Canonical Question
Body Fluids
Master answer
DEFINITION
Body fluid compartment is defined as collections of physiologically significant fluid characterised as →
(i) being easily defined, largely separate from another compartment by some form of physical barrier
(ii) having a similar composition within the compartment
(iii) behaving predictably to certain fluid interventions
DISTRIBUTION OF TOTAL BODY WATER (TBW)
- TBW is 60% of body weight (42 L in 70 kg ♂ adult) → BUT this varies according to:
- Body fat content – ↑ fat content → ↓ %TBW of body weight
- Age – Aging causes ↓ %TBW of body weight (due to ↑ fat content)
- Gender – ♀ TBW is 55% body weight (due to ↑ fat content)
- TBW is broken into various “body fluid compartments” as per the table below:
| COMPARTMENT | % TBW | Volume (L) | % Body Weight | |
|---|---|---|---|---|
| TBW | 100% | 42 L | 60% | |
| ICF | 55 | 23 | 33 | |
| ECF | 45 | 19 | 27 | |
| ECF Components | Interstitial Fluid | 20 | 8.4 | 12 |
| Plasma Volume | 7.5 | 3.2 | 4.5 | |
| Dense CT H2O | 7.5 | 3.2 | 4.5 | |
| Bone H2O | 7.5 | 3.2 | 4.5 | |
| Transcellular fluid | 2.5 | 1 | 1.5 | |
Composition
| ICF | ECF | |
|---|---|---|
| Consists of all fluids within the cell membrane | consists of all fluids outside the cell membrane | |
| % TBW | 55% (23L) | 45% (19L) |
| % Body Weight | 33% | 27% |
| Mainly | Mainly K+ (150 mmol/L), PO43- (100 mmol/L) and proteins (+++) | Mainly Na+ (140 mmol/L) and Cl- (100-105 mmol/L) |
| Smaller amounts | Na+ (10 mmol/L) Cl- (3 mmol/L) HCO3- (10 mmol/L) Mg2+ (0.5 mmol/L) Ca2+ (< 0.01 mmol/L) | HCO3- (27 mmol/L) K+ (3.5-5 mmol/L) Ca2+ (1.1 mmol/L) PO43- (1.1 mmol/L) Mg2+ (0.5 mmol/L) protein (+) |
| Osmolality | 290 mosm/kg H2O | 290 mosm/kg H2O |
| does NOT exist not a single united fluid compartment → exists as 1014 discrete separate packets (cells) of solution |
Extracellular fluid (ECF) Sub-compartments
| Compartment | TBW | %Bdy Wt | |
|---|---|---|---|
| Interstitial fluid | 20% (8.4L) | 12% | Fluid that bathes all cells in body and links their ICF with Plasma volume (PV) |
| role in transfer of metabolic substrates (O2/nutrients), waste products and chemical messengers | |||
| Similar composition to PV → BUT very ↓ protein composition cf. PV | |||
| Includes “lymph” → role in returning excess ISF and protein to circulation | |||
| Plasma fluid | 7.5% (3.2L) | 4.5% | Role in transport function within body (Ie. of metabolic substrates, waste products, chemical messengers) → relies on high “bulk flow” |
| Similar composition to ISF → BUT ↑↑↑ protein content | |||
| Dense Connective tissue (CT) H2O | 7.5% (3.2L) | 4.5% | |
| Bone H2O | 7.5% (3.2L) | 4.5% | |
| Transcellular fluid | 2.5% (1L) | 1.5% | Diverse group of small fluid collections |
| Eg. CSF, bile, joint fluid, aqueous humour, bowel fluid, bladder urine, body cavity fluids, Etc. | |||
| This type of fluid is specially characterised by: – having special physiological roles in body – being in contact with ICF across an epithelial cell membrane (rather than ISF), – and being formed by specific cellular transport activity in epithelial-lined spaces |
There are two groups of ECF:
- “Functional ECF” (PV + ISFV → 27.5% TBW)
- Kinetically active (fast) ECF → vital in determining compartment distribution of acutely infused fluids
- Explains 2:1 ECF:ICF ratio with acute IVF intervention (as functional ECF:ICF ratio, rather than total ECF:ICF ratio which is 55:45)
- “Non-functional ECF” (TCF + bone H2O + dense CT H2O → 17.5% TBW)
- Kinetically inactive (slow) ECF → minor role in determining compartment distribution of acutely infused fluids
Blood volume
- Blood volume (5 L) consists of BOTH ECF and ICF compartments → PV from ECF (3.2 L) and red cell volume from ICF (1.8 L)
- Blood volume = PV (from ECF) + red cell volume (from ICF)
Factors controlling distribution of TBW
- All body fluid compartments are isotonic (or iso-osmotic in terms of “effective”
osmoles) as H2O easily and rapidly moves (via osmosis) across cell membranes - Distribution of TBW in body fluid compartments is due to various factors (see
below) that cause H2O to shift across membranes into a specific compartment
TBW distribution between ICF and ECF
→ determined by ECF content of Na+ because:
- ECF Na+ content is main determinant of ECF osmolality → this is because
Na+ and its associated anion (Cl-) are the main osmotically active solutes in ECF (90% ECF osmolality) → but since ∆ in ECF Cl- content occur 2° to ∆ in ECF
Na+ content, so ECF Na+ content is the true determinant of ECF osmolality- Remember → ECF Na+ is controlled by arterial, venous and cardiac baroreceptors that measure ECFV (which is determined by ECF Na+)
- ECF osmolality determines TBW distribution between ECF and ICF → this is
because cell membranes are H2O-permeable and thus ∆ ECF osmolality cause
H2O movement across it via “osmosis” until both ICF and ECF osmolalities
equalise (Ie. hypertonic ECF draws H2O out from ICF until ICF/ECF osmolality
are equal)- Remember → Semi-permeable membrane (permeable to H2O but impermeable to most solutes) allows osmotic gradients to develop on both sides of the membrane.
“Osmosis” is a process where H2O passively diffuses across the membrane from ↓ to ↑ osmolality until osmolality is same on both sides of the membrane
- Remember → Semi-permeable membrane (permeable to H2O but impermeable to most solutes) allows osmotic gradients to develop on both sides of the membrane.
TBW distribution between ISF and intravascular compartments
→ determined by “Starling forces” → balance of PHYDROSTATIC and PONCOTIC across the capillary membrane determines net ultrafiltration of H2O into ISF:
\[Net \; H_{2}O \; Flux = K_{f} \times { [(P_{CAP} – P_{ISF}) – \sigma \; (\pi_{CAP} – \pi_{ISF})]}\]
- Hydrostatic pressure gradient (PIV – PISF) → PIV at arterial end and venous end of capillary is 35 mmHg and 15 mmHg, respectively, while PISF is 0 mmHg → thus, a PHYDROSTATIC gradient exists favouring net H2O filtration into ISF (decreasing from 35 mmHg at start of capillary to 15 mmHg at the end)
- Oncotic pressure gradient (πIV – πISF) → plasma colloids (esp plasma proteins) cannot cross capillary membrane into ISF, so [protein] in capillaries is >> ISF (Ie. 80 vs 20 g/L) → πIV >>. πISF (28 mmHg vs 3 mmHg) → constant but small net oncotic pressure gradient throughout capillary length favouring H2O retention in intravascular space
- KF → = capillary surface area (↑↑↑) and its hydraulic permeability (membrane is semi-permeable → permeable to H2O and most solutes EXCEPT large proteins)
- σ → Reflection coefficient
- Thus → NFP at arterial end of capillary is +10 mmHg (favouring filtration in interstitial space) while at venous end it is – 10 mmHg (favouring reabsorption in capillary) → there is net 2 mL/min fluid filtered into interstitial space
TBW distribution between ECF sub-compartments
→ determined by:
- Starling forces → balance of PHYDROSTATIC and PONCOTIC across the membrane determines ultrafiltration of H2O across compartments (as above)
- Active and passive transport of solute across the membrane (Eg. passive diffusion, facilitated transport, active transport) → influences osmotic gradient across the membrane and causes osmosis of H2O between compartments
- Gibbs-Donnan effect → non-diffusible ion in one compartment causes equal concentration ratios of diffusion ions across the membrane at equilibrium → results in ↑ osmotic pressure in compartment with non-diffusible ion → leads to osmosis of H2O into it
Source: Bianca’s notes
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2022B Q11 | Describe the body fluid compartments. | historical_member | — |