SURGERY I
UNIT 5: FLUID AND ELECTROLYTE
CTEVT Health Science Second Year | Syllabus 2024
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💧 Introduction to Fluid & Electrolyte Balance
Maintaining fluid and electrolyte balance is fundamental to surgical care. This unit covers the distribution of body fluids, regulatory mechanisms (RAAS, ADH), assessment and management of dehydration, principles of IV fluid therapy, and understanding edema formation. Proper fluid management is crucial for patient recovery and prevention of complications.
⚖️ Normal Distribution & Composition of Body Fluid
💧 Total Body Water (TBW)
of body weight in adults
Example: 70 kg adult → ~42 liters water
📊 Daily Water Balance
Intake (≈2600 ml)
Drinking: 1500 ml
Food: 800 ml
Metabolic: 300 ml
📊 Fluid Compartments
🧬 Intracellular Fluid (ICF)
Volume: 2/3 of TBW (~28L in 70kg)
Main electrolytes: K⁺, Mg²⁺, HPO₄²⁻
Location: Inside cells
🧪 Extracellular Fluid (ECF)
Volume: 1/3 of TBW (~14L in 70kg)
Components: Plasma (3L) + Interstitial (11L)
Main electrolytes: Na⁺, Cl⁻, HCO₃⁻
🧪 Electrolyte Composition of Body Fluids
| Electrolyte | ICF (Intracellular) | ECF (Extracellular) | Normal Serum Level | Major Function |
|---|---|---|---|---|
| Sodium (Na⁺) | Low (10 mEq/L) | High (140 mEq/L) | 135‑145 mEq/L | Osmotic pressure, nerve conduction |
| Potassium (K⁺) | High (140 mEq/L) | Low (4 mEq/L) | 3.5‑5.0 mEq/L | Cardiac rhythm, muscle contraction |
| Chloride (Cl⁻) | Low | High (103 mEq/L) | 98‑106 mEq/L | Fluid balance, acid‑base |
| Bicarbonate (HCO₃⁻) | Low | High (24 mEq/L) | 22‑28 mEq/L | Acid‑base buffer |
🧪 Volume Regulating Hormones
Aldosterone
Source: Adrenal cortex (zona glomerulosa)
Stimulus for Release:
- ↓ Blood volume/pressure
- ↑ Angiotensin II
- ↑ Serum K⁺
Actions:
- ↑ Na⁺ reabsorption (DCT, collecting duct)
- ↑ Water retention (follows Na⁺)
- ↑ K⁺ excretion
- ↑ Blood volume & BP
ADH (Vasopressin)
Source: Posterior pituitary (from hypothalamus)
Stimulus for Release:
- ↑ Plasma osmolality (>290 mOsm/kg)
- ↓ Blood volume (>10% loss)
- Pain, stress, nausea
Actions:
- ↑ Water reabsorption (collecting ducts)
- ↓ Urine output (concentrated urine)
- Vasoconstriction at high doses
- Maintains plasma osmolality
📊 Comparison: Aldosterone vs ADH
| Parameter | Aldosterone | ADH (Vasopressin) |
|---|---|---|
| Primary Stimulus | ↓ Blood volume, ↑ K⁺ | ↑ Plasma osmolality |
| Primary Action | ↑ Na⁺ reabsorption | ↑ Water reabsorption |
| Effect on Urine | ↑ Volume, ↓ Na⁺ conc. | ↓ Volume, ↑ concentration |
| Electrolyte Effect | ↓ K⁺ (hypokalemia) | ↓ Na⁺ (hyponatremia if excess) |
🔄 Renin–Angiotensin–Aldosterone System (RAAS)
RAAS Activation Pathway
Stimulus
↓ BP, ↓ Renal perfusion, ↓ Na⁺
Renin Release
From JG cells (kidneys)
Angiotensin II
Via ACE in lungs
Effects
Vasoconstriction, Aldosterone, ADH
🎯 Actions of Angiotensin II
✓ Vasoconstriction
↑ Systemic vascular resistance → ↑ BP
✓ Aldosterone Release
↑ Na⁺ & water retention
✓ ADH Release
↑ Water reabsorption
✓ Thirst Stimulation
↑ Water intake
📝 Clinical Significance of RAAS
Hypertension: RAAS overactivity → target for ACE inhibitors, ARBs
Heart Failure: RAAS activation increases preload/afterload → worsen failure
Renal Disease: RAAS blockade slows progression (diabetic nephropathy)
Shock: Compensatory mechanism to maintain BP/perfusion
🔄 Hydrostatic & Oncotic Pressure & Edema
⚖️ Starling Forces at Capillary
Hydrostatic Pressure (Pc)
Definition: Pressure exerted by blood against capillary walls
Direction: Pushes fluid OUT of capillary → filtration
Normal values: Arterial end: 35 mmHg, Venous end: 15 mmHg
↑ in: Heart failure, venous obstruction, volume overload
Oncotic Pressure (πc)
Definition: Osmotic pressure due to plasma proteins (mainly albumin)
Direction: Pulls fluid INTO capillary → reabsorption
Normal value: ~25 mmHg
↓ in: Hypoalbuminemia (nephrosis, cirrhosis, malnutrition)
🦵 Edema
🎯 Causes of Edema
↑ Hydrostatic Pressure
- Heart failure
- Venous thrombosis
- Pregnancy
- Prolonged standing
↓ Oncotic Pressure
- Nephrotic syndrome
- Liver cirrhosis
- Malnutrition
- Protein‑losing enteropathy
Other Causes: ↑ Capillary permeability (burns, inflammation), Lymphatic obstruction (filariasis, tumor)
🩺 Examination & Classification
Types:
- Pitting: Depression persists (↑ ECF)
- Non‑pitting: No depression (lymphedema, myxedema)
- Localized: One limb/organ
- Generalized: Anasarca (whole body)
Examination:
- Inspection: Swelling, shiny skin
- Palpation: Pitting test (grade 1‑4)
- Measurement: Limb circumference
- Weight: Daily monitoring
📐 Starling Equation for Fluid Movement
Jv: Net fluid movement
Kf: Filtration coefficient
Pc: Capillary hydrostatic pressure
Pi: Interstitial hydrostatic pressure
πc: Capillary oncotic pressure
πi: Interstitial oncotic pressure
σ: Reflection coefficient
Edema: When Jv > lymphatic drainage
🚑 Dehydration
🎯 Causes & Classification
⚡ Causes of Dehydration
Fluid Loss:
- Diarrhea (most common)
- Vomiting
- Excessive sweating
- Polyuria (DM, DI)
- Burns
Inadequate Intake:
- NPO status
- Dysphagia
- Coma
- Elderly neglect
- Anorexia
📊 Classification of Dehydration
| Severity | Fluid Loss | Key Signs |
|---|---|---|
| Mild | 3‑5% body weight | Thirst, dry mouth |
| Moderate | 6‑9% body weight | ↓ Urine, sunken eyes, poor skin turgor |
| Severe | ≥10% body weight | Hypotension, tachycardia, lethargy, shock |
🩺 Diagnosis & Management
🔍 Diagnosis of Dehydration
Clinical Signs:
- Dry mucous membranes
- Sunken eyes
- Poor skin turgor (>2 sec)
- ↓ Urine output
- Tachycardia, hypotension
Laboratory:
- ↑ Hematocrit
- ↑ BUN:Cr ratio (>20:1)
- ↑ Serum osmolality
- ↑ Urine specific gravity
- ↑ Serum sodium (if hypertonic)
💊 Management of Dehydration
Mild Dehydration
ORS orally: 50‑100 mL/kg over 4 hours
Continue breastfeeding/normal diet
Moderate Dehydration
ORS + IV fluids: 100 mL/kg over 6 hours
Monitor urine output, vital signs
Severe Dehydration
IV fluids urgently: 20 mL/kg bolus NS/RL
Then 100 mL/kg over 6 hours
💉 Parenteral Fluid Replacement Therapy
A. Crystalloid Solutions
Normal Saline (0.9% NaCl)
Composition: Na⁺ 154 mEq/L, Cl⁻ 154 mEq/L
Osmolality: 308 mOsm/L (isotonic)
Indications:
- Hypovolemic shock
- Fluid resuscitation
- Hyponatremia
- Metabolic alkalosis with Cl⁻ loss
Caution: May cause hyperchloremic acidosis, fluid overload
Ringer Lactate (Hartmann’s)
Composition: Na⁺ 130, K⁺ 4, Ca²⁺ 3, Cl⁻ 109, Lactate 28 mEq/L
Osmolality: 273 mOsm/L (slightly hypotonic)
Indications:
- Burns, trauma
- Diarrhea with dehydration
- Mild metabolic acidosis
- Surgical fluid replacement
Contraindications: Liver failure (lactate metabolism), severe alkalosis
Dextrose Solutions
Types: D5W (5% dextrose), D10W, D25W, D50W
Mechanism: Provides free water after dextrose metabolism
Indications:
- Maintenance fluids
- Hypernatremia correction
- Hypoglycemia treatment
- Free water replacement
Caution: May cause hyponatremia, hyperglycemia, phlebitis
B. Colloid Solutions & Maintenance Therapy
Colloid Solutions
Albumin 5%/25%: From human plasma, ↑ oncotic pressure
Use: Burns, hypoalbuminemia, ascites
Dextran/Hetastarch: Synthetic colloids
Use: Volume expansion, prophylaxis for DVT
Risk: Anaphylaxis, renal impairment, bleeding
Maintenance Fluid Therapy
4‑2‑1 Rule (Holliday‑Segar):
- First 10 kg: 4 mL/kg/hr
- Next 10 kg: 2 mL/kg/hr
- Each kg >20: 1 mL/kg/hr
Example: 25 kg child → (10×4)+(10×2)+(5×1)=65 mL/hr
Common Maintenance Fluid:
D5W 0.45% NaCl + 20 mEq KCl/L (for adults)
Adjust for fever, sweating, GI losses
Selection of IV Fluid
Depends on:
- Age: Children need more precise calculation
- Weight: For dose calculation
- Degree of dehydration: Mild/moderate/severe
- Clinical condition: Cardiac/renal status
- Electrolyte imbalances: Na⁺, K⁺, acid‑base
- Type of loss: GI, renal, skin, third‑spacing
⚖️ Acid‑Base Balance (Lungs & Kidneys)
Lungs (Acute Control)
Mechanism: Regulation of CO₂ by changing ventilation
Response time: Minutes to hours
Equation: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
↑ Ventilation: ↓ PaCO₂ → alkalosis
↓ Ventilation: ↑ PaCO₂ → acidosis
Kidneys (Chronic Control)
Mechanism: H⁺ excretion & HCO₃⁻ reabsorption/generation
Response time: Hours to days
Processes:
- HCO₃⁻ reabsorption (proximal tubule)
- H⁺ excretion (distal tubule)
- NH₃ production (glutamine metabolism)
Powerful: Can excrete up to 100‑200 mEq H⁺/day
📄 ONE PAGE SUMMARY
Body Fluid Distribution
- TBW: 60% body weight
- ICF: 40% (K⁺, Mg²⁺, HPO₄²⁻)
- ECF: 20% (Na⁺, Cl⁻, HCO₃⁻)
- Daily balance: 2600 ml in & out
Regulatory Hormones
- Aldosterone: ↑ Na⁺, water; ↓ K⁺
- ADH: ↑ Water reabsorption
- RAAS: Activated by ↓ BP/volume
- Angiotensin II: Vasoconstriction, ↑ Aldo/ADH
Edema & Dehydration
- Edema causes: ↑ Hydrostatic, ↓ Oncotic pressure
- Dehydration: Mild (ORS), Moderate (IV+ORS), Severe (IV bolus)
- Signs: Dry mouth, sunken eyes, ↓ urine, poor turgor
IV Fluids
- Normal saline: Isotonic, for resuscitation
- Ringer lactate: Burns, diarrhea, mild acidosis
- Dextrose: Free water, maintenance
- Maintenance: 4‑2‑1 rule for children
🧠 Memory Aids & Mnemonics
Fluid Distribution “60‑40‑20”
TBW 60% → ICF 40% → ECF 20%
Easy to remember percentages
Electrolyte Distribution “Na‑Out, K‑In”
Na⁺ = ECF (Outside), K⁺ = ICF (Inside)
Sodium outside cells, Potassium inside
RAAS Activation “RAA”
Renin → Angiotensin → Aldosterone
Sequential activation pathway
Maintenance Fluids “4‑2‑1”
First 10kg:4, Next 10kg:2, Each kg >20:1
Holliday‑Segar rule for pediatric fluids
Download Complete Notes
Get a printable PDF with fluid compartments, electrolyte values, IV fluid guides, and management algorithms.
🏷️ Related Topics
Electrolyte Balance
RAAS System
Aldosterone
ADH
Edema
Hydrostatic Pressure
Oncotic Pressure
Dehydration
Oral Rehydration
IV Fluids
Normal Saline
Ringer Lactate
Acid‑Base Balance
Maintenance Fluids
CTEVT Syllabus 2024
Health Science Notes
Surgical Fluids
Fluid Management
Clinical Surgery
© CTEVT Health Science Second Year | HA Surgery I Unit 5 Fluid And Electrolyte Notes.
Source: CTEVT Syllabus 2024 | For academic use only | Essential for surgical patient management