Physiological Systems, Biological Membranes, and Homeostasis: Structured Study Notes

Physiological Systems

Internal vs. External Body Surfaces

Body surfaces are classified based on their continuity with the external environment or their separation by cellular membranes. This distinction is fundamental for understanding physiological barriers and compartmentalization.

• External body surfaces are continuous with the external environment and include:

  • Nasals and oral passages

  • Respiratory tract

  • GI tract

  • Urinary tract

  • Reproductive tract

  • Ducts of exocrine glands

• Internal body surfaces are only accessible from internal body fluid or by transport across an external face membrane. Examples include:

  • Most cell membranes

  • Cardiovascular and lymphatic membranes

  • Serous and connective tissue membranes

Multicellular membranes divide the body into compartments that maintain separate chemical environments. Lining and covering membranes have distinct sides:

• External face: faces the environment outside the body

• Apical (luminal) face: faces the lumen of a hollow structure

• Basolateral face: faces the interstitial environment

Extracellular Fluid Compartments

Fluid compartments are separated by multicellular tissues that form barriers to exchange both in the membranes of the cells and the spaces between the cells.

• Multicellular membranes are epithelial or connective tissues comprised of multiple cells and networks of extracellular proteins.

  • Epithelial membranes can limit movement between compartments by forming continuous layers.

  • Connective tissue membranes are used for binding and packaging, with cells separated by extracellular matrix.

Fluid Compartments of the Body

• Tissue-level compartments:

  • Blood plasma and lymph

  • Interstitial fluid

• Organ-level compartments:

  • Cerebrospinal fluid

  • Synovial cavities

  • Chambers in eyeballs and ears

• Cell-level compartments:

  • Intracellular fluid (cytosol)

  • Fluid in membranous organelles

Biological Membranes

Cell Membranes

Cell membranes are selectively permeable barriers that separate chemical environments inside and outside the cell.

• Basic structure: consists of phospholipids interspersed with cholesterol, leading to overall lipid quality.

• Lipid soluble molecules freely diffuse across the membrane.

• Water soluble molecules cannot pass through the membrane unless via channels or carriers.

Membranous Organelles

Organelles are specialized structures within cells that perform specific functions, often separated by their own membranes.

• Mitochondria: location of aerobic ATP synthesis.

• Endoplasmic reticulum (ER):

  • Smooth ER: involved in lipid synthesis and drug detoxification.

  • Rough ER (with ribosomes): involved with protein synthesis.

• Golgi apparatus: packages synthesized proteins in transport vesicles.

• Lysosomes: digest endocytosed molecules or damaged cell parts.

• Peroxisomes: digest fatty acids and foreign molecules.

Cell Membrane Structure

• Phospholipid bilayer: provides the basic structure and selective permeability.

• Membrane proteins: embedded or attached to the membrane, mediate interactions between the cell and extracellular environment.

• Membrane carbohydrates (glycocalyx): sugars attached to membrane proteins or lipids, often act as signal or recognition molecules.

Homeostasis

Definition

Homeostasis is the ability of the body to maintain optimal performance of a system under a given set of conditions.

Cannon's Postulates: Variables Under Homeostatic Control

• Environmental factors affecting cells:

  • Osmolarity

  • Temperature

  • pH

• Materials cells need:

  • Nutrients

  • Water

  • Inorganic ions

  • Oxygen

  • Internal secretions

Mass Balance

To maintain constant levels of substances, any gain must be offset by an equal loss.

• Total amount of substance = (intake + production) - (excretion + metabolism)

• Substances are gained through foods, beverages, or synthesis, and lost through excretion or chemical reactions.

• Chemical reactions progress toward even distribution of reactants and products.

Feedback Loops

Feedback loops allow communication between cells to maintain homeostasis in physiological systems.

• Elements of feedback loops:

  • Physical or chemical stimulus

  • Receptors: monitor environment for changes

  • Chemical or electrical signal: communicates changes

  • Target cells: receive signals and produce responses

• Types of reflexes:

  • Negative feedback: response reduces the magnitude of the stimulus, maintaining homeostasis. Most normal signaling reflexes are negative feedback.

  • Positive feedback: response amplifies the magnitude of the stimulus. Used to activate systems that counter a problem (e.g., childbirth, acid secretion in the stomach).

  • Feedforward reflexes: anticipate the need for change before it occurs (e.g., salivation, some digestive reflexes).

Water Balance, pH, and Temperature

Key Variables

The body closely regulates water balance, pH, and temperature to maintain proper function.

• Water balance: necessary for chemical reactions, action potentials, and signaling.

• pH and temperature: must be maintained within a narrow range for proteins to retain their shape and function.

Water Balance Priorities

• Maintain proper solute concentration of body fluids

• Maintain proper blood viscosity

Table: Types of Body Membranes and Their Functions

Membrane Type	Location	Main Function
Epithelial Membrane	Body surfaces, lining of organs	Barrier, selective transport, secretion
Connective Tissue Membrane	Joints, organ capsules	Binding, packaging, support
Phospholipid Bilayer	Cell membrane	Selective permeability, compartmentalization

Key Equations

• Mass Balance Equation:

  • $\text{Total amount of substance} = (\text{intake} + \text{production}) - (\text{excretion} + \text{metabolism})$

Additional info:

• Homeostasis is a central concept in physiology, ensuring stability of the internal environment despite external changes.

• Membrane structure and function are foundational for understanding transport, signaling, and compartmentalization in cells.