Physiology 209

H₂O

• 45% - 75%
• Key solvent; where metabolic reactions take place

• Variation by body composition
  Skin	70%
  Muscle	75%
  Organ - Heart, liver, brain, kidney	70%-80%
  Fat (adipose tissue)	10%

• Water & solid content are about the same in everyone, but the water percentage varies due to the varying fat mass

• Variation with age and gender
  Baby	75%
  Female : Male (body fat variation)	50% : 60%
  Seniors Female : Male	45% : 50%

• Body water content is greater in infants and males
• Significant for water-soluble medications (final concentration in body)
• In dynamic steady state – individual & environment, and amongst internal components
• Water balance
  • Intake – fluids, foods, oxidative water from metabolism
    • (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy)
  • Output – insensible (lungs, skin), kidneys (balancer), stool
    • Insensible – not noticeable, unavoidable
  • Obligatory vs facultative losses
    • Obligatory – 1.5L/day
      • Insensible: 1.0L
      • Urine + stool: 0.5L
    • Facultative – vary with intake
      • Urine – kidney is major homeostatic organ
  • Insensible perspiration
    • Pure water
    • Passive evaporation (affected by environment)
    • Entire skin surface
    • Continuous
  • Sweating
    • Electrolyte solution
    • Active secretion
    • Sweat glands
    • Activated by heavy work/high temp
• Volume relatively constant – helps maintain solute concentration & blood volume/pressure
• Water Balance
  • Negative water balance
    • Reduced intake
    • Excessive loss from gut
    • Excessive sweating
    • Excessive loss of expired air (dry air)
    • Excessive loss of urine
  • Water ‘intoxication’ (positive water balance)
    • Excessive intake
    • Renal system failure
• Average male is regarded as 70kg
• Body water compartments – 60% of body mass
• Water is freely moving; compartments are not rigidly isolated chambers

Intracellular Fluid (ICF)

• Body water compartments – 60% of body mass
• Aggregate of fluid bound by internal surface of cell membranes

Extracellular Fluid (ECF)

• 1/3 of body water compartments
• Major subcompartments
  • Plasma – fluid medium in which blood cells are suspended
    • 25% of ECF, 5% of total body water compartments
  • Interstitial Fluid (ISF)
    • 75% of ECF, 15% of total body water compartments
    • True ‘Milieu Intérieur’
    • Percolates between individual cells
• Minor subcompartments
  • Lymph
    • Lymphatic system – network of blind - ended terminal tubules
      • Coalesce to form larger lymphatic vessels → converge to large lymphatic ducts which drain into large veins in chest
    • 1 – 2% of ECF
  • Transcellular fluid
    • Aggregate of small fluid volumes secreted by specific cells into a number of body cavities (lined by epithelial cells) & having specialized functions
    • <1 – 2% of ECF
    • Intraocular, cochlear, cerebrospinal, pleural & pericardial, peritoneal, synovial, fluid in ducts of glands, bladder, etc
    • Does not affect body fluid balance; very local; however, has important functions
• Number recap: Total H₂O 60%, ICF 40%, ECF 20%, ISF 15%, Plasma 5% (of total body weight)
• For healthy individuals, total volume & relative distributions between compartments must remain constant (though they are in dynamic equilibrium)
• Hematocrit (Ht) – percentage of blood volume occupied by Red Blood Cells (erythrocytes)
  • Height of erythrocyte column/height of whole blood column
  • Ht = Packed Cell Volume (PCV)
  • Normal value for males = 45%

Methods to Determine Compartment Volumes

• Direct (ie centrifuge & measure)
• Indirect – indicator dilution method
  • #Know: total quantity of test substance, concentration of substance after dispersion
  • #Add quantity Q of indicator to vein → allow time to equilibrate → remove known volume of blood & centrifuge for plasma → measure concentration (c) of substance in plasma → calculate V = Q/c

  • {indicators

    Indicator choice}

    • #Non-toxic, diffuse readily, distribute evenly through compartments to be measured, induce no changes in distribution of water, easy to measure
    • Antipyrine, D₂O, T₂O – for total body water measurements
    • Radioactively labeled Inulin, sucrose, mannitol – for ECF (does not pass cell membrane)
    • Evans’ blue (T1824) or I¹³¹ – Albumin – for plasma (does not pass capillary wall)

Ionic Compositions

• ICF – high in K⁺ & Mg⁺⁺, low in Na⁺ & Cl^-
• ECF – high in Na⁺ and Cl^-, low in K⁺
• 0.9% NaCl – dilute sea water – matches proper concentration
• Artificial physiological solutions – substitute for plasma/ISF (examples, don’t need to memorize)
  • Physiological saline – 9g NaCl + H₂O to make 1L
  • Ringer’s solution – 8.6g NaCl, 0.3g KCl, 0.3g CaCl₂, + H₂O to make 1L
  • Locke-Ringer solution – 9g NaCl, 0.4g KCl, 0.2g CaCl₂, 0.2g MgCl₂, 0.5g NaHCO₃, 0.5g dextrose (simple sugar, energy), + H₂O to make 1L

Units of Concentration

• Amount (mass) of solute: 1g% = 1g solute in 1dL of water
• Number of solute molecules: 1mol = gram molecular weight/1L of H₂O
• Molality (m) – number of moles of solute dissolved in 1Kg solvent
• Molarity (M) – amount of solute in specific amount of solution
• Number of reactive units: Eq = molarity of ion * valency (ie Na⁺ 1Eq/mol, Ca²⁺ 2Eq/mol
• Viability of cells depends on relative constancy of internal environment (milieu intérieur) & requires exchanges with internal & external environment

• Plasma

  Capillary wall

  ISF

  Cell membrane

  ICF

• Important transports: lungs, skin, kidneys, GI tract

Cell Membrane

• Functions – support distinct compositions of ICF & ISF; be selectively permeable
• Highly permeable to H₂O, lipid-soluble substances, dissolved gases (O₂, CO₂), small uncharged molecules
• Less permeable to larger molecules, charged particles
• Impermeable to very large molecules
• Properties due to bimolecular phospholipid layers w/ molecules exhibiting lateral mobility
  • Phospholipid, hydrophilic heads, hydrophobic tails
  • Amphipathic – both hydrophilic & hydrophobic parts
• Cholesterol inserted in phospholipid layer adds stability & rigidity
• Proteins
  • Integral – closely associated with phospholipids, mostly cross the membrane (transmembrane)
  • Peripheral – loosely associated, mostly on cytoplasmic side
• Glycocalyx – carbohydrates & glycoproteins on outer side of membrane; gives distinct identity
• Fluid mosaic model
• Functions of plasma membrane proteins
  • Selective transport channel
  • Enzyme
  • Cell surface receptor
  • Cell surface identity marker
  • Cell adhesion
  • Attachment to cytoskeleton

Transmembrane Transport Pathways

• Via phospholipid bilayer
• Via interaction with protein cluster (channel/carrier)
• Factors – lipid solubility, particle size, electrical charge, available & number of carriers/ion channels
• Passive – energy independent     Active – energy dependent

Diffusion

• Resulting from random thermal molecular motion
• Net flux = high → low concentration
• If permeable, can go through membrane
• J = PA(C₀ – C_i)
  • J – net flux across membrane
  • P – permeability/diffusion coefficient of membrane
  • A – surface area of membrane
  • C₀ – C_i – concentration gradient of diffusing molecule
  • Diffusion time increases in proportion to the square of the distance
• Dissolve through lipid components for non-polar molecules and through channels for ions

Ion Channels

• Consist of single protein or clusters of proteins
• Show selectivity based on diameter & distribution of charges
• Movement affected by electrochemical (electrical & concentration) gradient
• Ligand-gated, voltage-gated, mechanically-gated
• Channels: Na⁺, K⁺, Ca⁺, Cl^-
• Number of ions flowing through channels generating ionic current depends on
  • Channel conductance
  • How often channel opens
  • How long channel stays open

Carrier-Mediated Transport

• Specificity – usually transports one type of molecule only
• Saturation – rate of transport reaches maximum when all binding sites on all transporters are occupied; limit – transport maximum (Tm) exists
• Competition – structurally similar substances compete for same binding site on carrier
• Facilitated diffusion – carrier enables solute to penetrate more readily – passive
  • Solute binds to carrier → changes carrier configuration → solute is delivered to other side of membrane → carrier resumes original configuration
  • Ie for glucose, insulin increases number of transporters
• Active transport
  • Requires chemical energy (usually ATP)
  • Susceptible to metabolic inhibitors
  • Can transport against concentration gradient
  • Primary active transport
    • Chemical energy transferred directly from ATP
    • Phosphorylation of transporter changes conformation & affinity of binding site
    • Ie sodium potassium pump – 3Na⁺ out, 2K⁺ in
• Active – energy dependent – carrier-mediated active transport (primary & secondary), pino/phagocytosis
• Sodium potassium pump
  • Phosphorylation of pump → change in conformation → affinity for Na⁺ decreases
• Other active transporter proteins – Ca-ATPase, H-ATPase, H/K ATPase
• Secondary Active Transport – relies on electrochemical gradient to bring along another solute
  • Ie glucose transported with Na⁺
  • Cotransport – both solutes move in the same direction
    • Na⁺/glucose, Na⁺/amino acids
  • Countertransport – solutes move in different directions
    • Na⁺/Ca⁺, Na⁺/H+, Cl^-/HCO₃^-
• Endocytosis – cell membrane invaginates, forming channel, pinching off to form vesicle
  • Pinocytosis (Fluid endocytosis) – ingestion of dissolved materials, liquid contents slowly transferred to the cytosol – not selective
  • Phagocytosis – ingestion of solid particles, particles pinched into phagocytic vacuole (phagosome) – fuses with lysosomes and then degraded
  • Receptor-mediated
    • Clathrin-dependent receptor-mediated endocytosis
    • Potocytosis
• Exocytosis – diffusion of vesicle, releasing content in ECF

Osmosis – diffusion of water

• Water diffuses freely across most cell membranes
• Aquaporins – groups of proteins facilitating osmosis – forms water permeable channels
• Osmosis – net movement of H2O across semipermeable membrane
• Osmotic pressure – pressure required to oppose the movement of water across a semipermeable membrane
• Osmolarity – total solute concentration of solute – 1 osmol = 1 mol of solute
  • 1 mol glucose = 1 osmol     1 mol NaCl = 2 osmol
• Osm = osmol/liter
• Ie saline drip – 0.9% saline = 0.15 NaCl = 300mOsm = same mOsm of body
  6.7 atm = 5100 mmHg
• Isosmotic – solutions with same number of osmotically active particles
• Hypoosmotic – solutions with lower number of osmotically active particles
• Hyperosmotic – solutions with higher number of osmotically active particles
• Only nonpenetrating particles can exert osmotic pressure
  • Extracellular Na⁺ is nonpenetrating as it is pumped out after moving in
• Isotonic – cell retains size
• Hypotonic – cell swells
• Hypertonic – cell shrinks

Capillaries

• Single layer of flattened endothelial cells & supporting basement membrane
• Diffusion through water filled channels & across cell membranes
• Pinocytosis/exocytosis – endocytosis & vesicle formation on luminal side → exocytosis & vesicle release on interstitial side
• Bulk flow – flow of molecules due to pressure difference (ultra filtration)
• Filtration – bulk flow across porous membrane (acts like sieve) withholding some particles

Blood

• Transport – nutritive, respiratory, excretory, hormone transport, temperature regulation
• Acid-base balance – normal pH range 7.30-7.45
• Protective
• Contains both ECF (plasma) & ICF (inside blood cells)
• May be studied in vivo & in vitro
• Accounts for ~7% of body mass = ~5L
• Normovolemia – normal blood volume
• Hypovolemia – lower blood volume
• Hypervolemia – higher blood volume
• Centrifuged blood – 55% plasma (yellow), ~0% buffy layer (WBCs, platelets), 45% RBCs
• Hematocrit

Plasma

• Similar to ISF
• > 90% water
  • 0.9g/dl NaCl Na⁺, K⁺, (Ca⁺⁺, Mg⁺⁺)     Cl^-, HCO₃^-, (PO₄^–
• Glucose, amino acids, lipids, O₂, CO₂

Plasma Proteins (Colloids)

• (unlike ISF) Proteins (colloids) = 7g% - albumins, globulins, fibrinogen
  • Separating plasma proteins
    • Differential precipitation by salts
    • Sedimentation in ultracentrifuge
    • Electrophoretic mobility
    • Immunological characteristics
• Electrophoresis – fraction method based on movement of charged particles along voltage gradient
  • Influenced by number, distribution of charges, & MW of each protein
  • Stained and scanned – area under graph peaks shows amount
  • In renal disease – albumin (big left peak) gets significantly smaller
  • In bacterial infection – γ globulins increases due to presence of antibodies
• Majority produced in liver
  • Albumin, fibrinogen, α₁, α₂, β globulins
• γ globulins produced in lymphoid tissue