Homeostasis of Body Fluids Chapter Notes | Physiology - NEET PG PDF Download

Homeostasis

Definition: Maintenance of a constant internal environment within cells, essential for cell survival. Every cell resists change to maintain a stable internal environment.

Key Contributors:

• Walter Cannon (1929): Coined the term "homeostasis."
• Claude Bernard (1865): Introduced the concept of the internal environment, emphasizing the role of extracellular fluid (ECF), particularly interstitial fluid (ISF), termed "milieu interieur."

Purpose: Ensures normal internal cell functions by maintaining consistency in the ECF environment.
Control Systems: Detect deviations from normal and make adjustments to restore the desired value. Operated via:
Feedback Control: Output modifies the next action.

• Positive Feedback: Amplifies change (vicious cycle), leading to instability (e.g., blood clotting, ovulation, childbirth, lactation, nerve action potential, cardiac muscle contraction).
• Negative Feedback: Primary homeostatic control; reverses change to restore normalcy (e.g., baroreceptors regulate blood pressure).

Feedforward Control: Anticipates disturbances to prevent changes.

Control Systems

Positive Feedback:

• Mechanism: Amplifies ongoing change, producing more of the accumulating product.
• Outcome: Leads to instability, potentially causing death.
• Examples:
  • Blood clotting.
  • LH surge during ovulation.
  • Uterine contractions (Ferguson reflex).
  • Lactation (suckling).
  • Nerve action potential generation.
  • Sarcoplasmic calcium release in cardiac muscle.

Negative Feedback:

• Mechanism: Returns the controlled variable to normal by counteracting the change.
• Example: Baroreceptors reduce elevated blood pressure.
• Disadvantages:
  • Incomplete compensation.
  • Slow and incomplete responses (persistent error).
  • Excessive feedback may cause instability.

• Loop Gain:
  • Formula: Gain = Correction / Error.
  • Example: If BP rises from 100 mm Hg to 175 mm Hg and corrects to 125 mm Hg:
    • Correction = 175 - 125 = -50 mm Hg.
    • Error = 125 - 100 = +25 mm Hg.
    • Gain = -50 / +25 = -2.

Sample Question (AIIMS May 2016):

• Problem: SBP reduced by 10 mm Hg on standing, regained 8 mm Hg.
• Solution: Correction = +8 mm Hg, Error = -2 mm Hg, Gain = +8 / -2 = -4.

Feedforward Control

Mechanism: Anticipates disturbances, generating corrective commands to prevent changes in the controlled variable.

Advantages:

• Faster response.
• Eliminates steady-state errors.
• Avoids system instability.

Example: Thermoregulation.

• Central Thermoreceptors: Act as feedback sensors; if core temperature drops to 36°C, restore it to 37°C setpoint.
• Peripheral Thermoreceptors (Skin): Act as feedforward sensors; detect environmental temperature drops, initiating responses before core temperature falls.

Other Examples:

• Increased heart and respiratory rate before exercise due to psychic stimulation.
• Rapid body movements controlled by the brain via feedforward (feedback too slow).

Body Composition

Overview: Human body composition at atomic, molecular, and tissue levels.

• Atomic Level:
  • Oxygen: 60%
  • Carbon: 20%
  • Hydrogen: 15%
  • Calcium: 1%
  • Others: 4%
• Molecular Level:
  • Water: 60%
  • Protein: 18%
  • Fat: 15%
  • Mineral: 6%
  • Glycogen: 1%
• Tissue Level:
  • Skeletal muscle: 36%
  • Non-skeletal: 29%
  • Adipose tissue: 25%
  • Bone: 10%

Body Water (as % of Body Weight):

Total Body Water (TBW): 60% (e.g., 42 L in a 70 kg man).

• Intracellular Fluid (ICF): 40% (28 L, 2/3 of TBW).
• Extracellular Fluid (ECF): 20% (14 L, 1/3 of TBW).
  • Interstitial Fluid (ISF): 15% (3/4 of ECF).
  • Plasma: 5% (1/4 of ECF).

Total Blood Volume: 8% (Plasma 5% + Blood cells 3%).

• Formula: Total blood volume = Plasma volume / (1 - Hematocrit).

Transcellular Fluid:

• Specialized ECF (1-2 L, 1-3% body weight).
• Includes: Pericardial (50 mL), Pleural (10-20 mL), Peritoneal (Male: none, Female: 5-20 mL), Synovial (0.5-1.5 mL per large joint, up to 100 mL in osteoarthritis), cerebrospinal, and intraocular fluids.

Mesenchymal Tissue Fluid: In dense connective tissue, cartilage, bones; ~6% of body water.

• Combined with ISF and transcellular fluid, forms 75% of ECF.

Key Notes:

• Females have ~10% less TBW due to higher adipose tissue.
• Infants have higher TBW (brain: 74-80% water, bones: 20% water) than adults, with more ECF (prone to dehydration).
• Maximum ICF is in muscles.
• At puberty, body fluid distribution matches adults, with male-female differences emerging.

Body Fluid Changes with Age

• Infants:
  • Higher ECF:ICF ratio, prone to dehydration.
  • Postnatal diuresis reduces ECF volume; ICF expands with growth.
  • By 3-4 months, ICF = ECF; by 1 year, ICF:ECF approaches adult levels.
• By Age 2: TBW ~60% of body weight.
• Puberty: Adult-like fluid distribution; sex differences appear.
• Table 1.2: Total Body Water by Age:
  • Premature infant: 90%
  • Full-term newborn: 70-80%
  • One year: 64%
  • Puberty to 39 years: Men 64%, Women 52%
  • 40-60 years: Men 55%, Women 47%

  • 60 years: Men 52%, Women 46%

• Tissue Water Content:
  • Blood: 83%
  • Kidney: 82.7%
  • Heart: 79.2%
  • Lung: 79%
  • Spleen: 75.8%
  • Adipose tissue: 10% (least).

Lean Body Mass: 71-72 mL water/100 g tissue, independent of sex.

Measurement of Body Fluid

• Principle: Volume of distribution (indicator dilution).
• Formula: v = (Q - e) / C
  • v = Volume of fluid
  • Q = Quantity of indicator
  • C = Concentration of indicator
  • e = Indicator lost or metabolized

Indicators:

• Plasma Volume: Evans’ blue (T1824), 125I-albumin.
• RBC Volume: 51Cr, 59Fe, 32P, antigenic tagging.
• Total Blood Volume: Plasma volume / (1 - Hematocrit).
• ECF Volume: Inulin (most accurate), sucrose, mannitol, sodium thiosulphate, sodium thiocyanate, 22Na, 125I-iothalamate.
• Interstitial Fluid: ECF volume - Plasma volume.
• ICF: Total body water - ECF.
• Total Body Water: D2O (most frequent), tritium oxide, antipyrine.

Normal Fluid Balance

Daily Intake: 2300 mL (2100 mL ingested fluids + 200 mL from metabolism).

Daily Loss:

• Insensible: 700 mL (skin 350 mL, lungs 350 mL).
• Sweat: 100 mL.
• Feces: 100 mL.
• Urine: 1400 mL.

Minimum Water Requirement: 1.5 L/day (adult).

Shifts of Water Between Compartments

Darrow-Yannet Diagram:

• Key Principles:
  • Water moves rapidly across cell membranes; ICF and ECF osmolarities are nearly equal.
  • Cell membranes are impermeable to many solutes; osmoles remain constant unless added/lost from ECF.
• Axes:
  • Y-axis: Osmolality (solute concentration).
  • X-axis: Volume (ICF 2/3, ECF 1/3).
• Normal State: Equal osmolality in ICF and ECF.
• Changes:
  • ECF volume/osmolality changes based on solution type (isotonic, hypotonic, hypertonic).
  • ICF volume varies with ECF osmolality; ICF osmolality changes inversely to volume.

Scenarios:

• Gain of Isotonic Fluid (B): Increases ECF volume, no change in osmolality or ICF volume (e.g., isotonic NaCl infusion).
• Loss of Isotonic Fluid (C): Decreases ECF volume, no change in osmolality or ICF volume (e.g., hemorrhage, diarrhea, vomiting).
• Gain of Hypotonic Fluid (D): Increases ECF volume, decreases ECF osmolality, increases ICF volume (e.g., SIADH, tap water drinking).
• Loss of Hypotonic Fluid (E): Decreases ECF volume, increases ECF osmolality, decreases ICF volume (e.g., sweating, diabetes insipidus).
• Gain of Hypertonic Fluid (F): Increases ECF volume and osmolality, decreases ICF volume (e.g., excessive NaCl, mannitol).
• Loss of Hypertonic Fluid (G): Decreases ECF volume and osmolality, increases ICF volume (e.g., adrenocortical insufficiency).

Clinical Problem:

Loss of 1 L:

• Water (Hypotonic): ICF 667 mL, ECF 333 mL.
• Isotonic NaCl: ECF 1000 mL, ICF 0 mL.
• Half-Isotonic NaCl: ECF 667 mL, ICF 333 mL.

Ionic Composition

Ionic Concentrations (mOsm/L):

• ICF:
  • K⁺: 140 (max cation)
  • Na⁺: 14
  • Cl^-: 4
  • HCO₃^-: 10
  • HPO₄, H₂PO₄: 11 (max anion)
  • Protein: 4
  • Ca²⁺: <0.0002
  • Mg²⁺: 20
  • Osmolar activity: 281
  • Osmotic pressure (37°C): 5423 mm Hg
• ISF:
  • K⁺: 4
  • Na⁺: 139
  • Cl^-: 108 (max anion)
  • HCO₃^-: 28
  • HPO₄, H₂PO₄: 2
  • Protein: 0.2
  • Ca²⁺: 1.2
  • Mg²⁺: 0.7
  • Osmolar activity: 281
  • Osmotic pressure: 5423 mm Hg
• Blood (ECF):
  • K⁺: 4.2
  • Na⁺: 145 (max cation)
  • Cl^-: 108 (max anion)
  • HCO₃^-: 24
  • HPO₄, H₂PO₄: 2
  • Protein: 1.2
  • Ca²⁺: 1.8
  • Mg²⁺: 1.5
  • Osmolar activity: 282
  • Osmotic pressure: 5443 mm Hg

Key Notes:

• Maximum Ca²⁺ concentration gradient (ECF:ICF ~12000:1).
• Anion differences due to intracellular molecules impermeable to cell membranes.
• Cation distribution (Na⁺, K⁺) driven by Na⁺-K⁺-ATPase pump.

Exchangeable Solutes:

• K⁺: 100% exchangeable.
• Na⁺: 65-70% exchangeable.
• Ca²⁺, Mg²⁺: Mostly nonexchangeable.

Body Content (70 kg adult):

• Na⁺: 3500-5000 mEq (~58 mEq/kg), 90% ECF, 10% ICF, 4% turnover.
• K⁺: 3000-3750 mEq (~53 mEq/kg), 98% ICF, 2% ECF, 2.3% turnover.
• Ca²⁺: 60,000 mEq, 0.01% turnover.
• Mg²⁺: 2000 mEq, 0.5% turnover.
• Phosphate: 18,000 mMol, 0.17% turnover.

Units of Measurement

Moles:

• 1 mole = molecular weight in grams, contains 6 × 10²³ molecules.
• Millimole (mmol) = 1/1000 mole, Micromole (µmol) = 1/1,000,000 mole.
• Example: 1 mol NaCl = 58.5 g (Na 23 g + Cl 35.5 g), 1 mmol = 58.5 mg.

Equivalents (Eq):

• 1 Eq = 1 mol / valence.
• Example: 1 mol NaCl = 1 Eq Na+ (23 g) + 1 Eq Cl- (35.5 g); 1 Eq Ca2+ = 40 g / 2 = 20 g.

Osmole:

• Measures osmotically active particles.
• 1 Osmole = Mol / Number of particles per molecule in solution.

Examples:

• 1 mol NaCl = 2 osmoles (1 Na⁺, 1 Cl^-).
• 1 mol Na₂SO₄ = 3 osmoles (2 Na⁺, 1 SO₄).
• 1 mol CaCl₂ = 3 osmoles (1 Ca²⁺, 2 Cl^-).

Formulas:

• mOsmol/L = [Weight (g/L) / Molecular weight (g)] × number of species × 1000.
• mEq = (mg × valence) / Molecular weight.

Example A: Convert 10 mg% Ca²⁺ to mEq/L and mOsmol/L.

• 10 mg% = 100 mg/L.
• mEq/L = (100 × 2) / 40 = 5 mEq/L.
• mOsmol/L = [0.1 / 40] × 1 × 1000 = 2.5 mOsmol/L.

Example B: mEq of 1.5 g KCl.

• Molecular weight KCl = 74.5.
• mEq = (1500 × 1) / 74.5 = 20 mEq/L.

Osmolality and Osmotic Pressure

Relation:

• At 37°C, 1 osmole/L = 19,300 mm Hg (25.4 atm).
• 1 mOsmol/L = 19.3 mm Hg.
• Body fluids (~300 mOsmol/L) = 5790 mm Hg osmotic pressure.

Tonicity:

• Osmolality of a solution relative to plasma osmolality.
• Examples: 0.9% NaCl (isotonic), 5% glucose (initially isotonic, later hypotonic).

Plasma Oncotic Pressure: 28 mm Hg (19 mm from proteins, 9 mm from Donnan effect).

Plasma Osmolality Calculation

Calculated Osmolality (CO):

• When Na, K in mEq/L or mmol/L, Glucose, BUN in mg/dL:
  • Plasma Osmolality = 2(Na + K) + Glucose/18 + BUN/2.8
  • Or: 2(Na) + Glucose/18 + BUN/2.8
• When all in mmol/L:
  • Plasma Osmolality = 2(Na + K) + Glucose + BUN
  • Or: 2(Na) + Glucose + BUN

Measured Osmolality (MO):

• Most accurate via freezing point depression.

Osmolal Gap:

• Difference between CO and MO.
• Indicates foreign substances (e.g., ethanol, methanol).
• Modified formula (with ethanol/mannitol):
  • Plasma Osmolality = 2(Na) + Glucose/18 + BUN/2.8 + Ethanol/3.7 + Mannitol/18

Normal Osmolality: 280-290 mOsm/kg.

• Na⁺, Cl^-, HCO₃^-: 270 mOsm.
• Glucose, urea: 20 mOsm.
• Plasma proteins (70 g/L): 2 mOsm.

Best Equation: Zander’s optimized equation (not commonly used).

Osmolal Gap: Normal within 10 mOsm/kg.

Intravenous Fluid Solutions

Fluid Summary:

Dextrose:

• 5%: Isotonic (physiologically hypotonic), 278 mOsm/kg, 50 g/L, 170 Cal/L.
• 10%: Hypertonic, 556 mOsm/kg, 100 g/L, 340 Cal/L.

Saline:

• 0.45%: Hypotonic, 154 mOsm/kg, 0 Cal.
• 0.9%: Isotonic, 308 mOsm/kg, 0 Cal, safe with blood products.
• 3%: Hypertonic, 1026 mOsm/kg, 0 Cal.

Dextrose in Saline:

• 5% in 0.225%: Isotonic, 355 mOsm/kg, 50 g/L, 170 Cal/L.
• 5% in 0.45%: Hypertonic, 432 mOsm/kg, 50 g/L, 170 Cal/L.
• 5% in 0.9%: Hypertonic, 586 mOsm/kg, 50 g/L, 170 Cal/L.

Multiple Electrolytes:

• Ringer’s: Isotonic, 309 mOsm/kg, 0 Cal, no sodium lactate.
• Lactated Ringer’s (Hartmann’s): Isotonic, 274 mOsm/kg, 0 Cal, similar to plasma, no Mg²⁺.