Physiological Systems

Internal vs. External Body Surfaces

Body surfaces are classified based on their continuity with the external environment and their accessibility. Understanding these distinctions is essential for studying 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 can only be accessed 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:

• The surface facing the environment outside the body is called the external face.

• The surface facing the lumen of a hollow structure is called the apical or luminal face.

• The surface facing the interstitial environment is called the basolateral face.

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 of closely packed cells.

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

Fluid Compartments of the Body

• Tissue-level compartments are separated by multicellular membranes:

  • Blood plasma and lymph

  • Interstitial fluid

• Organ-level compartments are separated by multicellular membranes:

  • Cerebrospinal fluid

  • Synovial cavities

  • Chambers in eyeballs and ears

• Cell-level compartments: Intracellular compartments are separated by phospholipid bilayers:

  • 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; the body cannot prevent this.

• Water soluble molecules cannot pass through the membrane unless via a channel or carrier.

Membranous Organelles

Membranous organelles are hollow structures that contain solutes required for specific cellular functions.

• 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: Soluble proteins embedded in or attached to the inner or outer surface of the membrane, mediating specific interactions between the cell and the extracellular environment.

• Membrane carbohydrates (Glycocalyx): Sugars attached to either 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 that affect cells:

  • Osmolarity

  • Temperature

  • pH

• Materials cells need to maintain homeostasis:

  • Nutrients

  • Water

  • Inorganic ions

  • Oxygen

  • Internal secretions

When conditions change, homeostatic set points change to maintain optimal system performance.

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, and lost through excretion or chemical reactions.

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

Equation:

Feedback Loops

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

Elements of Feedback Loops

• Physical or chemical stimulus: A monitored component of the environment.

• Receptors: Structures that monitor the environment for changes.

• Chemical or electrical signal: Secreted potential that communicates changes.

• Target cells: Cells that receive and respond to signals.

Types of Reflexes

• Negative Feedback: In response to a change, a signal is produced to cause target cells to change their function, maintaining homeostasis.

  • Most normal signaling reflexes exert negative feedback.

• Positive Feedback: In response to a change, a signal is produced to amplify the magnitude of the stimulus.

  • Often used to activate systems that counter a problem (e.g., childbirth, acid secretion in the stomach).

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

Water Balance, pH, and Temperature

Regulation of Body Fluids

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

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

• pH and temperature: Must be maintained in 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.

Example: If blood pH drops below 7.35, acidosis occurs; if it rises above 7.45, alkalosis occurs. Water balance is essential for maintaining blood pressure and cellular function.

*Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and examples have been added to support understanding of physiological principles.*