Master Physiology
for PMDC NLE Step 1

Core Concepts

Physiology is the study of the normal functioning of living organisms and their parts. It focuses on maintaining homeostasis through complex regulatory mechanisms.

• Cellular Physiology:
  • Homeostasis: Maintenance of a relatively stable internal environment. Essential for survival.
  • Feedback Loops:
    • Negative Feedback: Most common. Counteracts stimulus to restore set point (e.g., blood pressure regulation, temperature control, hormone regulation).
    • Positive Feedback: Amplifies stimulus, moving variable away from set point (e.g., parturition, blood clotting, action potential generation).
  • Membrane Transport:
    • Passive: Diffusion (simple, facilitated), Osmosis (water movement across semi-permeable membrane). No ATP.
    • Active: Primary (direct ATP use, e.g., Na+/K+ pump), Secondary (uses energy stored in ion gradients, e.g., SGLT). Requires ATP.
  • Action Potentials: Rapid, transient changes in membrane potential. Generated by voltage-gated ion channels. Phases: Depolarization (Na+ influx), Repolarization (K+ efflux), Hyperpolarization. All-or-none principle.
• Nervous System:
  • Neurons: Functional unit. Transmit electrical signals. Components: dendrites, soma, axon, synaptic terminals.
  • Neurotransmitters: Chemical messengers at synapses. Key examples: Acetylcholine (NMJ, ANS parasympathetic), Norepinephrine (ANS sympathetic), Dopamine, Serotonin, GABA (inhibitory), Glutamate (excitatory).
  • Autonomic Nervous System (ANS): Involuntary control.
    • Sympathetic: "Fight or flight." Thoracolumbar origin. Short preganglionic (ACh), long postganglionic (NE, except sweat glands ACh).
    • Parasympathetic: "Rest and digest." Craniosacral origin. Long preganglionic (ACh), short postganglionic (ACh).
• Endocrine System:
  • Hormone Types: Peptide (insulin, GH), Steroid (cortisol, estrogen), Amine (thyroid hormones, epinephrine).
  • Key Glands/Hormones: Hypothalamus (releasing/inhibiting hormones), Pituitary (GH, TSH, ACTH, FSH, LH, Prolactin, ADH, Oxytocin), Thyroid (T3, T4, Calcitonin), Parathyroid (PTH), Adrenal (cortisol, aldosterone, catecholamines), Pancreas (insulin, glucagon).
  • Regulation: Negative feedback is crucial.
• Cardiovascular System:
  • Cardiac Cycle: Systole (contraction, ejection), Diastole (relaxation, filling). ECG: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).
  • Cardiac Output (CO): HR x SV. Regulated by preload, afterload, contractility.
  • Blood Pressure (BP): CO x SVR. Regulated by baroreceptors (short-term), Renin-Angiotensin-Aldosterone System (RAAS), ADH, ANP (long-term).
  • Starling Forces: Govern fluid movement across capillaries (hydrostatic vs. oncotic pressure).
• Respiratory System:
  • Lung Volumes & Capacities: Tidal volume, Vital capacity, Residual volume, Total lung capacity.
  • Gas Exchange: Occurs in alveoli-capillary membrane. Driven by partial pressure gradients (PO2, PCO2). V/Q mismatch is key pathology.
  • O2/CO2 Transport: O2 primarily by hemoglobin. CO2 primarily as bicarbonate, also bound to hemoglobin, dissolved.
  • Control of Breathing: Medullary rhythmicity center. Chemoreceptors (central for CO2/pH, peripheral for O2/CO2/pH).
• Renal System:
  • Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute. Dependent on renal blood flow and pressure gradients. Autoregulated.
  • Tubular Functions:
    • Proximal Tubule: Bulk reabsorption (Na+, H2O, glucose, amino acids).
    • Loop of Henle: Countercurrent multiplier for urine concentration (ADH acts on collecting duct).
    • Distal Tubule/Collecting Duct: Fine-tuning of Na+, K+, H2O, acid-base balance (aldosterone, ADH).
  • Acid-Base Balance: Regulated by buffers (bicarbonate), lungs (CO2 excretion), kidneys (HCO3- reabsorption/generation, H+ excretion).
• Gastrointestinal System:
  • Motility: Peristalsis, segmentation. Regulated by enteric nervous system.
  • Secretions: Saliva, gastric acid (parietal cells), pancreatic enzymes (digestion), bile (fat emulsification).
  • Digestion & Absorption: Carbohydrates, proteins, fats broken down into smaller units for absorption.
• Musculoskeletal System:
  • Muscle Contraction: Excitation-contraction coupling. Action potential leads to Ca2+ release from SR, binding to troponin, exposing actin binding sites for myosin. ATP-dependent cross-bridge cycling.

Clinical Presentation (Physiological Derangements & Manifestations)

• Hypotension/Hypertension: Derangements in BP regulation (e.g., low CO, high SVR).
• Respiratory Distress: Impaired gas exchange (V/Q mismatch, diffusion defects), ventilatory failure.
• Edema: Imbalance of Starling forces (e.g., low plasma oncotic pressure, high capillary hydrostatic pressure).
• Electrolyte Imbalances: Renal dysfunction, hormonal issues (e.g., hyponatremia, hyperkalemia).
• Acid-Base Disorders: Respiratory (CO2), Metabolic (HCO3-).
• Endocrine Dysfunctions: Hypo/hypersecretion of hormones (e.g., diabetes mellitus, thyroid disorders).
• Cardiac Arrhythmias: Abnormalities in cardiac conduction system (e.g., ion channel defects).
• Neurological Deficits: Impaired neurotransmission, neuronal damage (e.g., stroke, demyelination).

Diagnosis (Assessment of Physiological Function)

Physiological assessment involves measuring parameters to evaluate organ system function and identify deviations from normal.

• ECG: Cardiac electrical activity, arrhythmias, ischemia.
• Spirometry: Lung volumes and airflow rates.
• Blood Tests: Electrolytes, creatinine (GFR estimation), blood gases (ABGs for acid-base, O2/CO2), hormone levels.
• Urinalysis: Renal function, specific gravity, solute excretion.
• Nerve Conduction Studies/EMG: Peripheral nerve and muscle function.
• Imaging: Echocardiography (cardiac function), X-ray/CT/MRI (structural assessment of organs affecting function).
• Dynamic Tests: Oral Glucose Tolerance Test (OGTT), ACTH stimulation test, water deprivation test.

Management (Physiological Interventions)

Aims to restore or support normal physiological function, often by correcting underlying derangements or supporting failing systems.

• Fluid and Electrolyte Management: IV fluids, electrolyte supplementation/restriction.
• Acid-Base Correction: Bicarbonate administration, ventilatory adjustments.
• Ventilatory Support: Oxygen therapy, mechanical ventilation.
• Cardiovascular Support: Vasopressors/inotropes (increase BP/contractility), diuretics (reduce fluid overload), antiarrhythmics.
• Hormone Replacement/Antagonism: Insulin for diabetes, thyroid hormone for hypothyroidism, corticosteroids for adrenal insufficiency.
• Renal Replacement Therapy: Dialysis for severe renal failure.
• Pharmacological Modulation: Drugs targeting specific receptors (e.g., beta-blockers, ACE inhibitors).

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