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Homeostasis

Homeostasis of Body Fluids Chapter Notes | Physiology - NEET PG

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

Homeostasis of Body Fluids Chapter Notes | Physiology - NEET PG

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

Homeostasis of Body Fluids Chapter Notes | Physiology - NEET PG

  • 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:

Homeostasis of Body Fluids Chapter Notes | Physiology - NEET PG

  • 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+.
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FAQs on Homeostasis of Body Fluids Chapter Notes - Physiology - NEET PG

1. What is homeostasis and why is it important for body fluids?
Ans.Homeostasis refers to the maintenance of a stable internal environment within the body despite external changes. It is crucial for body fluids because it regulates the balance of electrolytes, temperature, pH, and fluid levels, ensuring optimal functioning of cells and organs.
2. How do control systems function in maintaining homeostasis?
Ans.Control systems in the body consist of sensors, control centers, and effectors. Sensors detect changes in the internal environment, the control center (often the brain) processes this information, and effectors (like muscles or glands) enact changes to restore balance. This feedback loop is essential for maintaining homeostasis.
3. What is the difference between feedforward control and feedback control in homeostasis?
Ans.Feedforward control anticipates changes and initiates responses before the changes occur, helping to prepare the body for upcoming events. In contrast, feedback control responds to changes after they have been detected, adjusting the body’s functions to return to homeostasis. Both are important for effective regulation.
4. How does body composition change with age and what are its implications for fluid balance?
Ans.As individuals age, body composition typically shifts, leading to a decrease in lean body mass and an increase in body fat. This change can affect fluid balance, as lean tissue holds more water than fat. Older adults may experience altered fluid distribution and increased risk of dehydration.
5. What methods are used to measure body fluid levels and what is considered normal fluid balance?
Ans.Body fluid levels can be measured using methods such as bioelectrical impedance analysis, dual-energy X-ray absorptiometry, and various laboratory tests. Normal fluid balance refers to the equilibrium between fluid intake and output, ensuring that the body maintains adequate hydration for physiological functions.
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