Skin Structure and Function

Metabolic Functions of the Skin

The skin is not only a protective barrier but also plays a role in metabolic processes such as vitamin D synthesis and excretion of waste products.

• Vitamin D Synthesis: The skin converts 7-dehydrocholesterol to vitamin D3 when exposed to UV light.

• Excretion: Sweat glands help remove urea, salts, and water from the body.

Different Layers of the Skin

The skin consists of three main layers, each with distinct functions.

• Epidermis: Outermost layer, provides a waterproof barrier and creates skin tone.

• Dermis: Middle layer, contains connective tissue, hair follicles, and sweat glands.

• Hypodermis (Subcutaneous Layer): Deeper subcutaneous tissue made of fat and connective tissue.

Components of the Skin

• Cells: Keratinocytes, melanocytes, Langerhans cells, and Merkel cells.

• Structures: Hair follicles, sweat glands, sebaceous glands, blood vessels, and nerves.

Cleavage Lines

Cleavage lines, or Langer's lines, are patterns of collagen fiber orientation in the dermis. Surgical incisions made parallel to these lines heal better.

Skin Proteins

• Keratin: Provides strength and waterproofing.

• Collagen: Gives skin its structure and elasticity.

• Elastin: Allows skin to stretch and recoil.

Cellular Transport and Membrane Dynamics

Osmosis and Active Transport

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Active Transport: Movement of substances against their concentration gradient using energy (ATP).

Secondary Active Transport

Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Exocytosis

Process by which cells expel materials in vesicles by fusing with the plasma membrane.

Tissues: Epithelial and Connective

Properties of Epithelial Tissues

• Cellularity: Composed almost entirely of cells.

• Polarity: Has an apical (top) and basal (bottom) surface.

• Avascularity: Lacks blood vessels.

• Regeneration: High capacity for renewal.

Components of Connective Tissue

• Cells: Fibroblasts, adipocytes, macrophages, mast cells.

• Fibers: Collagen, elastic, and reticular fibers.

• Ground Substance: Gel-like material that fills space between cells and fibers.

Types of Epithelial Tissue

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; found in glands and kidney tubules.

• Simple Stratified Epithelium: Multiple layers; provides protection, found in skin and lining of mouth.

• Simple Squamous Epithelium: Single layer of flat cells; found in alveoli and blood vessels.

Cellular Organelles and Functions

Nucleus

The nucleus is the control center of the cell, containing genetic material (DNA) and directing cellular activities.

Phagocytosis

Process by which cells engulf large particles or pathogens; important in immune defense.

Golgi Apparatus

Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Smooth Endoplasmic Reticulum (ER)

Involved in lipid synthesis, detoxification, and calcium storage.

Exocrine Glands

Glands that secrete substances onto epithelial surfaces via ducts (e.g., sweat, salivary glands).

Cellular Communication and Junctions

Endocrine and Exocrine Glands

• Endocrine: Release hormones directly into the bloodstream.

• Exocrine: Release secretions through ducts to epithelial surfaces.

Types of Cell Junctions

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap Junctions: Allow communication and passage of ions between cells.

Cellular and Molecular Biology

Valence Shell Electrons and Covalent Bonds

• Valence Electrons: Electrons in the outermost shell; determine chemical reactivity.

• Covalent Bonds: Formed when atoms share pairs of electrons.

Atomic Mass and Atomic Number

• Atomic Number: Number of protons in an atom's nucleus.

• Atomic Mass: Sum of protons and neutrons in the nucleus.

Isotopes

Atoms of the same element with different numbers of neutrons.

Avogadro's Number

The number of particles in one mole of a substance: .

Body Chemistry and Solutions

pH and Buffers

• pH: Measure of hydrogen ion concentration; scale ranges from 0 (acidic) to 14 (basic).

• Buffers: Substances that minimize changes in pH by absorbing or releasing H+ ions.

Preparation of Solutions

• Percent Solution: Amount of solute per 100 mL of solution.

• Molar Solution: Moles of solute per liter of solution.

Potential and Kinetic Energy

• Potential Energy: Stored energy due to position or structure.

• Kinetic Energy: Energy of motion.

Chemical Equilibrium

State in which the rate of the forward reaction equals the rate of the reverse reaction.

Chemical Reactions

• Types: Synthesis, decomposition, exchange, and reversible reactions.

• General Equation: (synthesis), (decomposition)

Additional Topics

• Body Fluid Compartments: Intracellular and extracellular fluids.

• Basic Elements in the Body: Oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus.

• Skin Cancer: Uncontrolled growth of abnormal skin cells; types include basal cell carcinoma, squamous cell carcinoma, and melanoma.