Introduction to Human Physiology

Overview and Etymology of Physiology

• Definition: Physiology is defined as the study of the normal functioning of a living organism and its individual constituent parts, incorporating all associated physical and chemical processes.

• Etymological Roots:

  • Derived from the Latin root physio-, which broadly translates to "physical".

  • Derived from the Greek root logia-, which translates to "study".

  • Combining these roots, physiology represents the comprehensive knowledge of that which is natural in a broad sense.

Sub-Disciplines and Specialized Branches

• Human Physiology: Focuses specifically on the operational and functional mechanisms of the human body.

• Animal Physiology: Focuses on the functional processes of non-human animal species.

• Plant Physiology: Studies the internal functions, metabolic pathways, and physical processes of plants.

• Environmental Physiology: A biological discipline that investigates how an organism's physiological processes adapt and respond to specific environmental conditions.

• Comparative Physiology: Investigates and leverages the functional diversity across various organismal species to understand fundamental biological principles.

• Evolutionary Physiology: Studies the adaptation and historical transformation of biological systems and their component structures across multiple generations. Key focus areas include:

  • The explicit role of behavioral traits in driving evolutionary change.

  • The physiological mechanics of sexual selection.

  • Physiological adaptations associated with geographic variation.

Historical Foundations and Evolution of Medical Science

• Early Medical Concepts and Supernatural Beliefs:

  • Illness was historically attributed to divine wrath or supernatural displeasure.

  • Communities routinely sought to appease deities during severe disease outbreaks to avert widespread death.

  • Herbal preparations were utilized as early natural therapeutics to counter disease symptoms.

• Hippocrates ($460\text{ BC} - 377\text{ BC}$):

  • Recognized as the "father of medicine".

  • Began shifting medical understanding away from supernatural models starting around $420\text{ BC}$.

  • Posited that human disease was not inflicted by angry gods, but resulted directly from poor diet, environmental factors, and detrimental daily habits.

  • Established the Hippocratic School of Medicine.

  • Instructed his sons as well as his son-in-law, Polybus.

  • Polybus and Thessalus (son of Hippocrates) subsequently established the Dogmatic School of Medicine.

• Galen ($129\text{ AD} - 200\text{ AD}$):

  • Became a foundational figure in early modern medicine and conducted extensive anatomical studies.

  • Primary anatomical research relied on monkey dissections because human body dissection was legally prohibited.

  • Authored detailed mammalian structural drawings that served as standard anatomical texts until the $1500\text{s}$.

  • Formulated the theory that muscle movements are directly controlled by the brain.

  • Delineated functional differences between the cranial nervous system and the peripheral nervous system.

• Andreas Vesalius ($1514 - 1564$):

  • Renowned anatomist and practicing physician who performed direct, extensive human dissections.

  • Published De humani corporis fabrica, considered a premier masterwork on human anatomy.

  • Disproved key inaccuracies in Galen's systemic circulation model, though retained the mistaken belief that arterial and venous vessels contained fundamentally different types of blood.

  • Appointed official personal physician to Holy Roman Emperor Charles V.

• William Harvey ($1578 - 1657$):

  • Served as personal physician to King James I and King Charles I.

  • Correctly demonstrated systemic blood circulation, detailing the propulsive mechanics of the heart and physical properties of blood.

  • Overturned centuries-old theories claiming that blood recirculated exclusively within the pulmonary tissue.

  • Discovered the precise mechanical role of venous valves in directing blood flow toward the heart.

  • Pioneered the application of quantitative empirical analysis to physiological research:

  • Calculated that the human heart pumps $245\,\text{kg}$ of blood daily.

  • Utilized this metric to refute Galen's model, demonstrating that Galen's theory would require the liver to manufacture an impossible $245\,\text{kg}$ of new blood every day.

• Claude Bernard ($1813 - 1878$):

  • Established formal experimental methodology within medical science.

  • Introduced experimental medicine and implemented double-blind testing procedures to eliminate experimental bias and achieve objective empirical standardisation.

  • Developed the foundational concept of the Milieu intérieur (internal environment), stating: "The constancy of the internal environment is the condition for a free and independent life."

• Walter Bradford Cannon ($1871 - 1945$):

  • Served as Chair of the Department of Physiology at Harvard Medical School.

  • Formulated and coined the physiological term "fight or flight".

  • Conducted pioneering X-ray experiments using metallic compounds to enhance diagnostic imaging of gastrointestinal pathways (the direct predecessor to modern barium meal procedures).

  • Formalized and expanded Claude Bernard's work into the central biological concept of homeostasis, publishing his landmark framework in $1932$

Core Applications and Importance of Physiology

• Understanding Systemic Mechanics:

  • Provides insight into how individual organs and full body systems function in an integrated, coordinated manner.

  • Clarifies physiological mechanisms required to adapt to environmental stressors, such as extreme temperatures or zero-gravity environments.

• Pathophysiology and Therapeutics:

  • Explains how underlying diseases disrupt normal biological functions.

  • Informs the development of effective targeted disease treatments and therapeutic modalities.

• Interdisciplinary Connections:

  • Serves as a foundational discipline directly interconnected with Medicine, Pharmacology, Cell Biology, and Biochemistry.

Major Divisions and Organ Systems

• Primary Divisions:

  • Cell Physiology: Focuses on the functional mechanisms, transport, and biochemical processes occurring at the individual cellular level.

  • Systemic Physiology: Studies the complex functional dynamics of fully integrated organ systems throughout the body.

• Ten Major Human Organ Systems:

  • Circulatory System: Consists of the heart, blood vessels, blood, and the lymphatic system.

  • Gastro-intestinal System: Handles food ingestion, enzymatic breakdown, nutrient absorption, and waste elimination.

  • Endocrine System: Coordinates systemic body functions through chemical signaling molecules and hormones.

  • Immune System: Protects the organism against foreign pathogens and cellular damage.

  • Integumentary System: Provides external structural protection via skin, hair, and associated structures.

  • Musculoskeletal System: Maintains structural support, bodily protection, and generates mechanical movement.

  • Nervous System: Mediates rapid signal transduction, environmental sensing, and motor processing.

  • Renal System: Regulates fluid balance, blood pressure, osmoregulation, and metabolic waste removal.

  • Reproductive System: Facilitates gamete generation and procreation.

  • Respiratory System: Conducts respiratory gas exchange between the organism and external environment.

Biological Hierarchy and Disciplines

• Sequential Levels of Biological Organization:

  1. Atom: Fundamental chemical unit (e.g., lithium).

  2. Molecule: Bound combination of atoms (e.g., water / $\text{H}_2\text{O}$).

  3. Macromolecule: Complex organic polymers (e.g., $\text{DNA}$).

  4. Cell: Basic functional unit of living structures (e.g., Euglena).

  5. Tissue: Aggregation of specialized cells performing unified functions (e.g., cuboidal epithelium).

  6. Organ: Structure composed of multiple functional tissues (e.g., brain).

  7. Organ System: Group of organs acting in concert to accomplish major bodily functions (e.g., respiratory system).

  8. Organism: Single individual living entity (e.g., cat).

  9. Population: Group of co-occurring individuals belonging to the same species (e.g., lion pride).

  10. Community: Interacting populations of distinct species living together (e.g., lions and zebras).

  11. Ecosystem: Combined biotic community and abiotic environmental components (e.g., lions, zebras, trees, grass, soil, etc.).

  12. Biosphere: Global sum of all ecosystems supporting life across Earth.

• Mapping Academic Disciplines to Biological Levels:

  • Chemistry: Spans atoms and simple molecules.

  • Molecular Biology: Focuses on molecules, macromolecules, and initial cellular structures.

  • Cell Biology: Focuses on internal cellular mechanisms and cell populations.

  • Physiology: Focuses on biological processes across tissues, organs, organ systems, and whole organisms.

  • Ecology: Spans broader biological scales from populations, communities, and ecosystems up to the biosphere.