Human Body Systems: A Comprehensive Overview

Human Body and Homeostasis

Cells and Their Functions

Cytoplasm: The material between the plasma membrane and the nucleus, composed of cytosol (largely water with dissolved proteins) and cytoplasmic organelles.

• Mitochondria: Provide ATP (energy).
• Ribosomes: Sites of protein synthesis.
• Endoplasmic Reticulum: Manufactures proteins and catalyzes reactions.
• Golgi Body: Concentrates and packages proteins.
• Lysosomes: Contain digestive enzymes; degrade unwanted substances.
• Cytoskeleton: Provides structural support.
• Nucleus: Control center of the cell; contains genes.
• Centriole: Plays an active role in cell division.
• Microvilli: Increase surface area for absorption.
• Vacuole: Membrane-bound organelle that stores and excretes substances.

Plasma Membrane: Separates intracellular fluids from extracellular fluids; plays a vital role in cellular activity (e.g., transport). Composed of a lipid bilayer with embedded and dispersed proteins.

Types of Cellular Transport

• Passive Transport: Filtration and diffusion (no energy required).
• Active Transport: Processes that use energy (ATP) and occur against a concentration gradient.
• Osmosis: Diffusion of water down a concentration gradient.

• Isotonic Solution: No change in cell size.
• Hypertonic Solution: Cell loses water.
• Hypotonic Solution: Cell gains water and may rupture (lysis).

Tissues

Epithelial Tissue

Composed almost entirely of cells, supported by connective tissue; contains no blood vessels; rapidly replaces lost cells by cell division; functions in protection, absorption, filtration, and secretion.

• Simple Epithelium: One layer.
• Stratified Epithelium: Two or more layers.

Connective Tissue

Most abundant tissue type; binds, supports, protects, insulates, and transports; contains ground substance (ECM), protein fibers, and loosely packed cells. Ground substance, clear and composed of water, fills the space between cells and fibers. Protein fibers provide mechanical support.

Muscle Tissue

• Skeletal Muscle: Initiates and controls voluntary movement.
• Cardiac Muscle: Propels blood into the circulation.
• Smooth Muscle: Propels substances along internal passageways.

Nervous Tissue

Branched cells with long cellular processes and support cells; transmit electrical signals from receptors to effectors; found in the brain, spinal cord, and peripheral nerves.

Integumentary System

Membranes

• Epithelial Membranes: Simple organs containing epithelial and connective tissue layers.
• Cutaneous Membrane (Skin): Dry, outermost protective layer; composed of keratinized stratified squamous epithelium.
• Mucous Membrane: Moist membranes that line all body cavities open to the exterior surface; functions in absorption and secretion.
• Serous Membrane: Line body cavities closed to the exterior; occur in pairs (e.g., peritoneum [abdominal], pleura [around lungs], pericardium [around heart]).
• Connective Tissue Membrane (Synovial Membrane): Loose areolar tissue that lines fibrous capsules around joints; secretes lubricating fluid to cushion organs moving against each other.

Skin

The integumentary system consists of the skin and skin appendages. It insulates and cushions deeper body organs, protects the body from damage (mechanical, chemical, thermal, ultraviolet radiation, microorganisms, and desiccation), aids in the loss and retention of body heat, aids in the excretion of urea and uric acid, and creates vitamin D.

Skin Structure

• Epidermis: Outer layer; capable of being hard and tough; stratified squamous epithelium; keratinocytes composed of five strata (layers).
• Dermis: Strong, flexible connective tissue below the epidermis; two layers (papillary and reticular) separated by a basement membrane.
• Hypodermis (Subcutaneous Layer): Directly below the dermis; not part of the skin; composed of adipose tissue; well vascularized; serves as a shock absorber and insulates deeper tissues.

Cells of the Epidermis

• Melanocytes: Mostly in the stratum basale; produce melanin (pigment; color ranges from yellow to brown to black).
• Epidermal Dendritic Cells: Alert and activate immune cells to threats.
• Merkel Cells: Touch receptors.

Layers of the Epidermis (Deepest to Outermost)

• Stratum Basale
• Stratum Spinosum
• Stratum Granulosum
• Stratum Lucidum
• Stratum Corneum

Layers of the Dermis

• Papillary Layer: Upper dermal region; contains projections called dermal papillae that indent the epidermis above; contains capillary loops; some house pain and touch receptors; on the palms and soles, they increase grip.
• Reticular Layer: 80% of skin thickness; contains dense irregular connective tissue with collagen and elastic fibers, blood vessels, sweat and oil glands, and a rich nerve supply; pressure and vibration receptors.

Skin Color

Influenced by melanin (yellow, reddish-brown, or black pigment), carotene (orange-yellow pigment from some vegetables), and hemoglobin (red color).

Musculoskeletal System

Cardiovascular System

Nervous System