Sodium and Water Balance

Intracellular: 66%
Extracellular: 33%
--75% interstital
--25% circulating plasma

Water follows solute

amount of solute in compartment

freely permeable to water, unrestricted flow, equal osmolality in most body fluids

Plasma 142 mmol/L
Intracellular: 12 mmol/L

plasma: 4.4 mmol/L
intracellular 140 mmol/L

plasma: 104 mmol/L
intracellular 4

1000ml Water
---intracel 670 ml
---interstitial 250 ml
--plasma 80 ml

1000 ml + 150 mmol NaCl
---interstitial 750 ml
--plasma 250 ml

if dehydrated, nothing in extracellular--give NS and whole L will stay in extracellular
if gave H20--1/3 would leave extracellular

sodium balance regulates ECF volume
free water balance regulates osmolality

Dietary intake
renal filtration--GFR depends on renal blood flow and function
Reabsorption--adjusted by aldosterone (increases) and by atriopeptin (decreases)
Excretion: net of filtration and reabsorption

65-100 mmmol/day
minimum 30 mmol/day

40-60 mmol/day

70% reabsorbed: Na/H exchange, Na/glucose cotransport, angiotensisn II, NE, peritubular capillary flow

20% flow dependent, loop diuretics

5%, flow dependent, thiazide

increase in osmolality (solute excess or free water deficit)

decrease in circulating volume or in BP
--hemodynamic changes associated with heart failure or liver failure may also trigger

other non-osmotic stimuli: pain, nausea, angio 2, acute pyschosis, drugs

H20 follows sodium
sodium stays in ECF
Water balance via ADH adjusted by kidneys to match Na balance, maintaining normal osmolality

Sodium balance determines ECF volume, therapeutic target for ECF volume is sodium blanace
So to get rid of extra fluid, get rid of Na+

Determined from clinical assessment of ECF volume
Serum Na and osmolality are indicators of water balance relative to Na
Serum [Na] is not an indicator for Na balance---it indicates H20 balance

changes in total body water 1 L = 1 kg
weight change is sensitive
Hard to detect changes in ECF of less than 3 L

pitting edema
pulmonary congestion
--esp in heart failure, dyspnea, orthopnea,
ascites

Increased CVP, jugular venous distension
Hepato-jugular reflux (HJR)(press on liver_
Left ventricular filling pressure S3 sound

occurs when body water is not matched with body electrolyte solute
results in abnormal osmolality of body fluids
free water imbalances are distributed throughout both ICF and ECF
Assessed as Na serum outside of normal range

135-144 mmol/L
so water imbalanced if outside of range

safe to remove .5-1 kg/day of Excess ECF
Need to excrete more sodium than intake--reduce intake, start diuretic, monitor for signs of over dieresis