Body Water Distribution😊

Body Water Distribution

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Total Blood Volume:

Approx. 8% of total body weight.

Plasma → 5%
RBC volume → 3%

Total Body Water (TBW):

Demographic	% Total Body Weight (Water)
Children (General)	75%
Adult: Male	60%
Adult: Female	50-55%

Fluid Compartments

Fluid Compartment	% Total Body Weight (TBW)	% Total Body Water (TBWtr)	Indicator used	Example Volume
Total Body Water	60%	100%	• Deuterium oxide • Tritium oxide • Antipyrine.	42L (70kg)
Intracellular Fluid (ICF)	40%	2/3 (66%)	Total Body Water volume - ECF volume.	28 liters
Extracellular Fluid (ECF)	20%	1/3 (33%)	• Inulin • Sucrose • Sodium isotope • Iodine isotope • Mannitol	14 liters
Plasma:	5%	1/12	• Evan blue • RISA [Radioactive Iodine 125 labelled albumin]	ㅤ
Interstitial fluid	15%	3/12	ECF volume - Plasma volume.	ㅤ
RBC	ㅤ	ㅤ	• 51Cr, 59Fe tagged RBC	ㅤ

Example (PV): TBW=48L.

PV = (5% body weight / 60% body weight) * TBW
= (5/60) * 48L = 4L.

Mnemonic:

TBW → Water

Pani → Antipyrine
H2O → D2O, T2O

ECF → good for swimming → I SSIM in ECF

Inulin
Sucrose
Sodium isotope
Iodine 125 isotope
Mannitol

Plasma

Blue () Plasma (Plasma Protein) TV idu (iodine)

RBC

Crafted RBC → Cr51, Fe59

Transcellular Compartment:

Fluid inside certain cavities.

Fluid	Volume
CSF	150 ml
Synovial fluid	1 ml per large joint
Pleural fluid	10-20 ml
Pericardial fluid	Approx. 50 ml

Measurement of Body Water

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Principle: Volume of Distribution

Using dilution principle

Formula: V = (Q - E) / C

V: Volume of distribution (compartment volume).
Q: Amount of substance injected.
E: Excreted or metabolized portion.
C: Concentration of substance in plasma after distribution.

Total Blood Volume = Plasma Volume
(1-Hematocrit)

Example (ECF): 500mg dose, 3mg/dL conc., 10% excreted → 450mg in body

Volume = 15 L.

Water excreted

Water excreted in	Volume
Stool	100 mL
Skin	Insensible loss ↳ 50 mL/hr (1.2L /day)
Urine	• > 500ml/day • Minimum → 50ml/hr

McArdle’s Maximum Allowable Sweat Rate (P4SR)

McArdle → Muscle → sweating

Zone	Predicted 4-hr Sweat Rate
Comfort	1 – 3 L
Just tolerable	3 – 4.5 L
Intolerable	> 4.5 L

Darrow-Yannet Diagram

Purpose:

Illustrates changes in ECF and ICF volume and osmolality.

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Change	Examples
Loss of Isotonic Fluid	Hemorrhage, Vomiting
Gain of Isotonic Fluid	0.9% NaCl, Ringer's lactate
Gain of Hypotonic Fluid	Pure water, SIADH, 0.45% NS
Loss of Hypotonic Fluid	DI
Gain of Hypertonic Fluid	3% NaCl infusion
Loss of Hypertonic Fluid	Adrenal insufficiency

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Osmosis and Volume Shift

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Osmotic movement:

From compartment with lower solute (X)
To compartment with higher solute (Y).

Sodium

Osmotically active.
Drags H2O.

Serum Osmolarity

Osmolarity of Na+: 300 mosm/L.

Sodium > BUN > Glucose

Mnemonic: Sodayum (200 Rs) Bunum (36 rs) Sugarum (5 rs)

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Sodium deficit (mEq) in Hyponatremia

= (desired - actual sodium) x TB Water

= (Desired Sodium - Current Sodium) × Body Weight (kg) × 0.6

TBW = 60% BW

Desired = 140

Free Water deficit in Hypernatremia

(Na - 140) x TBW
Na

Osmolarity vs. Osmolality

Osmolarity	Osmolality
mosm/L of solvent	mosm/kg of solvent
Most common	ㅤ

Osmoles

Concentration of osmotically active substance.

Starling Forces

Definition

Pressures that govern fluid movement across capillary walls

Determining net filtration or reabsorption.

Net Filtration Pressure Formula

(Forces favoring filtration) - (Forces opposing filtration)

Net Filtration Pressure = (Pc + πi) - (πc + Pi)

Components & Typical Values

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Pressure	ㅤ	Function
Capillary hydrostatic (Pc)	25 mmHg	Pushes fluid out of capillary.
Capillary oncotic (πc)	25 mmHg	Pulls fluid into the capillary
Interstitial oncotic (πi)	3 mmHg	Pulls fluid out of capillary
Interstitial hydrostatic (Pi)	-2 mmHg	Pushes fluid into the capillary

Pi can be Positive in:

Skeletal muscle

Liver

Brain

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