The document discusses body water and fluid compartments. It describes that total body water is about 42L in healthy adults. Water is contained within two main fluid compartments - intracellular fluid and extracellular fluid. Extracellular fluid is further divided into plasma and interstitial fluid. Various indicator dilution methods are used to measure the volumes of these fluid compartments. Fluid movement between compartments is regulated to maintain osmotic equilibrium and water balance in the body. Disruptions can lead to edema, an excess accumulation of fluid in tissues.

1. Body water and fluid
compartments

2. Outline
• Total body water
• Fluid intake and output
• Fluid compartments and composition
• Tonicity, osmolarity and osmolality of fluids
• Overview of edema

3. Body water content
• TBW -Total amount of water in body
• 42L in a 70kg health adult
• varies with age and sex
• Infants: 73% (low body fat, low bone mass)
• Adult males: ~60%, females: ~50% water
• Water content declines to ~45% in old age

4. Fluid intake and output
Source: Guyton and Hall(2015); Textbook of medical Physiology, P306

5. The Fluid compartments…

6. Fluid compartments
Total body water = 42 L
Two main fluid compartments
–Intracellular fluid (ICF) compartment: 2/3 in cells
–Extracellular fluid (ECF) compartment: 1/3 outside cells
•Plasma: 3L (20%)
•Interstitial fluid (IF): 11L (80%) in spaces btn cells
–Usually considered part of IF: lymph, CSF, humors of
the eye, synovial fluid, serous fluid, and gastrointestinal
secretions

7. ECF
Fluid outside cells may be;
• Intravascular;
in blood vessels. Contains electrolytes and a protein-rich fluid (plasma) esp.
albumin.
• Interstitial:
surrounds cell. Is the location of most ECF; ¾. Contains little or no protein
• Transcellular :
within specialized body cavities e.g. CSF, synovial, pleural.

8. Composition of body fluids
• Water and solids
• Solids; organic and inorganic substances
• Organic; Glucose, amino acids and other proteins, fatty acids and
other lipids, hormones and enzymes
• Inorganic; Na, K, Ca, Mg, Cl, HCO3, phosphate and sulfate

9. ECF Vs ICF composition
ICF
Cations:
Na+ (14)
K+ (140)
Mg2+ (20)
Anions:
HPO4 (50)
HCO3
- (10)
SO4 (10)
Cl- (4)
Nutrients:
High concentrations of proteins.

10. Movement of fluid btn compartments
• Difference btn relative osmolarity →fluid shifts
• Fluid shifts end at eqbm
• At eqbm, a new steady-state is achieved
• Changes in osmolarity typically affect the ECF first;
- compartment directly in contact with outside envt
- Solutes and water can be added to it from GI tract
- can be subtracted from it by action of kidneys
- loss from perspiration, breathing, and the feces

11. Water balance and ECF Osmolarity
• Water intake must = water output = ~ 2500 ml/day
• Water intake: beverages, food, and metabolic water
• Water output: urine (60%), insensible water loss (lost through skin
and lungs), perspiration, and feces

12. Measurement of fluid volume
• TBW and the volume of different compartments of the body fluids are
measured by indicator dilution method or dye dilution method.

13. Indicator dilution method
• A known quantity of a substance such as a dye is
administered into a specific body fluid compartment.
• These substances are called the marker substances or
indicators.
• After administration into the fluid compartment, the
substance is allowed to mix thoroughly with the fluid.
• Then, a sample of fluid is drawn and the
concentration of the marker substance is determined.
• Radioactive substances or other substances whose
concentration can be determined by using
colorimeter are generally used as marker substances

14. Formula of IDM
• Quantity of fluid in the compartment is measured using the formula;
V=
𝑴
𝑪
Where;
• V = Volume of fluid in the compartment.
• M = Mass or total quantity of marker substance injected.
• C = Concentration of the marker substance in the sample fluid
• However this is not so accurate (some fluid is lost in urine)

15. Fomula for IDM
• Correction factor, (to cater for loss in urine)
V =
𝑀 −𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑚𝑎𝑟𝑘𝑒𝑟 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 𝑒𝑥𝑐𝑟𝑒𝑡𝑒𝑑
𝐶

16. X-tics of a marker substance for IDM
• Must be nontoxic
• Must mix with the fluid compartment thoroughly within reasonable
time
• Should not be excreted rapidly
• Should be excreted from the body completely within reasonable time
• Should not change the color of the body fluid
• Should not alter the volume of the body fluid.

17. Applications of IDM
• There are 3 main applications;
i-measurement of plasma volume
ii-measurement of ECF volume
iii-measurement of volume of TBW

18. Indicators used
Fluid compartment Marker substances used
Total body water 1. Deuterium oxide (D2O)
2. Tritium oxide (T2O)
3. Antipyrine
Extracellular fluid 1.Radioactive sodium, chloride,
bromide, sulfate and thiosulfate.
2.Non-metabolizable saccharides
like inulin, mannitol, raffinose
and sucrose
Plasma 1. Radioactive iodine (131I)
2. Evans blue (T-1824)

19. Applications of IDM
i-Measurement of TBW
• Volume of TBW (fluid) is measured by using a marker substance which is
distributed through all the compartments of body fluid
• Deuterium oxide and tritium oxide mix with fluids of all the compartments
within few hours after injection.
• Since plasma is part of TBW, the concentration of marker substances can be
obtained from sample of plasma.
• The formula for IDM is applied to calculate total body water.
• Antipyrine is also used to measure TBW.But as it takes longer time to
penetrate various fluid compartments, the value obtained is slightly low.

20. Applications of IDM..
ii-measurement of ECF volume
• Substances which pass through the capillary membrane but do not
enter the cells, are used to measure ECF volume.
• These substances remain only in ECF and do not enter the cell (ICF).
• When any of these substances is injected into blood, it mixes with the
fluid of all subcompartments of ECF within 30 minutes to 1 hour.
• Indicator dilution method is applied to calculate ECF volume
• Also here, a sample of plasma is used (its part of ECF)

21. Example of ECF volume measurement
Consider the following results;
• Quantity of sucrose injected (Mass) : 150 mg
• Urinary excretion of sucrose : 10 mg
• Concentration of sucrose in plasma: 0.01 mg/mL
ECF volume =
𝑀 −𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑚𝑎𝑟𝑘𝑒𝑟 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 𝑒𝑥𝑐𝑟𝑒𝑡𝑒𝑑
𝐶
=
150−10𝑚𝑔
0.01𝑚𝑔/𝑚𝐿
= 14,000mL
= 14L

22. Applications of IDM…
iii-measurement of plasma volume
• The substance which binds with plasma proteins strongly and diffuses
into interstitium only in small quantities or does not diffuse is used to
measure plasma volume

23. Procedure;
-known volume of dye is injected intravenously
- after 10mins a blood sample is collected
-also 4 samples may be collected at 10min interval
-all samples are centrifuged and plasma is separated
- concentration of dye is measured in each plasma
sample
- average concentration is calculated
- urine is collected and amount of dye excreted is
measured.

24. • Then plasma volume is calculated from;
Volume =
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑑𝑦𝑒 𝑖𝑛𝑗𝑒𝑐𝑡𝑒𝑑 −𝑎𝑚𝑜𝑢𝑛𝑡 𝑒𝑥𝑐𝑟𝑒𝑡𝑒𝑑
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑑𝑦𝑒 𝑖𝑛 𝑝𝑙𝑎𝑠𝑚𝑎

25. Applications of IDM…
iv- measurement of interstitial fluid
• Volume of interstitial fluid cannot be measured directly.
• It is calculated from the values of ECF volume and plasma volume.
• Interstitial fluid volume = ECF volume – Plasma volume

26. Applications of IDM
v. Measurement of ICF
• Volume of ICF cannot be measured directly. It is calculated from the
values of total body water and ECF.
• ICF volume = Total fluid volume – ECF volume.

27. Concentration of body fluids
• Concentration of body fluids is expressed in three ways:
1. Osmolality
2. Osmolarity
3. Tonicity.

28. Osmolality
• Measure of a fluid’s capability to create osmotic pressure or
• Its also the osmotic (osmolar) concentration of a solution.
• Simply; it is the concentration of osmotically active substance in the
solution.
• Osmolality is expressed as the number of particles (osmoles) per
kilogram of solution (osmoles/kg H2O).

29. Osmolarity
• Osmolarity is another term to express the osmotic concentration.
• It is the number of particles (osmoles) per liter of solution
(osmoles/L).
• Osmotic pressure in solutions depends upon osmolality.

30. In practice, the osmolarity and not osmolality is used to determine the
osmotic pressure because of the following reasons:
i. Measurement of weight (kilogram) of water in solution is a difficult
process
ii. Difference between osmolality and osmolarity is very much
negligible and it is less than 1%.

31. • Change in osmolality of ECF affects the volume of both ECF and ICF.
• When osmolality of ECF increases, water moves from ICF to ECF.
• When the osmolality decreases in ECF, water moves from ECF to ICF.
• Water movement continues until the osmolality of these two fluid
compartments becomes equal.

32. Mole and osmole
• mole (mol) is the molecular weight of a substance in gram.
• Millimole (mMol) is 1/1000 of a mole.
• One osmole (Osm) is the expression of amount of osmotically active
particles.
• It is the mw of a substance in grams divided by number of freely
moving particles liberated in solution of each molecule.
• One milliosmole(mOsm) is 1/1000 of an osmole.

33. Tonicity
• Larger molecules sodium and glucose, which cross the cell membrane
slowly, can influence the movement of water
• Such molecules are; effective osmoles
• Osmolality that causes movement of water from one compartment to
another is called effective osmolality and the effective osmoles are
responsible for this.

34. Tonicity …
• Tonicity is the measure of effective osmolality.
• In terms of tonicity, the solutions are classified into three;
i. Isotonic fluid
ii. Hypertonic fluid
iii. Hypotonic fluid.

35. Tonicity…
i-isotonic fluid;
• Fluid which has the same tonicity as body fluids.
• Examples are; 0.9% NaCl solution (normal saline) and
5% glucose solution.
• RBCs or other cells placed in isotonic fluid (normal
saline) neither gain nor lose water by osmosis
• This is because of the osmotic equilibrium between
inside and outside the cell across the cell

36. Tonicity…
ii- Hypertonic fluid;
• Has greater tonicity than the body fluids.
• Example is; 2% NaCl solution.
• When RBCs or other cells are placed in hypertonic
fluid, water moves out of the cells (exosmosis)
resulting in shrinkage of the cells (crenation).

37. Tonicity …
iii- Hypotonic fluid
• Has less tonicity than the body fluids.
• Example is; 0.3% sodium chloride solution.
• When RBCs or other cells are placed in hypotonic
fluid, water moves into the cells (endosmosis)
• This causes swelling of the cells
• The RBCs become globular (sphereocytic) and get
ruptured (hemolysis).

38. Tonicity …
Effect of isotonic, hypertonic and hypotonic Solns on RBCs

39. Tissue fluid formation

40. Edema
• Edema refers to the presence of excess fluid in the body tissues.
• It may be generalized or local.
• Edema that involves the entire body is called generalized edema.
• Local edema – occurs in specific areas of the body such as
abdomen, lungs and extremities like feet, ankles and legs.
• occurs mainly in the ECF compartment; may involve ICF as well.
• Thus; two types;
i) Intracellular edema
ii) Extracellular edema

41. Intracellular edema
• Due to accumulation of fluid inside the cell
• Mainly caused by;
(1) depression of the metabolic systems of the tissues (due to
poor blood supply hence lack of O2)
(2) lack of adequate nutrition to the cells
(3) in inflamed tissues.
• Inflammation usually has a direct effect on the cell
membranes to increase their permeability, allowing sodium
and other ions to diffuse into the interior of the cell, with
subsequent osmosis of water into the cells.

42. Extracellular edema
• Excess fluid accumulation in the extracellular spaces
(outside the cell).
• 2 general causes:
(1) abnormal leakage of fluid from the plasma to the
interstitial spaces across the capillaries, and
(2) failure of the lymphatics to return fluid from the interstitium
back into the blood.
• The most common clinical cause of interstitial fluid
accumulation is excessive capillary fluid filtration.

43. Conditions that lead to extracellular edema
• two main abnormalities stated earlier:
1. Heart failure
2. Renal failure
3. Decreased amount of plasma proteins.
4. Lymphatic obstruction.
5. Increased endothelial permeability.

44. References
• Guyton and Hall (2015): Textbook of Medical Physiology. 13th Edition.