AHS C&T4
11/2/12
C&T4 — Body Fluids
Learning Outcomes
Themes
To be able to:
Importance of body fluids
(homeostasis)
Fluid compartments
(volumes)
Boundaries and movement of
materials
Pathophysiology
1.  describe the composition of the
three principal fluid
compartments in the body
2.  explain their respective
compositions in terms of the
nature of the compartments’
boundaries
3.  explain the importance of the
movement of material between
compartments.
4.  explain the effects of tonicity on
cell volume.
Body fluids — importance
Primary transport system
between cells
Nutrition
Waste
Signals
A large part of the
Internal environment
Maintain
Body systems
Homeostasis
Is essential
for
survival
of
Make
up
Fluid composition critical
for cell function.
Cells
Sherwood Fig. 1-5
1
AHS C&T4
11/2/12
Fluid Compartments
42 L
(60% of body mass)
3L
28 L
14 L
11 L
Volumes in L are those
of average 70kg male
NB 1 litre H2O weighs 1 kg
Water can move between compartments
Differences between Compartments?
Boundaries
Intracellular Fluid/Interstitial Fluid
Composition
Interstitial Fluid (IF)
Intracellular (ICF)
Na+ high
Cl- high
K+ low
Na+ low (pump)
Cl- low
K+ high (pump)
Boundary is cell membrane
selectively permeable
ion pumps*
Interstitial Fluid/Plasma
Boundary is capillary wall
Permeable to small molecules
Principal Anions
PO43-, Proteins (Pr-)
2
AHS C&T4
11/2/12
Osmosis
Water moves…
From lower to higher solute
concentration
Cell membrane is
semipermeable:
permeable to water but
not solute
Movement is passive.
Tonicity
Effect of solute concentration on cell volume
Solute cannot cross
membrane
Hence H2O moves
(a)  hypotonic solution
Cell gains water — swells
(b) hypertonic solution
Cells loses water — shrinks.
Important consideration in fluid replacement:
Haemorrhage: Replace blood with isotonic saline (NaCl)
No change in cell volume
Rehydration salts (diarrhoea): Replace water and ions
3
AHS C&T4
11/2/12
Lymphatic system
Fluid leaks out of
cardiovascular system.
Lymphatic system returns
lymph to the cardiovascular
system
Systemic capillaries: allow
exchange of materials
between blood and body
tissues – nutrients, gases (eg.
O2, CO2), water
Concept: Fluid Compartments (inputs and outputs)
Lymphatic drainage
Sherwood Fig 10-25"
3 litres per day not reabsorbed in capillaries/venules
Enters lymph vessels
Pumped to lymph nodes
Re-enters circulation near right atrium.
4
AHS C&T4
11/2/12
Arrows indicate direction of force
Fig 14.19
———————————————————————————————
———————————————————————————————
Arteriole
Capillary BP Colloid OP*
37 mmHg
25 mmHg
Capillary BP
Venule
17 mmHg
———————————————————————————————
———————————————————————————————
NET OUTWARD PRESSURE ~12mmHg
NET INWARD PRESSURE~8 mmHg
OUTWARD PRESSURE > INWARD PRESSURE:
Fluid leaves the capillary and enters the interstitial fluid
*Colloid OP — osmotic pressure of plasma proteins
Pathophysiology
Clue: Liver synthesises
most plasma proteins
Liver Failure
———————————————————————————————
———————————————————————————————
Arteriole
Capillary BP
Colloid OP
37 mmHg
25 mmHg
Capillary BP
Venule
17 mmHg
———————————————————————————————
———————————————————————————————
Fall in plasma proteins
Colloid OP falls
Reduced inward flow
Accumulation of fluid in tissues (oedema).
5
AHS C&T4
11/2/12
Starling’s Law — Importance
Interstitial Fluid (IF)"
Capillary wall is very permeable
Bulk flow of water and solutes
Rapid Plasma and Interstitial Fluid
interchange
BUT plasma is carefully regulated (kidney)
Hence Interstitial Fluid composition is
carefully regulated
Interstitial Fluid is the environment for all
cells.
Normal Daily Input and Output
Inputs
Ingestion
Fluid (1.25 litres)
Food (1 litre)
Metabolism (350 ml)
Total 2.6 litres
Outputs
Gut (Faeces 100 ml)
Urine (1.5 litres)
Breathing/Skin (900 ml)
Sweating (100 ml)
Total 2.6 litres
Which ones are used to regulate fluid volume?
6
AHS C&T4
11/2/12
Normal Daily Input and Output
Inputs
Ingestion
Fluid * (1.25 litres)
Food (1 litre)
Metabolism (350 ml)
* Regulated
Outputs
Gut (Faeces 100 ml)
Urine *(1.5 litres)
Breathing/Skin (900 ml)
Sweating (100 ml)
[Only regulated for heat balance,
NOT water]
[for water balance.]
Abnormal Inputs and Outputs
Inputs
Clinical
–  Injection/infusion
Excessive drinking
(‘polydipsia) *
Outputs
Gut (vomiting, diarrhoea)
Urine (diabetes insipidus *)
Breathing/Skin (burns)
Sweating (‘hyperhydria’ *)
Haemorrhage.
* Failure of regulation
7
AHS C&T4
11/2/12
Learning Outcomes !
1.  describe the composition of the three principal fluid compartments in the body
Intracellular (ICF) — high [K+], low [Na+]; PrInterstitial (IF) — high [Na+], low [K+]
Plasma — as IF, plus Proteins
2.  explain their respective compositions wrt the nature of the compartments’
boundaries
ICF/IF — cell membrane: semipermeable, Na+/K+ pump
Plasma/IF — capillary wall: bulk flow (not proteins).
3. explain the importance of the movement of material between compartments
Plasma/IF —plasma closely regulated (kidney) hence IF regulated
ICF/IF — cell membrane regulates ICF
4. explain the effects of tonicity on cell volume
Osmotic effect on cell volume of solutions of different concentrations of nonpenetrating solute: hypertonic— cell loses water…
Body Fluids — Summary!
Key importance — homeostasis"
Fluid balance (outline)!
Inputs/outputs"
Compartments [Core concept]!
Volumes"
Exchanges (Starling’s Law)"
Pathophysiology (oedema)"
Estimation."
8