Cellular Physiology - Chapter 1

Anatomy vs. Physiology vs. Pathophysiology

• Anatomy: study of the structure and description of the human body.
  • Micro-anatomy vs. macro-anatomy.
• Physiology: study of biological functions and processes of the human body under basal (normal) conditions.
  • Cellular physiology vs. systemic physiology.
  • Cellular physiology studies biochemical and biophysical processes within cells; systemic physiology studies regulation of physiological processes within the body by homeostatic reflexes.
• Pathophysiology: study of abnormal biochemical and biophysical processes in disease; analysis of these abnormal processes is used for diagnosis and treatment.
• Anatomy vs. Physiology Example:
  • Example: The heart’s structure (anatomy) enables its pumping function (physiology); dysfunction in structure (e.g., valve malformation) affects function (e.g., ineffective blood flow).

Homeostasis

• Definition: the dynamic constancy of the internal physiological environment while buffering the challenges of the external environment.
• All organs and systems interact in an orderly and synergistic manner to maintain homeostasis.
• It reflects the ability of the human body to maintain relatively constant internal conditions despite environmental changes.
• Core concept: keep internal variables within a narrow range necessary for cell function and organism survival.

Feedback Control Mechanisms

• Control system components:
  • Receptor (sensor)
  • Control center
  • Effector
  • Afferent pathway (input information)
  • Efferent pathway (output information)
• Process summary:
  1) Stimulus produces a change in a variable.
  2) Change detected by the receptor.
  3) Input: information sent along the afferent pathway.
  4) Output: information sent along the efferent pathway.
  5) Effector response feeds back to influence the magnitude of the stimulus and returns the variable to homeostasis.

Negative Feedback System

• Definition: the response of the control system is negative or opposing to the stimulus.
• Examples:
  • Regulation of blood pressure:
  • Stimulus: decrease in blood volume → decrease in blood pressure.
  • Receptor: baroreceptors in carotid arteries sense the drop.
  • Control center: brain (baroreceptor reflex/cardiovascular center).
  • Effector: vessels constrict, heart rate increases, etc.
  • Response: blood pressure returns toward normal.
  • Regulation of body temperature:
  • Stimulus: increased body temperature OR decreased body temperature (context-dependent).
  • Receptor: thermoreceptors in skin and hypothalamus.
  • Control center: thermoregulatory center in the hypothalamus.
  • Effector: sweat glands, cutaneous blood vessels (vasodilation/vasoconstriction), shivering (when cold).
  • Response: temperature moves back toward set point.

Positive Feedback System

• Definition: the response of the control system is positive or promoting the stimulus.
• Characteristic: amplifies initial response until an endpoint is reached.
• Examples:
  • Childbirth:
  • Cervical stretching stimulates sensory receptors.
  • Sensory input to brain triggers release of oxytocin from posterior pituitary.
  • Oxytocin promotes stronger uterine contractions, increasing cervical stretch and further stimulation until birth occurs.
  • Blood coagulation:
  • Cascade activation accelerates clot formation until the bleeding is stopped.
• Question: How do positive feedback systems stop?
  • They are self-limiting because they terminate when the endpoint is reached or when counter-regulatory mechanisms shut down the stimulus (e.g., birth concludes when delivery completes; clotting is limited by inhibitors and feedback controls).

Plasma Membrane

• Function: external cell barrier; selectively permeable.
• Structure: phospholipid bilayer marks the cell boundary; amphipathic in nature.
• Phospholipid molecules:
  • Hydrophilic heads are what? = hydrophilic (polar, phosphate-containing).
  • Hydrophobic tails are what? = hydrophobic (nonpolar, fatty acid tails).

Transport Mechanisms: Passive – Simple Diffusion

• Passive transport: movement down the concentration gradient (high to low).
• Simple diffusion: natural movement from high to low concentrations; unassisted transport (no integral protein).

Transport Mechanisms: Passive – Facilitated Diffusion

• Channel-mediated facilitated diffusion:
  • Special transport proteins create hydrophilic tunnels in the lipid bilayer.
  • Facilitates the transport of small, polar molecules and ions.
• Carrier-mediated facilitated diffusion:
  • Transport proteins carry the substance across.
  • Facilitates the transport of large, polar molecules.
• Key idea: substance moves down its concentration gradient with the help of a protein.

Transport Mechanisms: Active Transport

• Active transport moves substances against their concentration gradient (low to high).
• Primary Active Transport:
  • Carrier proteins pump molecules against the gradient.
  • Direct use of cellular energy (ATP).
• Secondary Active Transport:
  • Downhill movement of one molecule drives uphill movement of another.
  • Indirect use of energy: utilizes established gradient of molecule A to power transport of molecule B.

Transport Mechanisms: Vesicular Transport

• Endocytosis: substances are taken into the cell by modifying the plasma membrane.
  • Phagocytosis: "cell eating"; large particles engulfed, forming a phagosome.
  • Pinocytosis: "cell drinking"; invagination of membrane to bring extracellular fluid and solutes into a vesicle.
• Vesicular transport: bulk transport of substances into or out of the cell.
• Exocytosis: substances released from the cell into the extracellular environment; accounts for most secretion processes.

Osmosis

• Osmosis: movement of water across a selectively permeable membrane.
• Hypotonic solution:
  • Lower solute concentration; hypo- means 'less than'.
  • Higher water (solvent) concentration.
  • Net movement of water toward the side with higher solute concentration (into the more concentrated solution/into the cell if the cell interior is more concentrated).
• Hypertonic solution:
  • Higher solute concentration; hyper- means 'greater than'.
  • Lower water concentration.
  • Net movement of water toward the hypertonic side (out of the cell).
• Isotonic solution:
  • Equal solute and solvent concentration on both sides of the membrane.
  • Water molecules continue to cross, but there is no net movement.
• Note: In the provided content, the visual depiction may have labeling that could reverse the perceived direction; the standard interpretation is as listed above.

Tonicity of Solutions

• Tonicity: measure of the potential difference in osmotic pressure gradient between two solutions separated by a semipermeable membrane.
• It is influenced only by non-penetrating solutes (solutes that cannot cross the membrane and therefore exert osmotic pressure).
• Based on the image (when shown):
  • Solutes that exert osmotic pressure are non-penetrating solutes.
  • Water moves toward the solution with higher osmotic pressure.
• Clinical prompts (from the content):
  • If both solutions have the same solute concentration, there is no net water movement.
  • The solution outside the cell that is hypertonic to the inside causes the cell to lose water (cell shrinks).
  • The solution outside the cell that is hypotonic to the inside causes the cell to gain water (cell swells).

Clinical Applications

• When two solutions have the same total solute concentration, there is no net water movement (isotonic).
• A solution with higher solute concentration than the cell is hypertonic to the cell; the external environment is hypertonic relative to the cell interior.
• A solution with lower solute concentration than the cell is hypotonic to the cell; the external environment is hypotonic relative to the cell interior.

Red Blood Cells

• Hypotonic solution: cells swell and may lyse.
• Isotonic solution: cells maintain normal shape and volume.
• Hypertonic solution: cells shrink as water leaves the cell.

Future Direction of Medical Physiology

• Traditional view: DNA blueprint as primary controller of function and survival.
• Reality: Proteins expressed by the cell’s DNA power cellular and organ functions (muscle contraction, brain activity, metabolism, oxygen transport).
• Epigenetics: studies how lifestyle and environmental signals modify and regulate gene activity without changing the DNA sequence.
  • Epigenetics links environment, genetics, and function; lifestyle factors such as exercise, nutrition, stress, trauma, substance use, and social interactions can modify gene function.
  • Definition: mechanisms by which genes can be switched on and off without altering the genes themselves or their genetic code.
  • Practical implication: research suggests roughly 20% of health, disease propensity, and aging is due to genetics, while ~80% is influenced by epigenetic changes driven by lifestyle and environment.
• Ethical, philosophical, and practical implications:
  • Responsibility: the balance between genetic predisposition and modifiable lifestyle factors.
  • Public health: policies aimed at improving lifestyle factors could have large impacts on population health.
  • Privacy and equity: how epigenetic information might be used (or misused) in employment, insurance, or stigma; access to lifestyle interventions.