L3 Body Water

Question 1

How is body water distributed?

What is the 60-40-20 rule?

Answer 1

2/3 in ICF, 1/3 in ECF (20% of ECF = plasma, 80% of ECF = ISF)

Interstitial fluid (ISF) = layer between ICF and plasma

60% of body weight is water, 40% of body weight is ICF and 20 % body weight is ECF.

Question 2

How do you calculate Total Body Water?

Answer 2

Total Body Water = 0.7LBM + 0.1AT
where LBM = lean body mass (in kg) and AT = adipose tissue (in kg)

Increased body fat = decreased water %

Women and elderly have decreased TBW because of increased fat.

If LBM is not known, estimate: TBW = .6(weight)

Question 3

What are the relative concentration of Na, K, Cl and protein

Answer 3

Inside cell: lower Na, higher K, lower Cl, higher protein

Question 4

Estimating Plasma Osmolarity

Answer 4

Plasma Osmolarity (mOsm/kg H2O) =
 2Na+ (mEq/L) + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8

BUN = blood urea nitrogen, measure of nitrogen in blood and of renal function

Question 5

What is hematocrit?

How can volume changes (ECF or ICF) affect hematocrit?

Answer 5

Hematocrit = fraction of plasma occupied by red blood cells (RBCs) after centrifugation

= Height of RBCs/Total height (in column)

Question 6

How do volume changes in ECF or ICF affect total plasma protein and hematocrit?

Answer 6

Loss in ECF volume will increase plasma protein concentration, gain in ECF with will reult in decreased plasma protein (by dilution)

Volume changes that increase hematocrit:

1. Loss in ECF volume
2. Gain in ICF volume (RBCs swell)

Volume changes that decrease hematocrit:

1. Gain in ECF volume
2. Loss in ICF volume (RBC’s shrink)

Question 7

What are the affects of adding isotonic solution (e.g. saline infusion)?

Answer 7

Increase TBW from increased ECF volume, decrease plasma protein concentration and hematocrit (both from dilution)

Question 8

Loss of isotonic solution (e.g. diarrhea)

Answer 8

Decrease TBW

Decreases ECF volume

Increase [plasma protein] and hematocrit

Question 9

Gain of hypotonic solution (e.g. pure water)

Answer 9

Increase in ECF volume

Decrease in ECF osmolarity

Shift in H2O from ECF to ICF

Increase in ICF volume

Decrease in ICF osmolarity

Increase in TBW

Decrease plasma protein concentration

No change in hematocrit (no net change)

Question 10

SIADH Syndrome of Inappropriate Anti-diuretic Hormone Secretion

Answer 10

In SIADH, inappropriately high levels of ADH are secreted

Excess water is added to the ECF and ICF (in proportion to original values)

Plasma protein concentration is decreased, by dilution.

Hematocrit unchanged

Question 11

Loss of Hypotonic Solution

(e.g. water depravation, sweating)

Answer 11

Decrease in ECF volume

Increase in ECF osmolarity

Shift in H2O from ICF to ECF

Decrease in ICF volume

Increase in ICF osmolarity

Decrease in TBW

Increased plasma protein concentration (by fluid loss)

No change in hematocrit (no net change)

Question 12

Gain of Sodium (e.g. high NaCl intake)

Answer 12

Increase in ECF osmolarity

Shift in H2O from ICF to ECF

Decrease in ICF volume

Increase in ECF volume

Increase in ICF osmolarity

No change in TBW

Decrease plasma protein concentration (by dilution)

Decrease hematocrit
RBC concentration decreased by dilution and also because of water shift out of RBCs decreasing cell volume.

Question 13

Loss of Sodium (e.g. Aldosterone Insufficiency)

Answer 13

Decrease in osmolarity
Shift in H2O from ECF to ICF
Increase in ICF volume
Decrease in ECF volume
Decrease in ICF osmolarity
No change in TBW
Increase plasma protein concentration (by fluid loss)
Increase hematocrit
RBC concentration increased by ECF fluid loss and also because of shift of H2O into RBCs, increasing cell volume.