Cellular Physiology, Pathophysiology & Fluid-Electrolyte Basics
Cellular Transport Mechanisms
- Three broad categories
- Passive Transport – no ATP required; substances move "downhill" along their concentration gradient.
• Metaphor: boat drifting with the river current.
• Types
– Osmosis: water moves across a semipermeable membrane from high to low water concentration (high solute ➔ low solute).
▫ Think “fluid/water only.”
▫ Example: extracellular fluid highly concentrated ➔ water shifts into the cell until equilibrium.
– Diffusion: solute particles move from high to low concentration.
▫ “Dry” particles (e.g., O₂, CO₂) spreading out in a room. - Facilitated Transport – still passive but large/charged molecules need carrier proteins, pumps, or channels.
• Example: glucose entry via GLUT transporter. - Active Transport – requires ATP generated by mitochondria; can move substances against their gradient ("boat pulled upstream by a tugboat").
• Classic example: \text{Na}^+ / \text{K}^+‐ATPase pump cycling 3 \text{Na}^+ out & 2 \text{K}^+ into the cell per ATP.
Cellular Ingestion & Secretion
- Endocytosis (cellular ingestion)
• Phagocytosis – “cell eating”; membrane engulfs large solids (bacteria, viruses). “Phago” = eat, “cyto” = cell.
• Pinocytosis – “cell drinking”; uptake of fluid/vesicles (think "Pinot Grigio").
• Receptor-mediated endocytosis – selective uptake using ligand-receptor complexes (less test emphasis). - Exocytosis (secretion) – vesicles fuse with membrane and expel material after Golgi/lysosomal processing ("cell spitting it back out").
Cellular Respiration
- Only two metabolic modes
• Aerobic – with \text{O}2 (e.g., long-distance jogging).
• Anaerobic – without \text{O}2 (e.g., sprinting downhill, heavy muscle work at altitude).
Cell Division & Differentiation
- Mitosis – somatic cell division yielding two identical daughter cells (most common).
- Meiosis – gamete formation (sperm/ova).
- Differentiation – cells mature into specific types; loss of normal differentiation (anaplasia) underlies cancer.
Patterns of Cellular Adaptation (vocabulary)
| Term | Definition | Everyday Example / Disease |
|---|
| Atrophy | ↓ size/number | Bed-bound muscle wasting |
| Hypertrophy | ↑ cell size | Weight-lifter skeletal muscle; LV hypertrophy in HTN |
| Hyperplasia | ↑ cell number | Endometrial thickening; acromegaly soft-tissue growth |
| Metaplasia | Reversible substitution of one mature cell type for another | Cervical changes with HPV, smoking-induced bronchial changes |
| Dysplasia | Disordered growth, size, shape – precancerous | Severe cervical dysplasia (CIN III) |
Cell Injury & Death
- Free radicals (e.g., from charred foods, microplastics) damage DNA → mutations.
- Apoptosis – programmed “self-destruct”; protective, scheduled (≈14 days lifespan for many cells).
- Necrosis – uncontrolled death from ischemia/toxins; triggers inflammation, irreversible.
Disease Examples Tied to Adaptation
- Cerebral Atrophy – seen in Alzheimer’s; ventricles enlarge as brain mass shrinks.
- Cardiac (LV) Hypertrophy – response to chronic HTN/CHF; thicker wall, ↓ compliance ➔ chest pain, syncope, JVD, edema.
- Acromegaly – post-epiphyseal excess growth hormone; tissue hyperplasia (large hands, jaw, organs).
- Cervical Metaplasia/Dysplasia – HPV-induced; reversible metaplasia can progress to dysplasia & cancer; risk ↑ with multiple partners, early intercourse, smoking.
Body Fluid Compartments
- Total body water ≈ 60 % body weight.
• \text{ICF} = 0.4 \times \text{weight} (40 %).
• \text{ECF} = 0.2 \times \text{weight} (20 %). - ECF sub-compartments
• Interstitial – between cells ("Pam spray").
• Intravascular – plasma, lymph.
• Transcellular – CSF, synovial, pleural; excess = “third spacing” (ascites).
Fluid Movement Forces
- Osmosis – passive water flow down gradient; \text{Osmotic Pressure} \propto \Delta C_{solute}.
- Hydrostatic Pressure – physical push generated by fluid column (e.g., blood pressure).
- Tonicity mnemonic
• Isotonic: equal – no net shift.
• Hypotonic: cells Hippo (swell).
• Hypertonic: cells skinny/hyper – shrink.
Hormonal Control of Fluid Balance
- All triggered by hypovolemia ↓BP sensed by kidneys.
- ADH (vasopressin) – reabsorbs free water (anti-diuresis).
- Aldosterone – reabsorbs \text{Na}^+ (water follows). “Where sodium goes, water flows.”
- Renin–Angiotensin–Aldosterone System (RAAS) – renin → angiotensin II (vasoconstrict) → aldosterone.
Fluid Volume Excess (Hypervolemia)
- Causes
• High sodium diet; renal failure; hyperaldosteronism, Cushing, SIADH.
• Psychogenic polydipsia (water intoxication) – dilute electrolytes, cell lysis.
• Excess IV or tube-feeding water; hypertonic fluid errors. - Clinical Findings
• Peripheral, periorbital, scrotal edema; anasarca (whole-body).
• Pulmonary: dyspnea, crackles, ↓ O₂ sat.
• JVD (visible jugular veins).
• Bounding pulse, ↑ BP, S₃ gallop.
• Rapid weight gain; polyuria.
• Infants: bulging fontanelle. - Basic Interventions (preview)
• Elevate limbs, compression hose, loop diuretics, restrict Na⁺/fluids.
Fluid Volume Deficit (Dehydration / Hypovolemia)
- Etiologies: poor intake, GI losses (vomit/diarrhea), profuse sweating, prolonged hyperventilation, hemorrhage, renal wasting (nephrosis), diabetes insipidus.
- Signs/Symptoms
• Thirst, dry mucosa, ↓ skin turgor (tenting).
• Hypotension + tachycardia (weak/thready).
• Flat neck veins, oliguria (<30 mL/hr).
• Weight loss, sunken fontanelle.
• Confusion, dizziness (cerebral under-perfusion).
Key Electrolytes
| Electrolyte | Normal Range | Hallmark Clinical Concerns |
|---|
| Na⁺ | 135–145\;\text{mEq·L}^{-1} | Neuro: seizures in hyponatremia; confusion in hypernatremia |
| Cl⁻ | 96–106 | Follows Na⁺ trends |
| K⁺ | 3.5–5.0 | Cardiac arrhythmias in hypo- or hyper-kalemia; leg cramps (low) |
| Ca²⁺ | 8.5–10.5\;\text{mg·dL}^{-1} | Nerve conduction, tetany, Chvostek/Trousseau signs |
| Mg²⁺ | 1.5–2.5 | Neuromuscular excitability; works with Ca²⁺ |
| PO₄³⁻ | 2.5–4.5 | Inverse to Ca²⁺; similar symptom set |
Hormonal & Stress Physiology
- Hypothalamus ➔ Pituitary (“master gland”) ➔ Peripheral glands (thyroid, adrenals, gonads, etc.).
• Posterior pituitary: ADH, oxytocin, prolactin.
• Anterior pituitary: ACTH, TSH, GH, LH, FSH, etc.
Feedback Loops
- Negative Feedback (common) – deviation triggers opposite response to restore baseline.
• Cold → shiver → heat ↑ → normothermia.
• Hot → sweat → heat ↓ → normothermia. - Positive Feedback (rare) – amplifies change until event completes.
• Labor: fetal head stretch → oxytocin ↑ → stronger contractions → delivery.
Acute Stress Response & General Adaptation Syndrome (Hans Selye)
- Alarm – hypothalamus-pituitary-adrenal (HPA) activation, catecholamines surge.
- Resistance – body copes, attempts homeostasis.
- Exhaustion – reserves depleted → fatigue, immune suppression, chronic disease risk.
Study/Resource Tips
- Handout “Fluid & Electrolyte cheat sheet” – rewrite in your own words.
- Video reviews recommended: SimpleNursing (16-min RAAS/ADH), Nurse Mike/Nurse Sarah series.
- Mnemonics
• "Where sodium goes, water flows."
• "Hippo = Hypo (cells swell)."
• Bananas: 3.5–5 bunches ➔ K⁺ normal.
These bullet-point notes integrate definitions, physiology, clinical relevance, and educator anecdotes to replace the original 3-hour lecture while retaining every essential detail for exam prep.
