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Anatomy & Physiology: Foundations, Homeostasis, and Cellular Mechanisms

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Levels of Organization

Chemical Level

The chemical level is the simplest level of organization in the human body, consisting of atoms and molecules such as ions, proteins, carbohydrates, lipids, and water.

  • Atoms: Basic units of matter (e.g., C, H, O, N).

  • Molecules: Combinations of atoms (e.g., H2O, proteins, lipids).

Cellular Level

The cell is the smallest functional unit of life, capable of performing all life processes.

  • Cells: Basic living units (e.g., muscle cells, neurons).

Tissue Level

Tissues are groups of similar cells that perform a specific function. There are four primary tissue types:

  • Nervous

  • Connective

  • Muscular

  • Epithelial

Organ Level

Organs are structures composed of at least two types of tissues working together to perform specific functions (e.g., heart, liver).

System Level

Organ systems consist of multiple organs that coordinate to achieve unified functions (e.g., digestive system).

Organism Level

The organism is the highest level, representing all systems working together to maintain life.

Human Body Systems

There are eleven major organ systems in the human body, each with specialized functions:

  • Muscular

  • Skeletal

  • Nervous

  • Reproductive

  • Integumentary

  • Digestive

  • Cardiovascular

  • Respiratory

  • Endocrine

  • Urinary

  • Immune/Lymphatic

Four Themes in Physiology

1. Homeostasis

Homeostasis is the maintenance of a stable internal environment. It involves:

  • Set point and range (e.g., blood pH 7.35–7.45)

  • Three compartments:

    • ICF (Intracellular Fluid)

    • ECF (Extracellular Fluid = Plasma + ISF)

2. Energy

All physiological processes require energy, which is obtained from chemical bonds and gradients.

3. Structure-Function Relationship

Form determines function at all levels. For example, the shape of protein receptors determines their binding specificity.

4. Communication

Cells communicate via chemical and electrical signals (e.g., ligands, cytokines, neurons, hormones).

Control Systems

Control systems maintain homeostasis through feedback mechanisms.

  • Components:

    1. Input signal (stimulus/ligand)

    2. Integrating center (CNS, receptor, or organ/gland)

    3. Target (effector)

    4. Response (physiological process/result)

  • Negative Feedback: Opposes stimulus to maintain homeostasis (most common).

  • Positive Feedback: Amplifies stimulus until an event ends (e.g., childbirth, blood clotting).

Mole & Concentrations

  • 1 mole = particles

  • Molarity = mol/L

  • Osmolarity considers dissociation:

    • NaCl → 2 particles (Na+ + Cl-)

    • Glucose → 1 particle (non-dissociable, permeable)

  • Normal human osmolarity ≈ 300 mOsm

Chemical Bonds

  • Covalent (shared electrons)

  • Ionic (transferred electrons)

  • Hydrogen (weak, between polar molecules)

Ligand vs. Receptor

A ligand is a signal molecule (e.g., hormone, neurotransmitter) that binds to a receptor (usually a protein) to initiate a response.

Osmolarity vs. Tonicity

  • Osmolarity: Number of dissolved particles per liter (measured before movement).

    • Hyperosmotic: > 300 mOsm

    • Iso-osmotic: = 300 mOsm

    • Hypo-osmotic: < 300 mOsm

  • Tonicity: Effect of solution on cell volume (after movement).

    • Hypertonic: Water leaves cell → cell shrinks

    • Hypotonic: Water enters cell → cell swells/bursts

    • Isotonic: No net water movement

Additional info: Tonicity depends on non-permeable solutes only.

Hydrostatic Pressure vs. Osmotic Pressure

  • Hydrostatic Pressure (HP): Pressure from fluid/particles inside vessel, pushes out.

  • Osmotic Pressure (OP): Pressure from non-permeable solutes in plasma, pulls water in.

  • Analogy: HP = osmolarity; OP = tonicity.

Additional info: HP/OP apply to vessels; osmolarity/tonicity apply to cells.

Organic vs. Inorganic Molecules

  • Inorganic: Water (most abundant in body)

  • Organic: Carbohydrates, lipids, proteins, nucleic acids

Protein Interactions

Proteins are the most versatile group of molecules, involved in:

  • Enzymes

  • Membrane transporters

  • Signal molecules

  • Receptors

  • Binding proteins

  • Regulatory proteins

  • Immunoglobulins

Solubility: Water-soluble and non-water-soluble proteins differ in their ability to cross membranes.

Membrane Transport

Movement of substances across the plasma membrane is essential for cell function.

  • Plasma membrane: Phospholipid bilayer (selective permeability)

  • Lipid-soluble compounds: Diffuse freely

  • Polar compounds: Need carriers/channels

  • Glucose: Needs carrier (uniport or Na+ symport), permeable because cells use it quickly

  • Na+: Non-permeable unless specific channels open

  • Water: Moves mainly via aquaporins (osmosis)

  • Gases (O2, CO2): Diffuse freely

Ligand-Receptor Binding Concepts

  • Specificity: Shape-matching (puzzle piece); ability of ligands to bind to their receptor due to molecular shape/structure

  • Affinity: Which ligand a receptor prefers

  • Isoforms: Ligands that can bind the same receptor

  • Activation: Extra molecules that bind to receptors as cofactors to activate

  • Inhibition: Extra molecules that bind to receptors as cofactors to inhibit

  • Lysis: Explosion of a cell

  • Modulation: Increase/decrease the response

Energy and Thermodynamics

Definition

Energy is the capacity to do work.

Types of Energy

  • Potential: Stored (chemical bonds, gradients)

  • Kinetic: In use (movement, active processes)

Laws of Thermodynamics

  • First Law: Energy is transformed, not created/destroyed.

  • Second Law: Systems move toward entropy (disorder).

Transduction

Conversion of one energy form to another (e.g., ligand → receptor = signaling pathway → cell response).

Cellular Work

  • Chemical Work: Making/breaking chemical bonds (regulated by enzymes)

  • Transport Work: Moving solutes/water across membranes

  • Mechanical Work: Physical movement (muscle contraction, cell movement)

Metabolism

Definition

All chemical reactions in the body.

  • Catabolism: Breakdown of molecules

  • Anabolism: Synthesis of new molecules

Regulation

  • Enzymes, modulators, reversible reactions

  • Compartmentalization in organelles

  • Maintain ATP/ADP ratio (supply = demand)

Membrane Transport (Expanded)

  • Passive (no energy, down gradient):

    • Simple diffusion (lipid-soluble, gases)

  • Primary Active: ATP directly powers transport (e.g., Na+/K+ pump)

  • Secondary Active: Gradient of one solute drives another (symport/antiport)

  • Vesicular Transport:

    • Endocytosis: Pinocytosis (cell drinking), phagocytosis (cell eating), receptor-mediated

    • Exocytosis: Release contents out of cell

    • Transcytosis: Vesicle transports material across cell intact (common in capillaries)

Diffusion

Properties

  1. Passive (high → low concentration)

  2. Lipid-soluble crosses freely; water-soluble needs channel

  3. Proceeds until concentrations equalize

  4. Faster over short distances

  5. Faster at higher temperatures

  6. Slower for larger molecules

  7. Can occur across membranes or within cytoplasm

Rate of Diffusion Depends On

  1. Concentration gradient

  2. Size (smaller = faster)

  3. Surface area (larger = faster)

  4. Membrane thickness (thicker = slower)

  5. Temperature (warmer = faster)

Membrane Proteins

  • Structural proteins: Anchors, cytoskeleton, junctions

  • Functional proteins:

    • Transporters: Carrier proteins change conformation to move ligand

    • Receptors: Bind ligands to initiate cell response

    • Enzymes: Catalyze reactions at membrane

    • Cell adhesion molecules (CAMs): Mediate cell-cell interactions

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