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Introduction
1. Anatomy
2. Microbiology
3. Physiology
3.1 Nervous system and special senses
3.2 Cardiovascular system
3.3 Respiratory system
3.4 Gastrointestinal system
3.5 Renal and urinary system
3.5.1 Overview
3.5.2 Glomerular filtration
3.5.3 Tubular reabsorption and secretion
3.5.4 Renal tubules
3.5.5 Urine concentration
3.5.6 Body fluid compartments
3.5.7 Additional information
3.6 Endocrine system
3.7 Reproductive system
4. Pathology
5. Pharmacology
6. Immunology
7. Biochemistry
8. Cell and molecular biology
9. Biostatistics and epidemiology
10. Genetics
11. Behavioral science
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3.5.6 Body fluid compartments
Achievable USMLE/1
3. Physiology
3.5. Renal and urinary system

Body fluid compartments

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Our body is made up of about 50-70% water. This total body water (TBW) is distributed between two main compartments:

  • Intracellular fluid (ICF): about 2/3 of TBW (fluid inside cells)
  • Extracellular fluid (ECF): about 1/3 of TBW (fluid outside cells)

A helpful way to remember these proportions is the 60-40-20 rule:

  • 60% of body weight is water (TBW)
  • 40% of body weight is ICF
  • 20% of body weight is ECF

The ECF is further divided into:

  • Plasma: 1/4 of ECF
  • Interstitial fluid: 3/4 of ECF

Plasma makes up about 55% of blood volume. Interstitial fluid is similar to plasma, except it lacks plasma proteins and blood cells.

The ratio of TBW to total body weight varies widely between individuals. Most of this variation is explained by differences in adipose tissue, which has a low water content. Differences in the amounts of bone and plasma contribute much less.

  • Obese adults have a lower percent body water because they have more adipose tissue.
  • Differences in percent body water between males and females are mainly due to differences in adipose tissue.
  • Neonates have more water than adults: about 75-80% of body weight is water, with proportionately more ECF, especially interstitial fluid, than adults.

Markers for body fluid compartments

Compartment Marker
Total body water D2O, antipyrine, THO or tritiated water
ECF Mannitol, inulin, sulfate
Plasma Radiolabeled albumin, Evan’s blue

You can’t measure ICF directly using a marker, so it’s calculated indirectly:

  • ICF = TBW − ECF

You can also calculate interstitial fluid indirectly:

  • Interstitial fluid = ECF − plasma volume

Fluid shifts between compartments occur to equalize osmolarity across cell membranes (especially between ICF and ECF). Water moves freely across cell membranes to balance osmolarity.

For example, if the ECF becomes hypertonic, water shifts out of the ICF and into the ECF until osmolarity becomes equal in both compartments.

ECF osmolarity is estimated using plasma osmolarity:

Plasma osmolarity = 2 X Plasma Na+ + Glucose/ 18 + BUN/2.8

Where BUN is blood urea nitrogen.

Changes in body fluid compartments in different conditions

Type Example Osmolarity ECF ICF Hematocrit Plasma protein conc.
Isosmotic* volume contraction Diarrhea, burns No change Decreases No change Increases Increases
Hyposmotic volume contraction Hypoadrenalism Decreases Decreases Increases Increases Increases
Hyperosmotic volume contraction Sweating, fever, DI** Increases Decreases Decreases No change Increases
Isosmotic volume expansion Infusion of isotonic saline No change Increases No change Decreases Decreases
Hyposmotic volume expansion SIADH*** Decreases Increases Increases No change Decreases
Hyperosmotic volume expansion High salt intake Increases Increases Decreases Decreases Decreases

*ICF is not changing as there is no water shift owing to constant osmolarity.

**Diabetes insipidus

***Syndrome of inappropriate ADH secretion

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