Integumentary System

Overview and Functions

The integumentary system consists of the skin, hair, nails, and associated glands. It serves as the body's primary barrier against environmental hazards and plays a crucial role in protection, sensation, and regulation of body temperature.

• Physical Barrier: The skin acts as a direct physical barrier, protecting against injury, pathogens, and UV light.

• Water Loss Regulation: Reduces water loss from the body.

• Temperature Regulation: Helps maintain stable internal temperature.

• Vitamin D Synthesis: Sunlight exposure enables the skin to convert cholesterol-based molecules into vitamin D.

• Sensory Functions: Contains sensory receptors for touch, pain, and temperature.

Skin Structure

Layers of the Skin

The skin is composed of two major layers: the epidermis and the dermis.

• Epidermis: The outer layer, made of epithelial cells. Cells are produced deep within and pushed toward the surface, where they die and flake off.

• Keratination: Hardening of older cells due to keratin, producing a yellowish color.

• Dermis: The inner layer, containing blood vessels, nerves, and connective tissue. It has two regions:

  • Reticular Layer: More fibrous, containing collagen and elastin.

  • Papillary Layer: Contains blood vessels and ridges that create fingerprints.

Skin Color and Pigmentation

Skin color is determined by blood flow and pigments, primarily melanin.

• Melanocytes: Cells responsible for producing melanin.

• Variation: Different areas of skin have different quantities of melanin, resulting in darker patches such as moles, freckles, and nipples.

• Albinism: A recessive genetic trait resulting in lack of melanin production, leading to very pale skin and hair.

Hair

Structure and Growth

Hair is common to all mammals and consists of the shaft, root, and bulb. The hair follicle is an extension of the epidermis into the dermis.

• Shaft: Visible part of the hair.

• Root: Located below the surface.

• Bulb: The base of the hair follicle.

• Growth Cycles: Hair has different growth and resting periods.

Nails

Structure and Function

Nails are thin plates of dead cells called the stratum corneum, containing extra-hard keratin.

• Nail Body: The visible section of the nail.

• Nail Root: Extends under the skin.

• Eponychium: The cuticle, which grows onto the nail body.

• Growth: Unlike hair, nails grow continuously.

Glands of the Skin

Types and Functions

The skin contains two major types of glands: sebaceous glands and sweat glands.

• Sebaceous Glands: Connected to hair follicles, produce sebum (an oily substance) that lubricates and protects the skin.

• Sweat Glands: Two categories:

  • Eccrine (or merocrine) glands: Widely distributed, produce a water/salt mixture, and open directly to the skin surface.

  • Apocrine glands: Located in particular body regions (e.g., armpits), open into hair follicles, and secrete a mixture of organic substances and part of the cell body. These secretions can become odorous due to bacterial breakdown.

  • Holocrine glands: Release whole cells as part of the excretion process. Additional info: Holocrine glands are typically associated with sebaceous glands.

Thermal Regulation

Mechanisms of Heat Transfer

The integumentary system helps regulate body temperature through several mechanisms:

Thermal Disorders

• Hypothermia: Overcooling of the body due to exposure to extreme cold; can be fatal.

• Hyperthermia: Overheating of the body due to exposure to extreme heat.

Key Terms and Units

• Mu (μ): Greek letter representing micro- ($1 \times 10^{-6}$).

• Micrometer: $1,000$ micrometers = $1$ millimeter.

Summary Table: Skin Layers and Features

Additional info:

• Holocrine glands are typically associated with sebaceous glands, releasing entire cells as part of their secretion process.

• Melanin is the primary pigment responsible for skin color, but other pigments and blood flow also contribute.