8 Water Homeostasis Flashcards

Question 1

Q

Homeostasis

Answer

A

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

Question 2

Q

Normal water content & distribution

Total body water (TBW)
TBW distribution

Answer

A

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)

Question 3

Q

Water balance

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

Answer

A

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

Question 4

Q

Osmolarity, osmolality, & tonicity

Osmole
Osmolarity
Osmolality
Tonicity
Both osmolality & tonicity

Answer

A

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

Question 5

Q

Plasma tonicity

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

Answer

A

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

Question 6

Q

Plasma tonicity

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

Answer

A

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

Question 7

Q

Osmoreceptors

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

Answer

A

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

Question 8

Q

Homeostatic response to TBW deficit

Answer

A

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

Question 9

Q

Homeostatic response to TBW excess

Answer

A

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

Question 10

Q

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

Answer

A

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

Question 11

Q

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

Answer

A

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

Question 12

Q

Thirst

Originates in…
Triggered by…
Thirst vs. age

Answer

A

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

Question 13

Q

Renal water handling: absence vs. presence of ADH

Question 14

Q

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

Answer

A

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

Question 15

Q

Renal water handling: proximal tubule

Net water reabsorption force in the renal interstitium
P_c
During hypovolemia
π_c

Answer

A

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

Question 16

Q

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

Answer

A

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)

Question 17

Q

Renal water handling: water permeability

LOH
- TDL
- TnAL
- TkAL
DCT
CD

Answer

A

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

Question 18

Q

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

Answer

A

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

Question 19

Q

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

Answer

A

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,

Question 20

Q

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…

Answer

A

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

Question 21

Q

Renal water handling during TBW deficit

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

Answer

A

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

Question 22

Q

Renal water handling during TBW excess

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

Answer

A

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

Question 23

Q

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

Answer

A

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

Question 24

Q

Assessment of water balance

Electrolyte-free water clearance (C_eH2O)
C_eH2O equation
Urine/plasma electrolyte ratio
- Definition
- (U_Na + U_K) / P_Na > 1
  - C_eH2O
  - Free water
- (U_Na + U_K) / P_Na = 1
  - C_eH2O
  - Free water
- (U_Na + U_K) / P_Na < 1
  - C_eH2O
  - Free water
Under pathological circumstance

Answer

A

Electrolyte-free water clearance (C_eH2O)
- Amt of solute-free water excreted by kidneys in 24 hr
- Assessment of water balance
C_eH2O equation
- C_eH2O = V * { 1 - [ (U_Na + U_k) / P_Na ] }
- C_eH2O = electroyle-free water clearance
- V = urine volume in 24 hr
- U_Na = urine Na conc
- U_K = urine K conc
- P_Na = plasma/serum Na conc
Urine/plasma electrolyte ratio
- Used to predict electrolyte-free water clearance if info about urine volume (V) is unavailable
- (U_Na + U_K) / P_Na > 1
  - C_eH2O: negative
  - Free water: retained
- (U_Na + U_K) / P_Na = 1
  - C_eH2O: zero
  - Free water: neither retained nor excreted
- (U_Na + U_K) / P_Na < 1
  - C_eH2O: 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

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8 Water Homeostasis Flashcards

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