Overview
This lecture explores the structure, functions, and protective roles of the skin as part of the integumentary system.
Structure of the Skin
- Skin is the body’s largest organ, making up 16% of body weight and covering about 1.7 square meters.
- The integumentary system includes skin, hair, nails, glands, and nerves.
- Skin has three main layers: epidermis (outer), dermis (middle), and hypodermis (inner).
- Thickness ranges from 0.5 mm (thinnest) to 4 mm (thickest) depending on location.
Skin’s Protective Functions
- The skin is the first major barrier against the environment, preventing fluid loss and protecting against microbes.
- The dermis contains collagen, providing flexibility and shock absorption.
- Keratinocytes in the epidermis are regularly replaced and form a waterproof surface layer.
- Langerhans cells in the epidermis detect invaders and alert immune T-cells to trigger immune responses.
- Sebaceous glands in the dermis secrete sebum, supporting beneficial skin microbes that aid immunity.
Sensory and Regulatory Roles
- Merkel cells and other receptors in skin detect touch, with high concentrations in the fingertips.
- Skin senses temperature and relays data to the brain, which regulates blood flow to control heat loss or retention.
- Up to 25% of the body's blood can circulate through the dermis for temperature regulation.
- Sweat glands release sweat to cool the body in warm conditions.
Hair and Temperature Regulation
- Humans have about 5 million hair follicles, with 90,000–150,000 on the scalp for sun and damage protection.
- Arrector pili muscles cause goosebumps, trapping heat when cold.
Key Terms & Definitions
- Integumentary system — includes the skin and its associated structures (hair, nails, glands, nerves).
- Epidermis — outer layer of skin, mainly keratinocytes.
- Dermis — middle layer with collagen, nerves, glands, and blood vessels.
- Hypodermis — innermost skin layer, mainly fat and connective tissue.
- Keratinocytes — skin cells that produce keratin and make up most of the epidermis.
- Langerhans cells — immune cells in the epidermis that detect pathogens.
- Sebum — oily secretion from glands that supports skin microbes.
Action Items / Next Steps
- Review skin layer diagrams and the path of sensory signals.
- Read more about skin-related immune responses and temperature regulation.