Body Membranes

Types of Body Membranes

Body membranes are layers of tissue that cover surfaces, line body cavities, and surround organs. They play important roles in protection, secretion, and absorption.

• Cutaneous Membrane: Commonly known as skin; covers the body surface.

• Mucous Membranes: Line body cavities open to the exterior (e.g., digestive, respiratory, urinary, and reproductive tracts).

• Serous Membranes: Line closed ventral body cavities (e.g., peritoneum, pleura, pericardium).

Example: The skin acts as a cutaneous membrane, providing a barrier against pathogens and preventing water loss.

Glandular Epithelium

Definition and Classification

Glandular epithelium consists of one or more cells that produce and secrete an aqueous fluid called a secretion. Glands are classified based on where they release their products and their cell structure.

• Endocrine Glands: Ductless glands that secrete hormones directly into the blood or lymph. Example: Thyroid gland secreting thyroid hormones.

• Exocrine Glands: Glands that secrete their products onto body surfaces (skin) or into body cavities. Example: Sweat glands, salivary glands.

Unicellular exocrine glands (e.g., goblet cells) secrete mucus. Multicellular exocrine glands (e.g., salivary glands) have ducts and secretory units.

Endocrine vs. Exocrine Glands

• Endocrine glands release hormones into surrounding tissue fluid and blood, affecting distant organs.

• Exocrine glands release secretions onto body surfaces or into cavities; more numerous than endocrine glands.

Example: Pancreas functions as both an endocrine (insulin secretion) and exocrine (digestive enzyme secretion) gland.

Structure of Multicellular Exocrine Glands

Multicellular exocrine glands are classified by duct structure and secretory unit shape.

• Simple glands: Unbranched ducts.

• Compound glands: Branched ducts.

• Tubular glands: Secretory cells form tubes.

• Alveolar glands: Secretory cells form sacs.

• Tubuloalveolar glands: Both tube and sac structures.

Modes of Secretion

• Merocrine: Secrete products by exocytosis as produced. Example: Sweat glands.

• Holocrine: Accumulate products until cell ruptures. Example: Sebaceous (oil) glands.

• Apocrine: Accumulate products, apex ruptures (controversial in humans). Possible example: Mammary glands.

Functions of the Integumentary System

Protection

The skin provides multiple layers of protection against physical, chemical, and biological threats.

• Chemical Barriers: Skin secretes antimicrobial proteins and acids; sweat contains bactericidal substances.

• Physical Barriers: Keratinized cells and lipids block water and substances; stratum corneum is a major barrier.

• Biological Barriers: Epidermal dendritic cells and macrophages activate immune responses.

Body Temperature Regulation

The skin helps regulate body temperature tlhrough sweat production and blood vessel dilation/constriction.

• Insensible perspiration: Unnoticeable sweat loss (~500 mL/day).

• Sensible perspiration: Noticeable sweat loss during overheating.

• Vasodilation: Blood vessels widen to release heat.

• Vasoconstriction: Blood vessels narrow to conserve heat.

Cutaneous Sensation

Skin contains sensory receptors for touch, pressure, pain, and temperature.

• Tactile (Meissner's) corpuscles: Detect light touch.

• Lamellar (Pacinian) corpuscles: Detect deep pressure and vibration.

Metabolic Functions

• Skin synthesizes vitamin D precursor for calcium absorption.

• Chemical conversion of carcinogens and steroid hormones.

Blood Reservoir

• Skin holds up to 5% of the body's blood volume.

Excretion

• Skin excretes nitrogenous wastes (e.g., urea, ammonia) and salts.

Structure of the Skin

Layers of the Skin

• Epidermis: Superficial layer; composed of keratinized stratified squamous epithelium.

• Dermis: Deep layer; mostly dense irregular connective tissue.

• Hypodermis (subcutaneous layer): Not part of skin; mostly adipose tissue, anchors skin to underlying structures.

Cells of the Epidermis

• Keratinocytes: Produce keratin, a protein that gives skin its protective properties.

• Melanocytes: Produce melanin pigment, which protects against UV radiation.

• Dendritic (Langerhans) cells: Macrophages that activate immune responses.

• Tactile (Merkel) cells: Sensory receptors for touch.

Layers of the Epidermis

• Stratum basale (germinativum): Deepest layer; single row of stem cells; rapid mitosis.

• Stratum spinosum: Several cell layers thick; contains pre-keratin filaments.

• Stratum granulosum: Thin layer; cells begin to die; keratinization begins.

• Stratum lucidum: Only in thick skin (palms, soles); few rows of clear, flat cells.

• Stratum corneum: Outermost layer; 20-30 layers of dead, keratinized cells; protects against abrasion and penetration.

Example: The stratum corneum acts as the primary barrier to environmental damage and water loss.

Summary Table: Epidermal Layers

Key Terms and Definitions

• Keratin: A tough, fibrous protein that protects skin and underlying tissues.

• Melanin: Pigment produced by melanocytes; protects against UV damage.

• Desmosomes: Cell junctions that anchor keratinocytes together.

• Stratum: Layer of tissue, especially in the epidermis.

• Exocytosis: Process by which cells release substances to the exterior.

Formulas and Equations

• Rate of Sweat Production (example):

• Vitamin D Synthesis (simplified):

Additional info: Some content was inferred and expanded for clarity and completeness, including examples and definitions of key terms.