Anatomy & Physiology Exam 1 Review: Homeostasis, Physiology, Cell Structure, and Membrane Transport

Homeostasis and Reflex Pathways

Homeostasis: The Body's Internal Balance

Homeostasis refers to the maintenance of a stable internal environment within the body, despite changes in the external environment. It is essential for the survival and proper functioning of organisms.

• Definition: The process by which physiological systems maintain internal stability.

• Importance: Disruption of homeostasis can lead to illness or death.

• Examples: Regulation of body temperature, blood glucose, and pH.

Reflex Pathways: Steps in a Homeostatic Response

Reflex pathways are automatic, rapid responses to changes in the environment that help maintain homeostasis. They involve several key steps:

• Stimulus: A change in the environment (e.g., temperature increase).

• Sensor: Detects the change (e.g., thermometer senses temperature).

• Input Signal: Signal sent from sensor to integrating center.

• Integrating Center: Processes information and initiates response (e.g., control box programmed to respond to temperature).

• Output Signal: Signal sent to target (e.g., wire to heater).

• Target: Effector that carries out the response (e.g., heater turns on).

• Response: Action that restores homeostasis (e.g., water temperature increases).

Consequences of Failed Homeostasis

If the body cannot maintain homeostasis, it attempts to compensate. Failure to compensate leads to pathophysiology, illness, or death.

• Compensation Succeeds: Wellness is maintained.

• Compensation Fails: Illness or disease occurs.

The Science of Physiology

Physiology as an Experimental Science

Physiology is hypothesis-driven and relies on the scientific method to understand how the body functions.

• Hypothesis: A logical guess about how an event takes place.

• Experimentation: Testing hypotheses through controlled experiments.

• Variables:

  • Independent Variable: The factor manipulated by the experimenter (e.g., temperature).

  • Dependent Variable: The factor measured in response (e.g., food intake by birds).

Example: Testing if cold temperatures cause birds to eat more. The independent variable is temperature (27°C, 4°C, 0°C, -12°C), and the dependent variable is the amount of food intake.

Body Compartments: Cells and Tissues

Body Cavities and Fluid Compartments

The body is organized into anatomical and functional compartments to separate processes and maintain order.

• Anatomical Compartments: Cranial cavity, thoracic cavity, abdominal cavity, pelvic cavity.

• Functional Compartments:

  • Extracellular Fluid (ECF): Fluid outside cells (e.g., plasma, interstitial fluid).

  • Intracellular Fluid (ICF): Fluid inside cells.

Example: The lumen of the digestive tract is considered part of the external environment.

Types of Tissues

The human body is composed of four primary tissue types, each with specialized functions.

• Nervous Tissue: Internal communication (brain, spinal cord, nerves).

• Muscle Tissue: Contracts to cause movement (skeletal, cardiac, smooth muscle).

• Epithelial Tissue: Forms boundaries, protects, secretes, absorbs, filters (skin, lining of GI tract).

• Connective Tissue: Supports, protects, binds other tissues (bones, tendons, fat).

Cell Membranes and Membrane Transport

Structure of the Cell Membrane

The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Fluid Mosaic Model: Describes the dynamic arrangement of lipids and proteins.

• Functions: Physical barrier, regulation of exchange, communication, structural support.

Membrane Transport Mechanisms

Substances move across cell membranes by various mechanisms, depending on their properties and the membrane's permeability.

• Simple Diffusion: Movement of molecules from high to low concentration without energy input (e.g., O2, CO2).

• Facilitated Diffusion: Passive transport via membrane proteins (channels or carriers) for molecules that cannot diffuse directly (e.g., glucose, ions).

• Osmosis: Diffusion of water across a semi-permeable membrane.

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

Fick's Law of Diffusion

The rate of diffusion across a membrane is described by Fick's Law:

$J = -D \frac{dC}{dx}$

• J: Rate of diffusion

• D: Diffusion coefficient

• \frac{dC}{dx}: Concentration gradient

Osmolarity and Tonicity

Osmolarity is the concentration of solute particles per liter of solution. Tonicity describes how a solution affects cell volume.

• Isotonic: No net change in cell volume.

• Hypertonic: Cell shrinks (water leaves cell).

• Hypotonic: Cell swells (water enters cell).

Primary and Secondary Active Transport

• Primary Active Transport: Direct use of ATP to move ions (e.g., Na+/K+ ATPase pump).

• Secondary Active Transport: Uses energy from the movement of one substance down its gradient to move another against its gradient (e.g., sodium-glucose symporter).

Cell Junctions and the Extracellular Matrix

Types of Cell Junctions

Cell junctions connect cells to each other and to the extracellular matrix, allowing communication and structural integrity.

Junction Type	Function	Example
Gap Junction	Communication between cells	Cardiac muscle
Tight Junction	Prevent passage of substances between cells	Intestinal epithelium
Desmosome	Anchor cells to each other	Skin epithelium

Extracellular Matrix (ECM)

The ECM provides structural support and regulates cell behavior. It is composed of proteins (collagen, elastin), glycoproteins (fibronectin), and proteoglycans.

Membrane Potentials and Ion Transport

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the cell membrane, primarily due to the distribution of potassium (K+) and sodium (Na+) ions.

• Typical Value: -70 mV (inside negative relative to outside)

• Key Ion: Potassium (K+) leak channels are most important.

Nernst Equation: Calculates the equilibrium potential for a single ion:

$E_{ion} = \frac{61}{z} \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Eion: Equilibrium potential for the ion

• z: Charge of the ion

• [ion]out: Extracellular concentration

• [ion]in: Intracellular concentration

Cell Signaling and Signal Transduction

Types of Cell Communication

• Direct Contact: Gap junctions allow cytoplasmic exchange.

• Local Communication: Paracrine and autocrine signals act on nearby cells.

• Long-Distance Communication: Hormones travel through the bloodstream.

Signal Transduction Pathways

Signal transduction involves converting an extracellular signal into a cellular response, often through cascades and amplification.

• Ligand: A molecule that binds to a receptor (e.g., hormone, neurotransmitter).

• Receptor: Protein that binds ligand and initiates response.

• Second Messengers: Intracellular molecules (e.g., cAMP, Ca2+) that propagate the signal.

• Amplification: One signal molecule can activate many downstream molecules.

G Protein-Coupled Receptors (GPCRs)

• Gs Protein: Stimulates adenylate cyclase, increases cAMP.

• Gi Protein: Inhibits adenylate cyclase, decreases cAMP.

• Example: Epinephrine can bind to different adrenergic receptor isoforms, causing vessel constriction or dilation.

Tyrosine Kinase Receptors

• Ligand binding activates kinase activity, leading to phosphorylation of target proteins.

Calcium as a Second Messenger

• Calcium enters cells through voltage-gated channels or is released from intracellular stores (endoplasmic reticulum).

• Regulates processes such as muscle contraction, neurotransmitter release, and memory.

Hormones and Endocrine Regulation

Hormone Types and Functions

• Peptide Hormones: Chains of amino acids (e.g., insulin).

• Steroid Hormones: Derived from cholesterol (e.g., cortisol).

• Amine Hormones: Derived from single amino acids (e.g., epinephrine).

Hormones are classified by their source, receptor type, and mechanism of action.

Hormone Interactions

Interaction Type	Description	Example
Synergism	Combined effect is greater than sum of individual effects	Glucagon + epinephrine + cortisol on blood glucose
Permissiveness	One hormone enables another to act	Thyroid hormone permits reproductive hormones
Antagonism	One hormone opposes the action of another	Insulin vs. glucagon

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.