block all UV damage. Pain receptors warn of injury, and touch receptors help identify contact and pressure.

Vessels and sweat adjust heat exchange

When the body is hot, dermal vessels widen. This vasodilation brings more warm blood near the surface. Sweat glands release fluid, and its evaporation removes heat. When the body is cold, vasoconstriction reduces skin blood flow and heat loss. Skeletal muscles may also shiver to make heat.

Temperature control uses negative feedback. Receptors detect a rise or fall, the hypothalamus coordinates a response, and skin vessels and sweat glands act as effectors. Sweat cools only when it evaporates; sweat that drips away without evaporating removes less heat.

Direction check: vasodilation brings more warm blood near the surface and can increase heat loss; vasoconstriction reduces skin blood flow and conserves heat. Sweating cools only when water evaporates, so high humidity can reduce cooling even when sweat production is high. The hypodermis lies beneath the skin and is not another epidermal layer.

Appendages extend skin functions

Hair, nails, sweat glands, and oil glands are appendages of the skin. Hair offers light protection and helps sensation. Nails protect finger and toe tips. Sebaceous glands release oil that limits drying. Sweat glands aid cooling; some also release thicker secretions in selected regions.

Worked example: On a hot day, skin vessels widen and sweat increases. More warm blood reaches the surface, and evaporation removes energy. If humidity is high, sweat evaporates slowly, so cooling is less effective even though sweat production may be high.

Summary

• The epidermis forms the outer barrier; the dermis contains support structures.
• Sweat evaporation and vasodilation increase heat loss.
• Skin begins vitamin D production when exposed to UV light.

Hot: vessels widen and sweat rises. Cold: vessels narrow; these responses help but do not control every heat change.

Sources

• OpenStax Anatomy and Physiology 2e
• National Human Genome Research Institute educational resources
• NIH National Institute of General Medical Sciences science education