Foundations of Physiology and Cell Membranes: Homeostasis, Control Systems, and Membrane Structure

Physiology and Levels of Organization

Definition and Scope

Physiology is the study of the normal function of living organisms and their components, both chemical and physical. It is closely tied to anatomy, which focuses on the structure of cells, tissues, and organs that provide the basis for function.

• Levels of Organization:

  • Chemistry: Atoms & molecules

  • Molecular Biology: Smallest unit of structure that carries life processes

  • Cell Biology: Collection of cells carrying out related functions

  • Physiology: Organs, organ systems, populations of one species

  • Tissues: Formation into a structural and functional unit

  • Ecology: Ecosystem of different species, biosphere

• Emergent Properties: System properties that cannot be explained by knowledge of individual components. Example: Emotion or intelligence in humans cannot be predicted from knowing individual nerve cell properties.

• Function: The "why" approach (e.g., why RBCs transport oxygen).

• Mechanism: The "how" approach (e.g., how oxygen binds to hemoglobin molecules in RBCs).

Homeostasis

Definition and Importance

Homeostasis is the ability to maintain a relatively stable internal environment despite exposure to external variability. The term was coined by Walter Cannon in 1929.

• Homeo: Like or similar

• Stasis: Condition (not static)

Disruption of homeostasis can lead to a pathophysiological state, resulting in wellness (success) or illness (failure).

Internal and External Changes in Homeostasis

• Internal: Abnormal cell growth, autoimmune disorders, genetic disorders

• External: Chemical exposure, physical trauma, foreign invaders (bacterial and viral)

Internal Environment of the Body

• Most cells are not tolerant to changes in their surroundings.

• Extracellular fluid (ECF) surrounds cells, acting as a buffer between cells and the external environment.

Mass Balance and Homeostasis

• Law of Mass Balance: If the amount of a substance in the body is to remain constant, any gain must be offset by equal loss.

Equation:

$\text{Total amount} = \text{Intake} + \text{Production} - \text{Excretion} - \text{Metabolism}$

• Mass balance in an open system requires input to equal output.

• Input: Metabolic production (intestines, lungs, skin)

• Output: Metabolism to a new substance (kidney, liver, lungs, skin)

Homeostasis does not mean equilibrium; body compartments are in a dynamic steady state but not in equilibrium. Instead, a stable disequilibrium is maintained.

Control Systems and Homeostasis

Regulation and Feedback

• Regulated variables are kept within a normal range by control mechanisms.

• Control systems can be local or reflex (long-distance).

• General pathway: Input signal → Integrating center → Output signal → Response

Local Control

• Restricted to tissues or cells involved.

• Active cells reduce oxygen levels in tissue, causing endothelial cells to send local signals; vasodilation restores oxygen in tissue.

Reflex Control

• Long-distance signaling, involving nervous and/or endocrine systems.

• Uses more complex systems to maintain homeostasis.

• Reflex control refers to any long-distance pathway that uses the nervous and/or endocrine system.

Loops

• Response loop

• Feedback loop: Modulates the response loop, feeds back to influence the input ultimately.

Pathway: Stimulus → Sensor → Input signal → Integrating center → Output signal → Target → Response

Negative Feedback Loops

• A pathway in which the response opposes or removes the stimulus signal is known as negative feedback.

• This process stabilizes a system and can restore the initial state, but cannot prevent the disturbance/change from happening.

• Example: Blood glucose regulation: High blood glucose stimulates insulin release, which causes cells to take up glucose, lowering blood glucose and stopping insulin release.

Positive Feedback Loops

• Non-homeostatic

• Reinforce stimulus to drive the system away from a normal value rather than decreasing or removing it.

• Requires external action for the loop to cease the response.

Reflex Control Systems: Baroreceptor Reflex

• Baroreceptor reflex: Monitors blood pressure

• Artery wall stretch from increase in pressure (Stimulus)

• Baroreceptor acts as sensor

• Mechanical stretch → electrical signal → CNS (Input Signal)

• Medulla acts as integrating center

• Signals sent out toward tissues (Output Signal)

• Heart and peripheral arterioles (Target)

• Reduced HR and peripheral dilation (Response)

Feedforward Control and Biorhythms

• Some reflexes evolved to allow the body to predict a change about to happen (anticipatory).

• Example: Salivating reflexes → body gets ready for digestion.

• Biorhythms: Variables that change based on prediction and create repeating patterns/cycles of changes (e.g., hormone levels peaking in late afternoon and early evening).

Cell Membranes I

Composition

• Lipids

• Proteins

• Carbohydrates

Compartments

• Anatomical:

  • Cranial cavity

  • Thoracic cavity

  • Abdominopelvic cavity

• Body fluid:

  • ECF: Lies outside cells (blood plasma, interstitial fluid)

  • ICF: Inside cells (fat cells, serum, RBCs, smooth muscle)

Biological Membranes

• Cell membrane function:

  • Physical barrier between ICF and ECF

  • Environmental exchange regulation

  • Cell communication with environment

  • Structural support: Proteins make cell tissue connections that anchor to the cytoskeleton

• Average composition: 55% proteins, 45% lipids, trace amounts of carbohydrates

Cell Membrane Lipids

• Phospholipids (>50%)

  • Contain a polar head (hydrophilic) and a nonpolar fatty acid tail (hydrophobic), making them amphipathic

  • Phospholipids can arrange as bilayer (sheet), micelles (droplets, important in digestion), and liposomes (aqueous center)

• Sphingolipids (~30%)

  • Form lipid rafts

  • Anchor proteins, important for cell signal transduction

• Cholesterol (~20%)

  • Increases viscosity

  • Decreases permeability

Cell Membrane Proteins

• Integral proteins

  • Tightly bound to the membrane, crossing through (transmembrane)

  • Resting inside one side of lipid bilayer directly to fatty acid or external GPI anchor (sugar-phosphate chain)

  • Act as receptors, enzymes, and movement channels for atoms and minerals

• Peripheral proteins

  • Attach to integral proteins and are loosely attached to phospholipid heads

Cell Membrane Carbohydrates

• Glycoprotein (extracellular): Forms protective layer (glycocalyx) and regulates cell-cell interaction

• Glycolipid (intracellular): Has a glycocalyx and cell-to-cell interaction

Body Fluid Compartments

• Both intra- and extracellular compartments are in osmotic equilibrium

• ~60% of the body's water is distributed between these compartments

Cellular Solutes

• ICF: Mainly potassium ions and proteins, anions (HPO42-, H2PO4-)

• Interstitial Fluid (ECF): Mainly sodium ions and chlorine ions, no proteins present

• Plasma (ECF): Mainly sodium and chlorine ions, proteins, and small amount of HCO3-

Osmolarity and Tonicity

Definitions and Comparisons

• Isosmotic: Equal, identical osmolarities

• Hyperosmotic: Solution with higher osmolarity

• Hyposmotic: Solution with lower osmolarity

Tonicity vs Osmolarity

• Osmolarity describes the number of solute particles dissolved in solution (mOsm/L) and can be measured; tonicity has no units, it is a comparable term.

• Osmolarity can be used to compare two solutions; tonicity always compares a solution and a cell and describes the solution.

• Osmolarity does not tell you what happens to a cell placed in a solution; tonicity tells you what happens to cell volume when placed in a solution.

Examples of Tonicity Effects

• Isotonic solution: No net movement of water; cell volume remains unchanged.

• Hypotonic solution: Water moves into the cell; cell swells.

• Hypertonic solution: Water moves out of the cell; cell shrinks.

Summary Table: Membrane Composition

Membrane Type	Protein (%)	Lipid (%)	Carbohydrate (%)
Red blood cell membrane	49	43	8
Myelin membrane around nerve cells	18	79	3
Inner mitochondrial membrane	76	24	0

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and explanations have been inferred and supplemented for study purposes.