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 and their role in compartmentalization. Understanding these surfaces is essential for grasping how the body maintains distinct chemical environments.

• 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, which form barriers to exchange both in the membranes of the cells and the spaces between cells.

• Multicellular membranes are made of epithelial or connective tissues:

  • Epithelial membranes: limit movement between compartments by forming continuous layers

  • Connective tissue membranes: used for binding and packaging; cells are 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.

• Phospholipid bilayer: consists of phospholipids interspersed with cholesterol, giving the membrane its fluid 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: contains ribosomes; involved in protein synthesis

• Golgi apparatus: packages synthesized proteins into transport vesicles

• Lysosomes: digest endocytosed molecules or damaged cell parts

• Peroxisomes: digest fatty acids and foreign molecules

Cell Membrane Structure

• Membrane proteins: embedded in or attached to the membrane, mediating interactions between the cell and its environment

• Membrane carbohydrates (glycocalyx): sugars attached to membrane proteins or lipids, often acting 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 a constant amount of a substance, 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

• Substances are lost through excretion or chemical reactions

• Reactions progress in the direction that favors even distribution of reactants and products

Equation:

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

Feedback Loops

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

Elements of Feedback Loops

• Physical or chemical stimulus: monitored for changes

• Receptors: monitor the environment for changes

• Chemical or electrical signal: communicates changes between cells

• Target cells: receive signals and produce responses

Types of Reflexes

• Negative Feedback: signal causes target cells to change function to maintain homeostasis

  • Most normal signaling reflexes exert negative feedback

• Positive Feedback: signal 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

  • Examples: salivation, some digestive reflexes

Water Balance, pH, and Temperature

Key Variables

The body closely regulates water balance, pH, and temperature to ensure proper function of chemical reactions and cellular activities.

• Water balance: necessary for chemical reactions 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

Body Compartment	Barrier Type	Example Fluid
Tissue-level	Multicellular membrane	Blood plasma, lymph, interstitial fluid
Organ-level	Multicellular membrane	Cerebrospinal fluid, synovial fluid
Cell-level	Phospholipid bilayer	Cytosol, organelle fluid

Additional info: These notes expand on the original slides by providing definitions, examples, and context for each topic, ensuring a self-contained study guide suitable for college-level Anatomy & Physiology students.