Foundations of Anatomy & Physiology: Homeostasis, Chemistry, and Cell Structure

Homeostasis

Definition and Importance

Homeostasis refers to the maintenance of a stable internal environment within the body, despite external changes. This is essential for the proper functioning of cells and organs, and for overall survival.

• Definition: Maintaining a relatively constant internal environment (e.g., temperature, pH, glucose levels).

• Survival Needs: Nutrients, oxygen, water, normal body temperature, and appropriate atmospheric pressure are required for homeostasis.

• Homeostatic Imbalance: Too much or too little of any survival need can be harmful and may lead to disease or dysfunction.

Homeostatic Control Mechanisms

• Receptor: Detects changes (stimuli) in the environment.

• Control Center: Processes information and determines the appropriate response.

• Effector: Carries out the response to restore balance.

• Negative Feedback: The response reduces or eliminates the original stimulus (e.g., body temperature regulation).

• Positive Feedback: The response enhances or amplifies the original stimulus (e.g., labor contractions, blood clotting).

Basic Chemistry Concepts

Elements and Compounds

• Elements: Substances that cannot be broken down into simpler substances by chemical means. Major elements in the human body include Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N).

• Atoms: Smallest units of elements with unique properties.

• Molecules: Two or more atoms bonded together (e.g., O2).

• Compounds: Molecules composed of two or more different elements (e.g., H2O, C6H12O6).

Mixtures

• Solutions: Homogeneous mixtures; solute dissolved in solvent (e.g., glucose in blood).

• Colloids: Heterogeneous mixtures; particles do not settle (e.g., cytosol).

• Suspensions: Heterogeneous mixtures; particles settle out (e.g., blood cells in plasma).

Chemical Bonds and Reactions

• Ionic Bonds: Transfer of electrons from one atom to another, forming ions (cation = +, anion = -).

• Covalent Bonds: Sharing of electrons between atoms (single, double, triple bonds).

• Polar vs. Nonpolar: Polar molecules have unequal sharing of electrons; nonpolar have equal sharing.

• Reactions: Synthesis (anabolic, A + B → AB), Decomposition (catabolic, AB → A + B), Exchange (displacement, AB + C → AC + B).

• Energy Flow: Endergonic reactions absorb energy; exergonic reactions release energy.

• Rate Influencers: Temperature, concentration, particle size, and catalysts (enzymes) affect reaction rates.

Inorganic Compounds

• Water: High heat capacity, high heat of vaporization, solvent, cushioning, reactivity.

• Salts: Electrolytes (e.g., Na+, K+, Ca2+) vital for nerve impulses and muscle function.

• Acids & Bases: Acids release H+; bases accept H+ (release OH-).

• pH Scale: $0-6.9$ = acidic, $7$= neutral, $7.01-14$ = basic.

Organic Compounds

• Carbohydrates: C, H, O; main energy source. Includes monosaccharides (glucose), disaccharides (sucrose), polysaccharides (glycogen, starch).

• Lipids: C, H, O; energy storage, insulation, cell membrane structure. Includes triglycerides (fats), phospholipids (plasma membrane), steroids (cholesterol, vitamin D, hormones).

• Proteins: Polymers of amino acids; structural (fibrous) or functional (globular). Levels of organization: primary, secondary, tertiary, quaternary.

• Enzymes: Biological catalysts; lower activation energy, substrate-specific.

• Nucleic Acids: DNA (genetic blueprint), RNA (protein synthesis).

• ATP: Energy currency of the cell (adenosine triphosphate).

Cell Structure and Function (Cytology)

Cell Theory

• Cells are the structural and functional units of life.

• All living organisms are composed of cells.

• All cells come from pre-existing cells.

Generalized Cell Structure

• Plasma Membrane: Selectively permeable barrier; fluid mosaic model.

• Cytoplasm: Intracellular fluid and organelles.

• Nucleus: DNA control center.

Plasma Membrane Components

• Phospholipid Bilayer: Polar heads (hydrophilic) outside, nonpolar tails (hydrophobic) inside.

• Proteins: Integral (channels, carriers, receptors) and peripheral (enzymes, support, cell recognition).

• Glycocalyx: Identity markers on cell surface.

Cell Junctions

• Tight Junctions: Impermeable, prevent leakage.

• Desmosomes: Strong, allow flexibility (skin, heart).

• Gap Junctions: Allow communication (ions, small molecules).

Membrane Transport

• Passive Transport (no ATP):

  • Simple diffusion: movement of small, nonpolar molecules.

  • Facilitated diffusion: via carriers/channels (e.g., glucose, ions).

  • Osmosis: water movement (low solute → high solute).

  • Filtration: pressure-driven movement across capillary walls.

• Tonicity:

  • Isotonic: cell unchanged.

  • Hypertonic: cell shrinks (crenation).

  • Hypotonic: cell swells (lysis).

• Active Transport (uses ATP):

  • Primary: Na+/K+ pump, Ca2+ pump.

  • Secondary: uses gradient from primary to move molecules (symport/antiport).

  • Vesicular: endocytosis (phagocytosis, pinocytosis), exocytosis (secretion).

Table: Comparison of Membrane Transport Mechanisms

Type	Energy Required?	Examples	Direction
Simple Diffusion	No	O2, CO2	High → Low
Facilitated Diffusion	No	Glucose, ions	High → Low
Osmosis	No	Water	Low solute → High solute
Active Transport	Yes (ATP)	Na+/K+ pump	Low → High
Vesicular Transport	Yes (ATP)	Endocytosis, exocytosis	Varies

Example: Sodium-Potassium Pump

• Maintains electrochemical gradients essential for nerve and muscle function.

• Moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

• Equation: $3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Scientific terms are defined and contextualized for exam preparation.