Body Fluids & Fluid Balance

Learning Outcomes

• By the end of the lecture you should be able to:

  • Describe the distribution of total body water (TBW) in the body

  • List the ionic composition of different body compartments

  • Explain the principles and indicators used to measure the size of body-fluid compartments

  • Describe the formation of interstitial fluid

  • Discuss the pathophysiology of oedema (edema)

The Body as an Open System

• Continuous exchange of matter & energy with surroundings

  • Inputs: $H<em>2O$, $Na^+$, $K^+$, food, $O</em>2$

  • Outputs: $CO_2$, metabolic waste, heat, water, electrolytes

• Homeostasis requires matching input to output via renal, respiratory, integumentary, GI & endocrine systems

Daily Water Balance: Routes & Regulation

• Insensible loss (lungs): $0.3–0.4\,L\,day^{-1}$

  • ↑ with low atmospheric vapour pressure & ↑ temperature

• Insensible loss (skin): $0.35–0.4\,L\,day^{-1}$

  • ↑ ~$10\times$ after extensive burns (loss of stratum corneum)

• Sweat: $0.1–2\,L\,h^{-1}$ (can approach $14\,L\,day^{-1}$ during strenuous exercise in heat)

  • Regulated by temperature, sympathetic drive, adrenal steroids (alter electrolyte content)

• Faeces: $0.1–0.2\,L\,day^{-1}$; ↑ in diarrhoeal disease

• Urine: normal $0.5–1.4\,L\,day^{-1}$; can range $0.25–20\,L\,day^{-1}$ depending on fluid/electrolyte balance & ADH/aldosterone actions

Factors Affecting Total Body Water (TBW)

• Reciprocal relation with body fat

  • Newborn: $73–80\%$ of BW is water

  • Lean adult male: $\approx 60\%$

  • Adult female: $40–50\%$ (higher adipose fraction)

  • Obesity: ↓ TBW further

  • Elderly: $\approx 45\%$ due to ↑ fat & ↓ muscle mass

Overview of Fluid Compartments

• Conventional two-compartment model (by volume)

  • Intracellular fluid (ICF) ≈ $55\%$ of TBW

  • Extracellular fluid (ECF) ≈ $45\%$ of TBW

• Expanded model recognises sub-compartments of ECF

  • Interstitial fluid (ISF): $\approx 80\%$ of ECF

  • Plasma: $\approx 20\%$ of ECF

  • Transcellular fluids: CSF, ocular, pleural, peritoneal, synovial, digestive secretions (usually <$2–3\%$ of TBW but physiologically specialised)

Graphical Memory Aid

• Pie chart often taught: For a $70\,kg$ man—$42\,L$ TBW

  • $28\,L$ ICF (2⁄3)

  • $14\,L$ ECF (1⁄3) → $11\,L$ ISF + $3\,L$ plasma

Anatomical/Physiological Barriers

• Plasma membrane: separates ICF from ISF; selectively permeable to water, small nonelectrolytes; impermeable to proteins/most ions (unless channels exist)

• Capillary endothelium & basement membrane: separate plasma from ISF; freely permeable to water, ions, small solutes; retains plasma proteins (\pi_c)

Solute Classes

• Electrolytes: dissociate into ions → conduct electricity

  • Cations: $Na^+, K^+, Ca^{2+}, Mg^{2+}, H^+$

  • Anions: $Cl^-, HCO<em>3^-, HPO</em>4^{2-}, SO_4^{2-}, proteins^{-}$

• Non-electrolytes: urea, glucose, lipids, creatinine, O₂, CO₂, proteins (as colloids)

Ionic Composition of Major Compartments (Approx. mmol·L⁻¹)

• Plasma vs Interstitial vs ICF (skeletal muscle shown)

  • Cations

  • $Na^+$: 142 | 145 | 10

  • $K^+$: 4.3 | 4.4 | 140

  • $Ca^{2+}$ (ionised): 2.5 | 2.4 | 1

  • $Mg^{2+}$: 1.1 | 1.1 | 17

  • Anions

  • $Cl^-$: 114 | 117 | 4

  • $HCO_3^-$: 24 | 27 | 7

  • $HPO<em>4^{2-}/H</em>2PO_4^-$: 1 | 1.2 | 40

  • Proteins⁻: 1.5 | 0.1 | 3

  • Plasma is slightly hypertonic to ISF due to proteins (Gibbs-Donnan effect)

Functional Significance of Key Ions

• Sodium ($Na^+$)

  • Primary ECF cation (≈$90\%$ of ECF cations); determines ECF osmolarity & volume; essential for neuromuscular excitability, secondary active transport & acid-base balance.

• Chloride ($Cl^-$)

  • Major ECF anion; moves passively with $Na^+$ through leak channels; shifts with $HCO_3^-$ via “chloride shift” in RBCs; forms $HCl$ in gastric juice.

• Bicarbonate ($HCO_3^-$)

  • 2nd most abundant ECF anion; key buffer; concentration rises in systemic capillaries as $CO_2$ hydrates; regulated by kidney & exchange with $Cl^-$.

• Potassium ($K^+$)

  • Dominant ICF cation (≈$140\,mmol·L^{-1}$); sets resting membrane potential, repolarises action potentials, exchanged with $H^+$ for pH buffering; controlled by aldosterone.

• Magnesium ($Mg^{2+}$)

  • 2nd most common ICF cation; cofactor for >$300$ enzymes (e.g., Na⁺/K⁺-ATPase); required for neuromuscular transmission & myocardial stability.

Na⁺/K⁺ Pump

• Electrogenic ATPase maintains high $K^+<em>{in}$ / high $Na^+</em>{out}$; consumes ≈ $30\%$ of resting ATP; critical for osmotic equilibrium & excitability.

Specialised & Transcellular Fluids

Lymph

• Clear, colourless; $96\%$ water, $4\%$ solids.

  • Proteins (2–6% of solids): albumin, globulin, fibrinogen, antibodies, enzymes

  • Lipids (5–15% solids): chylomicrons & lipoproteins

  • Glucose, urea, creatinine, electrolytes ($Na^+, K^+, Ca^{2+}, Cl^-, HCO_3^-$)

• Functions: returns protein & fluid to blood, drains fats from intestine, removes microbes/foreign bodies, transports lymphocytes, maintains tissue integrity.

Milk (Human)

• Secreted by mammary glands; “complete” neonatal food.

  • Water 83–87%, solids 13–17%

  • Carbohydrate: lactose (yields galactose → structural glycoproteins; fermented to lactic acid → inhibits pathogens)

  • Lipids: triacylglycerols rich in palmitic, myristic, stearic, lauric, butyric & oleic acids

  • Proteins: casein (≈80%), lactalbumin, enzymes, immunoglobulins (passive immunity)

  • Minerals: $Ca^{2+}, Mg^{2+}, PO_4^{3-}, Na^+, K^+, Cl^-$

  • Vitamins: fat- & water-soluble except vitamin C (low)

Cerebrospinal Fluid (CSF)

• Clear, colourless; produced by choroid plexus; $\approx 500\,mL\,day^{-1}$.

  • Total circulating volume $120–150\,mL$; replaced ≈3×⁄day.

• Functions: hydraulic cushion (shock absorber), regulates intracranial pressure, may influence hunger/eating behaviour (hypothalamic access).

Amniotic Fluid (AF)

• Produced by amniotic membrane & fetus; volume increases with gestation.

  • Clear, contains desquamated fetal cells, minimal lipid.

• Functions: physical protection, medium for chemical exchange (nutrients, waste, hormones), permits movement & lung development.

Aqueous Humor

• Fills anterior & posterior chambers of eye; secreted by ciliary body.

  • Obstruction of outflow → ↑intraocular pressure → glaucoma.

  • Posterior cavity filled with vitreous humor (gel of hyaluronic acid from retina).

Sweat

• Secreted by eccrine glands; thermoregulatory.

  • Insensible perspiration: $0.8–1.2\,L\,day^{-1}$

  • Heavy exercise in heat: up to $10–14\,L\,day^{-1}$ → risk of water/electrolyte depletion.

  • Water content 99.2–99.7%; pH 4.7–7.5.

  • Electrolytes: $Na^+ 12.6–127\,mEq·L^{-1}$; $K^+ 5–32\,mEq·L^{-1}$; $Cl^- 8.5–85\,mEq·L^{-1}$.

  • Non-protein nitrogen (urea) 0.07–1% per hour during copious sweating.

  • Controlled by sympathetic cholinergic fibres & adrenal steroids (\uparrow aldosterone → lowers $Na^+$ in sweat).

Tears

• Produced by lacrimal glands; normally isotonic; becomes hypertonic after evaporation over cornea; copious flow remains isotonic.

  • Osmolarity at slow flow ≈$25\,mOsm$ hypertonic.

  • pH 7–7.6 (loss of $CO_2$).

  • Proteins 0.18–0.6 g·dL⁻¹; albumin:globulin ratio 1:5 → 2:1; mucin present.

  • Lysozyme breaks bacterial cell walls (innate immunity).

• Functions: lubrication, optical smoothing of corneal surface, antimicrobial protection, debris removal.

Capillary Fluid Filtration: Starling Forces

• Four primary forces determine net movement across capillary wall:

  1. Capillary hydrostatic pressure $P_c$ (pushes fluid out)

  2. Interstitial hydrostatic pressure $P_i$ (can oppose or favour outflow; usually ~1 mmHg)

  3. Capillary oncotic pressure $\pi_c$ (due to plasma proteins; pulls fluid in; ≈25 mmHg)

  4. Interstitial oncotic pressure $\pi_i$ (proteins in ISF; pulls fluid out; low)

• Net filtration/absorption described by Starling equation: $\text{Fluid\ movement} = K\big[(P<em>c - P</em>i) - (\pi<em>c - \pi</em>i)\big]$

  • $K$ = filtration coefficient (surface area × permeability)

• Typical muscle capillary profile

  • Arteriolar end: $P_c ≈ 37\,mmHg$ → filtration dominates

  • Venular end: $P_c ≈ 17\,mmHg$ → reabsorption dominates

  • Balance: filtration slightly exceeds reabsorption; excess (≈2–4 L·day⁻¹) returned by lymphatics.

Determinants of Interstitial Fluid Volume

• Magnitude of:

  • $P_c$ (raised in venous congestion, heart failure)

  • $P_i$ (may become negative in dehydrated tissues)

  • $\pi_c$ (falls with hypoalbuminaemia)

  • $K$ (↑ with inflammation, burns—greater permeability)

  • Number of perfused capillaries (active tissue ↑)

  • Lymph flow capacity

Oedema (Edema)

• Definition: abnormal, clinically apparent accumulation of fluid in interstitial spaces.

Major Aetiological Categories

1. Increased venous/capillary hydrostatic pressure

   • Congestive heart failure (right > left), deep-vein thrombosis, portal hypertension, effect of gravity (prolonged standing)

2. Decreased plasma oncotic pressure (↓ proteins)

   • Inadequate intake (malnutrition, Kwashiorkor)

   • Malabsorption

   • Impaired synthesis (cirrhosis, liver failure)

   • Excessive renal loss (nephrotic syndrome)

3. Increased capillary permeability

   • Inflammation, allergy (histamine, bradykinin), burns, sepsis

4. Lymphatic obstruction

   • Filariasis, malignancy, surgical lymph-node removal, congenital lymphoedema

Clinical Descriptors

• Generalised (anasarca) vs localised (e.g., pulmonary, cerebral, ascites, pleural effusion)

• Pitting oedema

  • Apply thumb pressure for ~5 s; persistent indentation ≡ pitting; indicates free interstitial water.

• Non-pitting oedema

  • Due to lymphatic obstruction or myxoedema (mucopolysaccharide accumulation).

Measurement of Fluid Compartments (Brief Principles)

• Indicator-dilution technique: $V = {\text{Amount\ administered} - \text{Amount\ lost} \over \text{Concentration\ after\ equilibration}}$

  • TBW: $^{3}H<em>2O$, $D</em>2O$, antipyrine

  • ECF: $^{22}Na^+$, thiosulfate, inulin

  • Plasma: Evans blue, radio-iodinated albumin

  • ISF: ECF – plasma

  • ICF: TBW – ECF

• Significance: allows calculation of shifts in pathology (e.g., dehydration types, SIADH, CHF).

Ethical / Practical Implications

• Recognition & prompt treatment of oedema can prevent organ dysfunction (e.g., pulmonary oedema threatens gas exchange).

• Understanding specialized fluids guides clinical interventions: e.g., lumbar puncture for CSF analysis, paracentesis for ascites, intra-ocular pressure screening for glaucoma.

• Maintenance of adequate hydration in infants/elderly essential due to differing TBW percentages and vulnerability to fluid shifts.

High-Yield Equations & Numbers (Memorise)

• $\text{Fluid\ movement} = K[(P<em>c - P</em>i) - (\pi<em>c - \pi</em>i)]$

• Adult lean male: TBW $\approx 0.6\times BW$; lean female $0.5–0.55\times BW$.

• Normal plasma osmolarity ≈ $285–295\,mOsm·kg^{-1}$ (dominated by $Na^+$ + associated anions).

• Normal lymph flow: $2–4\,L\,day^{-1}$.

• CSF production: $500\,mL\,day^{-1}$; total volume $120–150\,mL$.

Integrative Links to Previous / Future Topics

• Acid-base homeostasis (bicarbonate, phosphate & protein buffers) builds on ionic composition facts.

• Renal physiology: glomerular filtration, tubular reabsorption determine plasma & ECF composition.

• Cardiovascular lectures: Starling forces tie into microcirculation dynamics : ADH, aldosterone, natriuretic peptides modulate fluid compartment& venous return.

• Endocrinology volumes & osmolarity.

• Immunology: lymph composition & flow pivotal for antigen presentation & immune surveillance.