8 Water Homeostasis Flashcards Preview

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Flashcards in 8 Water Homeostasis Deck (24):
1

Homeostasis

  • Ability of the body to maintain a constant internal environment depsite fluctuations in diet, fluid intake, & other environmental conditions
    • Maintain a constant volume, stable electrolyte compositoin of body fluids, etc.
  • Kidney
    • Main organ responsible for homeostasis
    • Regulates water balance

2

Normal water content & distribution

  • Total body water (TBW)
  • TBW distribution

  • Total body water (TBW)
    • 60% of body weight in adult men
    • 50% of body weight in adult women due to higher proportion of adipose tissue
    • Decreases w/ age
  • TBW distribution (70kg adult man: 42L)
    • Intracellular fluid (ICF): 2/3 (28L)
    • Extracellular fluid (ECF): 1/3 (14L)
      • Interstitial fluid (ITF): 3/4 (10.5L)
      • Intravascular fluid (IVF): 1/3 (3.5L)

3

Water balance

  • Water balance
  • Water is added to the body via 3 major sources
  • Water is lost from the body via 4 major sources

  • Water balance
    • Water input = water output
  • Water is added to the body via 3 major sources
    • Ingested liquids
      • Ex. pure water
    • Ingested solid food
      • Ex. peanut butter (low content) & watermelon (high content)
    • Synthesized in the body from oxidized carbohydrates
      • Ex. metabolic water
  • Water is lost from the body via 4 major sources
    • Urine (most important)
      • Low urine volume (ex. dehydration, 0.5 L/day): need to retain water
      • High urine volume (ex. drink a lot of water, 18 L/day): need to excrete water
    • Sweat
      • Varies w/ physical activity & environmental temperature
    • Feces
      • Normally: small amt lost
      • Can be increased to several liters w/ severe diarrhea
    • Insensible losses
      • Evaporation in the respiratory tract & skin
      • Neither perceived nor directly measured

4

Osmolarity, osmolality, & tonicity

  • Osmole
  • Osmolarity
  • Osmolality
  • Tonicity
  • Both osmolality & tonicity

  • Osmole
    • # of moles of a compound that contribute to osmotic pressure
  • Osmolarity
    • # osmoles (mmol) / volume of solution (L)
    • Difficult to determine b/c volume changes w/ temperature & pressure
  • Osmolality
    • # osmoles (mmol) / weight of water (kg)
    • Easier to determine b/c amt of solvent remains constant despite changes in temperature & pressure
  • Tonicity
    • # particles in sol'n that can't cross a semi-permeable membrane
    • # particles that exert an osmotic effect driving water out of cells
  • Both osmolality & tonicity
    • Refer to the # of particles in 2 soln's separated by a semi-permeable membrane

5

Plasma tonicity

  • Main osmoles in plasma
  • Plasma osmolality (Posm) calculation
  • Normal plasma osmolality
  • Plasma tonicity (Pton) calculation
  • Normal plasma tonicity

  • Main osmoles in plasma
    • Na
    • Glucose
    • Urea (BUN)
  • Plasma osmolality (Posm) calculation
    • Posm (mmol/kg) = 2*[Na] (mEq/L) + serum glucose/18 (mg/dl) + BUN/2.8 (mg/dl)
    • Double Na for negative ions associated w/ na that contribute to plasma osmolality
    • Divide serum glucose by 18 & BUN by 2.8 for units
  • Normal plasma osmolality
    • 280-295 mOsm/kg
  • Plasma tonicity (Pton) calculation
    • Pton (mmol/L) = 2*[Na] (mEq/L) + serum glucose/18 (mg/dl)
    • Exclude BUN b/c urea freely crosses cell membranes & doesn't contribute to plasma tonicity
  • Normal plasma tonicity
    • 270-285 mOsm/kg

6

Plasma tonicity

  • Main determinant of plasma tonicity
  • Plasma tonicity vs. TBW
  • Plasma tonicity vs. water shifts among dif body fluid compartments

  • Main determinant of plasma tonicity
    • Na conc
    • Decrease Na conc --> plasma becomes hypotonic
    • Increase Na conc --> plasma becomes hypertonic
  • Plasma tonicity vs. TBW
    • Decrease TBW --> water deficit concentrates osmotic particles in plasma --> increase plasma tonicity --> hypertonic plasma
    • Increase TBW --> excess water dilute osmotic particles in plasma --> decrease plasma tonicity --> hypotonic plasma
  • Plasma tonicity vs. water shifts among dif body fluid compartments
    • Hypotonic plasma --> water flows from ECF to ICF --> cell swells
    • Hypertonic plasma --> water flows from ICF to ECF --> cell shrinks
    • Isotonic plasma --> no net water shift --> cell volume remains unchanged

7

Osmoreceptors

  • Osmoreceptors
  • Central osmoreceptors
    • Function
    • Location
    • Main osmoreceptor
    • Special group of structures
  • Peripheral osmoreceptors
    • Function
    • Location
    •  

  • Osmoreceptors
    • Sense changes in plasma tonicity b/c we don't have water sensors
      • Senses water imbalance since many clinical disorders can arise as a consequence of water excess or deficit
    • Activate homeostatic response to small changes in TBW
  • Central osmoreceptors
    • Sense changes in plasma tonicity
    • Located in the CNS
    • Organum vasculosum of lamina terminalis (OVLT)
      • Main CNS osmoreceptor
      • Part of circumventricular organs
    • Circumventricular organs
      • Lack BBB & are in direct contact w/ the ECF & plasma
      • Readily sense minimal changes in its composition (e.g. changes in plasma tonicity)
  • Peripheral osmoreceptors
    • Anticipate osmotic loads
      • Sense osmotic loads in food & trigger a response before actual changes in plasma tonicity occur
    • Located in peripheral structures
      • Pharynx
      • Esophagus
      • GI tract
      • Portal vein
      • Splanchnic mesentery

8

Homeostatic response to TBW deficit

  • Decrease TBW
    • Water is continuously lost in sweat, stool, etc.
    • If not replaced by drinking water, plasma tonicity will increase
  • Osmoreceptors in the OVLT sense increased plasma tonicity
    • Stimulates thirst
      • Drives drinking behavior
      • Increases water intake
    • Stimulates ADH release by the posterior pituitary
      • ADH --> kidneys
      • Increases water permeability of principal cells in the collecting duct
      • Increased water reabsorption --> decreased urine volume & increased urine osmolality --> concentrated urine
  • Increased thirst + increased water reabsorption in the kidney --> increased TBW back to normal --> decreased plasma tonicity back to normal

9

Homeostatic response to TBW excess

  • Increase TBW
    • Drink water --> water is distributed throughout the body
    • Amt of solute doesn't change
    • Excess water --> dilute solutes --> decrease plasma tonicity
  • Osmoreceptors in the OVLT sense decreased plasma tonicity
    • Inhibits thirst
      • Suppresses water drinking
      • Decreases water intake
    • Inhibits ADH release by the posterior pituitary
      • Decreased ADH delivery to kidneys
      • Decreased water permeability of principal cells in the collecting duct
      • Decreased water reabsorption --> increased urine volume & decreased urine osmolality --> diluted urine
  • Decreased thirst + decreaed water reabsorption in the kidney --> decreased TBW back to normal --> increased plasma tonicity back to normal

10

Antidiuretic hormone (ADH) / vasopressin

  • ADH synthesis
  • ADH storage
  • ADH release vs. plasma tonicity
  • ADH release vs. effective arterial blood volume (EABV)
    • EABV
    • Baroreceptors
    • Increase EABV -->
    • Decrease EABV -->
  • Set point for ADH release

  • ADH synthesis
    • Supraoptic & paraventricular nuclei in the hypothalamus
  • ADH storage
    • ADH is transported down axons from the hypothalamus to the posterior hypophysis where its stored in nerve terminals
  • ADH release vs. plasma tonicity
    • Increased plasma tonicity --> ADH release into the circulation
      • Plasma osmolality threshold: 280-285 mOsm/kg
    • Decreased plasma tonicity --> inhibits ADH release
  • ADH release vs. effective arterial blood volume (EABV)
    • EABV
      • Arterial blood volume that effectively perfuses organs
      • Can't be directly measured
        • Inferred from other physiological measurements (renin, aldo, urine Na, etc.)
    • Baroreceptors
      • Stretch-sensitive receptors in carotid & aortic sinus sense changes in EABV
      • Potent stimulus for ADH release even when plasma tonicity is decreased
    • Increase EABV --> neural impulses inhibit ADH release
    • Decrease EABV --> decrease discharge rate of stretch receptors --> ADH release
  • Set point for ADH release
    • As blood volume decreases (ex. hemorrhage), rates of ADH secretion increase
    • Set poitn for ADH release shifts to a lower plasma osmolality

11

ADH effect on distant organs

  • ADH effect on aquaporins
  • Main determinant of water flow through principal cells
  • ADH effect on Na/K/2Cl
  • ADH effect on urea
  • ADH effect on blood vessels

  • ADH effect on aquaporins
    • ADH binds to V2 receptors in the basolateral membrane of princiapl cells in the CD
      • Receptor is coupled to adenylylcyclase through G stimulatory protein (Gs)
    • Bindings increases intracellular cAMP --> activates protein kinase A (PKA) --> increases # of aquaporin 2 water (AQP2) channels in the apical membrane of principal cells
      • Allows water to move from tubules into principal cells
    • Water exits princiapl cells via AQP3 & AQP4 channels on the basolateral membrane
      • Not dependent on ADH
  • Main determinant of water flow through principal cells
    • Hyeprtonic medullary interstitium
    • Water: tubules --> principal cells --> medullary interstitium by osmosis
      • Driven by high tonicity of the renal medulla
  • ADH effect on Na/K/2Cl
    • ADH increases Na/K/2Cl activity in the TkAL
    • Increases NaCl transport into the medullary interstitium
    • Contributes to medullayr ypertonicity
  • ADH effect on urea
    • ADH increases expression of urea transporters (UTA1) --> increases permeability of the inner medullary CD to urea
    • ADH increases transport of urea into medullary interstitium & contribues to medullary hyeprtonicity
  • ADH effect on blood vessels
    • ADH binds to V1 receptors in smooth muscl ecells --> vasoconstriction

12

Thirst

  • Originates in...
  • Triggered by...
  • Thirst vs. age

  • Originates in thirst centers int eh brain
  • Triggered by an increase in plasma tonicity of as little as 2-3%
    • Plasma osmolality threshold = 295 mOsm/kg
  • Ability to feel thirsty decrease w/ age

13

Renal water handling: absence vs. presence of ADH

14

Renal water handling: proximal tubule

  • GFR
  • PT water reabsorption
  • PT permeability to water
  • Accumulation of fluid & solutes within the renal interstitium
  • Osmolality of reabsorbed fluid

  • GFR = 125 ml plasma / min (93% = water)
  • PT reabsorbes 65-80% of filtered water
    • PT reabsorbs Na & other solutes from tubular fluid into lateral intercellular spaces
    • Decreases the osmolality of tubular fluid
    • Increases the osmolality of hte lateral intercellular space
  • PT is highly permeable to water
    • Expresses AQP1 channels in the apical & basolateral membranes
    • Water is reabsorbed transcellularly by osmosis due to higher osmolality of fluid in the lateral intercellular space 9renal interstitium) than tubular fluid
    • Some water is also reabsorbed paracellularly
  • Accumulation of fluid & solutes within the renal interstitium
    • Increases hydrostatic pressure in this compartment
    • Forces fluid & solutes into peritubular capillaries
  • Osmolality of reabsorbed fluid
    • Reabsorbed fluid is iso-osmotic to plasma

15

Renal water handling: proximal tubule

  • Net water reabsorption force in the renal interstitium
  • Pc
  • During hypovolemia
  • πc

  • Net water reabsorption force in the renal interstitium
    • Filtration = Kf [ (Pc - Pi) - (πc - πi) ]
    • Kf = filtration coefficient
    • Pc = peritubular capillary hydrostatic pressure
    • Pi = renal interstitium hydrostatic pressure
    • πc = peritubular capillary oncotic pressure
    • πi = renal itnerstitium oncotic pressure
  • Pc
    • Determined by arteiral BP & vasuclar resistance in AffAs & EffAs
    • Increase arterial BP --> increase Pc --> decrease water reabsorption in PT
    • Increase vascular resistance in AffAs & EffAs --> decrease Pc --> increase water reabsorption in PT
  • During hypovolemia
    • Decrease arterial BP --> decrease Pc 
    • Release AII --> AffA & EffA constriction --> decrease Pc
    • Net effect: increase water reabsorption in Pt
  • πc
    • Determiend by serum albumin conc & FF
    • Increase serum albumin --> increase πc --> increase water reabsorption in PT
    • Increase FF --> more plasma filtered through glomerulus --> more concentrated albumin --> increase water reabsorption in PT

16

Normal hydrostatic & oncotic forces that determine water reabsorption by peritubular capillaries

  • Normal hydrostatic pressure in peritubular capillaries vs. renal interstitium
  • Normal oncotic pressure in peritubular capillaries vs. renal interstitium
  • Net result

  • Normal hydrostatic pressure in peritubular capillaries vs. renal interstitium
    • Peritubular capillaries: 13 mmHg
    • Renal interstitium: 6 mmHg
    • Net result: hydrostatic pressure gradient of 7 mmHg opposes water reabsorption
  • Normal oncotic pressure in peritubular capillaries vs. renal interstitium
    • Peritubular capillaries: 32 mmHg
    • Renal interstitium: 15 mmHg
    • Net result: oncotic pressure gradient of 17 mmHg favors water reabsorption
  • Net result
    • Net oncotic forces that favor reabsorption (17 mmHg) - net hydrostatic forces that oppose water reabsorption (7 mmHg) = net water reabsorption (10 mmHg)

17

Renal water handling: water permeability

  • LOH
    • TDL
    • TnAL
    • TkAL
  • DCT
  • CD

  • LOH
    • TDL: has AQP1 & reabsorbs 15% of filtered water
      • NaCl transporter: absent
      • NaCl permeability: no
      • Osmolality: increase
    • TnAL: lacks AQP1 & is impermeable to water 
      • NaCl transporter: Cl channel, Na channel?
      • NaCl permeability: yes
      • Osmolality: decrease
    • TkAL: lacks AQP1 & is impermeable to water
      • NaCl transporter: Na/K/2Cl co-transporter
      • NaCl permeability: yes
      • Osomlality: decrease
  • DCT
    • Lacks AQP1 & is impermeable to water
  • CD
    • Segments: cortical, outer medullayr, & inner medullary CD
    • Absence of ADH: impermeable to water
    • Presence of ADH: permeable to water

18

Countercurrent multiplication system

  • Corticopapillary osmotic gradient
  • Histological structure responsible for generating the corticopapillary osmotic gradient
  • 4 steps in the countercurrent multiplication process
    • Step 1: rest
    • Step 2: TnAl & TkAL NaCl transport
    • Step 3: TDL water transport
    • Step 4: additional fluid transport
  • Net effect

  • Corticopapillary osmotic gradient
    • Gradient of osmolarity in the interstitial fluid from the cortex to the tip of the papilla
    • Interstitial osmolarity increases from cortex --> outer medulla --> inner medulla --> papilla
  • Histological structure responsible for generating the corticopapillary osmotic gradient
    • LOH
  • 4 steps in the countercurrent multiplication process
    • Step 1
      • Tubular fluid in the LOH & medullary interstitum are isotonic (osmolarity = 300 mOsm/L)
    • Step 2
      • TnAL & TkAL transport NaCl from tubule --> medullary interstitium until interstitium is 200 mOsm/L (more concentrated than in tubule)
        • TnAL: passive process
          • Cl transported transcellularly via Cl channel & paracellularly via following Na
          • NaCl --> down conc gradient
          • Urea: inner medullary CD --> medullary interstitium
            • Reduces inner medullary interstitial NaCl conc --> establishes gradient
        • TkAL: active process
          • Via Na/K/2Cl transporter
          • Na --> down conc gradient from Na/K ATPase
      • Water impermeability --> salt leaves w/o water osmotically following
    • Step 3
      • Water follows the osmotic gradient out of the TDL until the osmolarities in the TDL & medullary interstitium become equal (400 mOsm/L)
      • TDL is impermeable to NaCl
    • Step 4
      • Additional fluid: PT (osmolarity = 300 mosm/L) --> TDL
      • Causes hyperosmotic fluid (400 mOsm/L): TDL --> TnAL & TkAL
  • Net effect
    • Add more solute to medulla --> increase conc gradient --> increase medullary interstitium osmolarity to 1200 mOsm/L
    • Gradually trap solutes in medulla

19

Urea recycling

  • Urea recyling contributes to...
  • Urea contributes to...
  • Plasma urea filration & reabsorption under normal GFR
  • Urea reabsorption in the PT
  • Urea conc in the TDL
  • Urea conc in the TnAL
  • Urea conc in the TkAL & DCT
  • Urea conc int he CCD, OMCD, & IMCD in the presence of ADH
  • Cycle continuation

  • Urea recyling contributes to...
    • Generation of the corticopapillary osmotic gradient
  • Urea contributes to...
    • 50% of the medullary interstitium osmolality when hte kidney forms max concentrated urine in the presence of ADH
    • Esp true in the IMCD where osmolality reaches 1200 mOsm/L
  • Plasma urea filration & reabsorption under normal GFR
    • All plasma urea is filtered
  • Urea reabsorption in the PT
    • 50% is reabsorbed
  • Urea conc in the TDL
    • Conc increases due to water reabsorption & urea secretion into the TDL via UT-A2 urea transporters
  • Urea conc in the TnAL
    • Conc increases due to urea secretion into the TnAL
  • Urea conc in the TkAL & DCT
    • Unchanged b/c tubules are impermeable to water
  • Urea conc int he CCD, OMCD, & IMCD in the presence of ADH
    • CCD & OMCD are permeable to water but impermeable to urea
      • Urea conc increases due to water reabsorption
    • IMCD is permeable to urea
      • High urea conc when fluid reaches IMCD
      • Urea is tranpsored out via UT-A1 urea transporters
      • Favors urea diffusion into the medullayr interstitium
  • Cycle continuation
    • A moderate share of the urea that moves into the interstitium eventually diffuses into the DTL and ATL,

20

Countercurrent exchanger

  • Purpose
  • Histological structure responsible for the countercurrent exchanger
  • Blood flow
  • Energy: active vs. passive process
  • Osmolality
    • Blood entering TDL
    • As blood descends...
    • Bend of vasa recta
    • As blood ascends...

  • Purpose
    • Maintain the medullayr gradient by mitigating washout
  • Histological structure responsible for the countercurrent exchanger
    • Vasa recta: capillaries that serve the medulla
    • Branch off EffA of juxtamedullary nephrons
    • Follow the LOH
  • Blood flow
    • RBF = 20% * CO
    • Medulla receives 5% of total RBF
    • Blood flow through the vasa recta is slow & has a low oxygen conc
  • Energy: active vs. passive process
    • Purely passive so no energy is spent
    • Contrasts countercurrent multiplication: energy is spent
  • Osmolality
    • Blood entering TDL: 300 mOsm/L
    • As blood descends, it's exposed to interstitial fluid w/ increased osmolality
      • Capillaries are freely permeable to water & solutes, so small solutes (ex. NaCl, urea) diffuse from the medulla --> descending capillary limb
      • Allows blood to equilibrate osmotically
    • Bend of vasa recta: osmolality = interstitial fluid = 1200 mOsm/L
      • Hairpin config doesn't allow medullary gradient to be washed out by solute leaving & water coming
    • As blood ascends the TnAL & TkAL, blood is exposed to interstitial fluid w/ decreasing osmolality
      • Solutes diffuse out into the interstitium & water diffuses into the TALs
      • Blood in the ascending limb equilibrates w/ surrounding interstitial fluid

21

Renal water handling during TBW deficit

  • Main job of kidneys during TBW deficit
  • PT
  • TDL
  • TnAL
  • TkAL
  • DCT
  • CD

  • Main job of kidneys during TBW deficit
    • Produce concentrated urine to maximize water reabsorption & preserve water balance
  • PT
    • Osmolality of glomerular filtrate = blood (300 mOsm/L0
      • Osmolarity remains at 300 mOsm/L b/c water is reabsorbed in proportion to solutes (iso-osmotic)
    • TBW deficit + total body Na deficit (hypovolemia): increase % of water reabsorbed (90%)
  • TDL
    • Permeable to water, impermeable to NaCl
    • Water is reabsorbed following the osmotic gradient of th ehypertonic medulla
    • As water --> medulla, tubular fluid osmolality increases & equilibrates w/ the osmolality of the medulla at the hairpin of the LOH (1200 mOsm/L)
  • TnAL
    • Impermeable to water so no osmotic equilibrium
    • Permeable to NaCl: passive process
      • NaCl conc in the medullayr interstitium has been diluted by water from the TDL
      • NaCl conc in TnAL > medulla
      • NaCl moves passivley down conc gradient --> decrease TnAL osmolality
  • TkAL (aka medullayr diluting segment, aka concentrating segment)
    • Impermeable to water
    • Permeable to NaCl: active process
      • NaCl is reabsorbed down its conc gradient into the medulla via the N/K/2Cl co-transporter
        • Gradient is produced by Na/K ATPase actively pumping Na out of the cell
        • ADh stimulates Na/K/2CL to contribute to the increasing gradient
      • osmolality of fluid leaving this segment = 100 mOsm/L
  • DCT (aka cortical diluting segment)
    • Impermeable to water
    • Permeable to NaCl
    • Osmolality of tubular fluid decreases to 80 mOsm/L
  • CD
    • Permeable to water only in the presence of ADH
      • As tubular fluids flow down, it's exposed to interstitial fluid w/ increasingly higher osmolality (corticopapillary osmotic gradient)
      • Water will be reabsorbed until tubular fluid equilibrates osmotically w/ surrounding medullary interstitial fluid
      • Final urine osmolality at the tip of the papilla = 1200 mOsm/L
    • ADH also increases urea permeability in the IMCD to facilitate urea transport
      • Urea contributes to the medullayr gradient in the inner medulla

22

Renal water handling during TBW excess

  • Main job of kidneys during TBW excess
  • PT
  • TDL
  • TnAL
  • TkAL
  • DCT
  • CD

  • Main job of kidneys during TBW deficit
    • Produce diluted urine to maximize water excretion & preserve water balance
  • PT
    • Osmolality of glomerular filtrate = blood (300 mOsm/L0
      • Osmolarity remains at 300 mOsm/L b/c water is reabsorbed in proportion to solutes (iso-osmotic)
    • TBW deficit + total body Na deficit (hypovolemia): decrease % of water reabsorbed (65%)
  • TDL
    • Permeable to water, impermeable to NaCl
    • Water is reabsorbed following the osmotic gradient of the hypertonic medulla
    • As water --> medulla, tubular fluid osmolality increases & equilibrates w/ the osmolality of the medulla at the hairpin of the LOH (600 mOsm/L) 
      • Lower than in TBW deficit
  • TnAL
    • Impermeable to water so no osmotic equilibrium
    • Permeable to NaCl: passive process
      • NaCl conc in the medullary interstitium has been diluted by water from the TDL
      • NaCl conc in TnAL > medulla
      • NaCl moves passivley down conc gradient --> decrease TnAL osmolality
  • TkAL (aka medullary diluting segment, aka concentrating segment)
    • Impermeable to water
    • Permeable to NaCl: active process
      • NaCl is reabsorbed down its conc gradient into the medulla via the N/K/2Cl co-transporter
        • Gradient is produced by Na/K ATPase actively pumping Na out of the cell
        • ADh stimulates Na/K/2CL to contribute to the increasing gradient
      • Osmolality of fluid leaving this segment = 120 mOsm/L
        • Higher than in TBW deficit b/c dilution step is diminished w/o ADH due to lack of N/K/2Cl stimulation
  • DCT (aka cortical diluting segment)
    • Impermeable to water
    • Permeable to NaCl
    • Osmolality of tubular fluid decreases to 100 mOsm/L
      • Higher than in TBW deficit
  • CD
    • Imermeable to water in the absence of ADH
      • As tubular fluids flow down, no osmotic equilibration is possible
      • NaCl continues to get reabsorbed
      • Urine osmolality decreases to 50 mOsm/L
    • ADH can't increase urea permeability in the IMCD
      • Urea doesn't contribute to the medullary gradient
    • Final urine osmolality at the tip of the papilla = 600 mOsm/L
      • Lower than in TBW deficit

23

Assessment of water balance

  • Urine osmolality (UOsm)
    • Definition
    • Normal vs. abnormal ranges
  • Urine specific gravity (USG)
    • Definition
    • Normal vs. abnormal ranges
    • USG vs. UOsm
    • Plasma USG & UOsm

  • Urine osmolality (UOsm)
    • # of particles in urine / kg of water volume
    • Normal range: 50-1200 mOsm/kg
      • UOsm < 100 --> max diluted urine + absent ADH --> TBW excess
      • UOsm > 600-800 --> concentrated urine + present ADH --> TBW deficit
  • Urine specific gravity (USG)
    • Ratio of density of urine w/ density of an equal volume of water
      • Proportional to the MW & # of particles in sol'n
    • Normal range: 1.005 - 1.035
      • Every 30 mOsm/kg increment in UOsm corresponds to an increase in USG of 0.001
      • USG > 1.030 --> concentrated urine --> TBW deficit
      • USG < 1.010 --> diluted urine --> TBW excess
    • Every 30 mOsm/kg increment in UOsm corresponds to an increase in USG of 0.001
    • Plasma: 1% heavier than water --> USG = 1.010 --> UOsm = 300 mOsm/kg

24

Assessment of water balance

  • Electrolyte-free water clearance (CeH2O)
  • CeH2O equation
  • Urine/plasma electrolyte ratio
    • Definition
    • (UNa + UK) / PNa > 1
      • CeH2O
      • Free water
    • (UNa + UK) / PNa = 1
      • CeH2O
      • Free water
    • (UNa + UK) / PNa < 1
      • CeH2O
      • Free water
  • Under pathological circumstance

  • Electrolyte-free water clearance (CeH2O)
    • Amt of solute-free water excreted by kidneys in 24 hr
    • Assessment of water balance
  • CeH2O equation
    • CeH2O = V * { 1 - [ (UNa + Uk) / PNa ] }
    • CeH2O = electroyle-free water clearance
    • V = urine volume in 24 hr
    • UNa = urine Na conc 
    • UK = urine K conc
    • PNa = plasma/serum Na conc
  • Urine/plasma electrolyte ratio
    • Used to predict electrolyte-free water clearance if info about urine volume (V) is unavailable
    • (UNa + UK) / PNa > 1
      • CeH2O: negative
      • Free water: retained
    • (UNa + UK) / PNa = 1
      • CeH2O: zero
      • Free water: neither retained nor excreted
    • (UNa + UK) / PNa < 1
      • CeH2O: positive
      • Free water: excreted
  • Under pathological circumstances
    • Homeostatic mechs that maintain water balance fail
    • TBW deficit & excess produce persistent change sin plasma tonicity
    • Changes can be assessed by measuring serum osmolality & serum Na conc