Physiology – High-Yield Concepts & Systems Overview

Introduction to Physiology

Definition
- Physiology is the scientific study of the functions and mechanisms operating within a living system.
- Focuses on how organs and systems cooperate to sustain life rather than merely describing their structure.
Scope & Connections
- Bridges basic biology and clinical medicine.
- Provides the functional context for disciplines such as anatomy, biochemistry, pharmacology, and pathology.

Diagnostics & Treatment
- Understanding normal physiological ranges allows clinicians to recognize pathological deviations (e.g.
  blood‐glucose homeostasis in diabetes).
Therapeutic Innovation
- Physiological insights underpin development of medical devices (e.g.
  pacemakers based on cardiac electrophysiology) and novel drugs (e.g.
  β-blockers modulating sympathetic tone).
Pharmacodynamics & Pharmacokinetics
- Explains how drugs alter body functions, receptor interactions, absorption, distribution, metabolism, and excretion.
Research & Translational Medicine
- Animal and cellular models of physiological processes guide translational breakthroughs.

Phospholipid Bilayer
- Amphipathic molecules self-assemble into two opposing layers.
- Provides selective permeability: lipid‐soluble entities cross readily, water-soluble require channels/transporters.
Homeostasis via Feedback Loops
- Negative feedback counteracts disturbances; positive feedback amplifies them.
- Essential for moment-to-moment regulation of the intracellular environment.
Transport Mechanisms
- Passive: diffusion & osmosis—move down electrochemical gradients, no ATP.
- Active: \text{Na}^+ / \text{K}^+ ATPase pumps 3\,\text{Na}^+ out / 2\,\text{K}^+ in per ATP hydrolyzed; maintains resting membrane potential.
- Clinical link: digitalis inhibits this pump, increasing intracellular \text{Ca}^{2+} in heart failure therapy.

Cells → Tissues → Organs → Organ Systems → Organism
Primary Tissue Types & Examples
- Nervous: brain, spinal cord, peripheral nerves; rapid electrochemical communication.
- Epithelial: GI tract lining, epidermis; protection, absorption, secretion.
- Muscle: skeletal (voluntary motion), cardiac (heart pump), smooth (visceral motility).
- Connective: bone, tendon, adipose; structural framework, energy storage, immunity.

Integumentary
- Organs: skin, hair, nails, sweat glands.
- Functions: barrier, thermoregulation, sensory interface.
Skeletal
- Organs: bones, cartilages, ligaments, marrow.
- Functions: support, mineral reservoir (e.g.
  \text{Ca}^{2+}), hematopoiesis.
Muscular
- Organs: skeletal muscles + associated tendons.
- Functions: movement, posture, heat production (shivering).
Nervous
- Organs: brain, spinal cord, peripheral nerves, sensory organs.
- Functions: rapid coordination, reflex arcs.
Endocrine
- Organs: pituitary, thyroid, adrenal glands, pancreas, gonads, plus endocrine tissue in other systems.
- Functions: long-term regulation via hormones (growth, metabolism, reproduction).
Cardiovascular
- Organs: heart, blood vessels, blood.
- Functions: transport of nutrients, gases, wastes; heat distribution.

Lipids (≈40–50 % of membrane mass)
- Phospholipids: primary structural units.
- Glycolipids: cell recognition & signaling.
- Cholesterol: modulates fluidity; ↓ fluidity at high temp, ↑ integrity at low temp.
- Lipid-soluble molecules that cross freely: \text{CO}2, \text{O}2, fatty acids, steroid hormones.
Proteins (≈50–60 %)
- Transporters & carrier proteins: glucose transporter (GLUT), amino-acid exchangers.
- Enzymes: adenylate cyclase, ATP synthase.
- Hormone receptors: insulin receptor (tyrosine kinase), GPCRs.
- Cell-surface antigens: ABO blood group determinants.
- Ion & water channels: voltage-gated \text{Na}^+, aquaporins.
- Provide water-soluble pathways for glucose, ions, amino acids.

Hydrophobic Tails
- Two fatty-acid chains.
- Unsaturated (double bonds) & short chains increase membrane fluidity—critical for cold adaptation.
Hydrophilic Head
- Phosphorylated glycerol backbone.
- Interfaces with aqueous ECF & ICF; enables amphipathic orientation.

Integral (Transmembrane): span bilayer; channels, pumps, receptors.
Peripheral: loosely attached to cytoplasmic or extracellular face; signaling scaffolds, cytoskeletal anchors.
Functional Examples
- Channel protein: voltage-gated \text{K}^+ channel—repolarizes neurons.
- Carrier protein: Na⁺-glucose symporter—intestinal glucose uptake.

Total Body Water (TBW)
- Approximately TBW = 0.6 \times \text{Body Weight}.
- For a 70 kg adult: 0.6 \times 70 = 42\,\text{L}.
Distribution
- Intracellular Fluid (ICF): \frac{2}{3} of TBW ≈ 28 L.
- Extracellular Fluid (ECF): \frac{1}{3} of TBW ≈ 14 L.
- Interstitial Fluid (ISF): \approx 75\% of ECF ≈ 10.5 L.
- Plasma: \approx 25\% of ECF ≈ 3.5 L.
Physiological Significance
- Electrolyte composition differs across compartments; e.g.
  Na⁺ high in ECF, K⁺ high in ICF.
- Capillary walls separate plasma & ISF; cell membranes separate ECF & ICF.
- Volume disturbances (dehydration, edema) analyzed with this model.

Definition
- Maintenance of a stable internal environment despite external fluctuations.
Regulatory Framework
- Sensor: detects change (e.g.
  thermoreceptors).
- Control Center: integrates data (hypothalamus).
- Effector: executes response (sweat glands, muscles).
Negative Feedback
- Opposes the original stimulus.
- Example: Thermoregulation
- Stimulus: body temp > 37^\circ\text{C}.
- Sensor: skin & brain thermoreceptors.
- Control center: hypothalamus.
- Effectors: sweat glands → evaporative cooling; cutaneous vasodilation.
- Outcome: temperature returns to set point.
Positive Feedback
- Amplifies the initiating stimulus; typically terminated by an external event.
- Example: Childbirth
- Stretch receptors in cervix send impulses → hypothalamus.
- Posterior pituitary releases oxytocin.
- Oxytocin ↑ uterine contractions, further stretching cervix.
- Loop ends with delivery of baby & placenta.
Clinical Correlate
- Unchecked positive feedback can be pathological (e.g.
  cytokine storm in sepsis).

Physiological Knowledge in Public Health
- Guides vaccination schedules (immune physiology), nutritional recommendations (metabolic physiology).
Technological Implications
- Development of artificial organs (dialysis machines emulate kidney filtration physiology).
Philosophical Dimensions
- Raises questions about homeostasis vs. enhancement in debates on transhumanism and bioengineering.

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