Body Fluids
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Transcript of Body Fluids
Body fluids
Dr. Deepthi de SilvaSenior Lecturer
Department of physiology
At the end of these lectures you should be able to
• Describe the normal composition of the body• Outline the distribution of water in the body• Explain the terms osmotic pressure,
osmolarity & osmolality• Outline the distribution of electrolytes in the
fluid compartments• List fluids that are hypo, iso and hypertonic to
plasma• Describe the outcome of expanding or
reducing the body fluid compartments• Outline the reason for using colloid, 0.9%
saline and 5% dextrose as fluid replacement
What does the body contain?
Average male (70 kg)Water 60%Fat 15%Protein 18%Minerals 7%
Total body water
Intracellular(fluid inside cells)
Extracellular
Interstitial fluid (fluid around cells)
Intra vascular fluid
(fluid inside blood vessels)
Transcellular fluid
Total body water (60% of body weight)
• Intracellular 40% of body
weight • Extracellular fluid (20% of body weight)
• Trans cellular fluid
Interstitial fluid (15% of body
weight)
Plasma (5% of body
weight)
Distribution of body fluids (70kg man)
Total body water(TBW)42L
Intracellular(ICF) ~ 28L (2/3rd)
Extracellular fluid(ECF) ~ 14L
(1/3rd)
Transcellular fluid 0.7L
Plasma~3.5L
Interstitial fluid
~ 10.5L
Physiological variations in TBW• Age: TBW as % of body weight decreases with age• Sex: Male > female (adult Male 55-60%; Female 45-50%)
Male Female10-18 y 59% 57%19-40 y 61% 51%41-60 y 55% 47%>60 y 52% 46%
• Fat content: greater the fat content less the TBW as a % of body weight
Measuring body fluids• Read up! (Ganong or Guyton
textbook of physiology)• Remember that we use a specified
substance & see how it is diluted in a fluid compartment [dilution principle]– Albumin stays in plasma– Inulin in ECF– Heavy water in ALL compartments
Units for measuring solute concentrations
• Mole (mol): molecular weight of a substance in grams
e.g. NaCl = 23+35.5 = 58.5g• Equivalent (eq) = 1 mol of ionised
substance/valence e.g. Ca2+ = 40/2 = 20g• Normality = gram equivalent in 1 Litre e.g. 1N solution of HCl = 1 +35.5g
=36.5g
Solutes diluted in body fluids
Plasma
ISF ICF
Volume 3.5L 12.0L 26.5LNa+ mmol/L 142 145 10K+ mmol/L 4 4 150Ca 2+ mmol/L 2 1 40Mg2+ mmol/L 101 114 15Cl- mmol/L 27 31 10HCO3
- mmol/L 1 1 100PO4 mmol/L 0.5 0.5 20Proteins 2 <0.1 60
Osmotic pressure
• Solution A has a lower concentration of solute and higher concentration of water (solvent)
• Water moves from the high to low water concentration area
• Called osmosis• Osmotic pressure: pressure required to stop
this movement of water
Solution A
Solution B
Solution A
Solution B
Membrane permeable to water
• Measured as Osmole = Gram molecular weight of a substance Number of free particles released in solution
• Osmolarity: number of osmoles/L of solvent
• Osmotic pressure number of particles temperature
P (osmotic pressure) = n (no. of particles). R.T(temp)
V (volume)
Osmolarity
• Osmolality: Number of osmoles per kg of solvent
• In plasma, usually expressed as osmoles/L
Measuring of osmolal concentration: - degree to which the freezing point is depressed - 1 mol ideal solution depresses it by 1.86oC - Number of mosm/L = freezing point
/0.00186
Osmolality
Tonicity• Osmolality of a solution with respect to plasma • Osmolal concentration of plasma 290mosm/L -
plasma freezing point is -0.54oC • Equal to 7.3 atmospheres pressure against water
• When an isotonic solution & plasma are separated by a semi permeable membrane, there is no net flow of water
• Fluids isotonic to plasma•0.9% NaCl•5% dextrose•King coconut water
Plasma osmolality
• Hypotonic fluids have a lower osmolality than plasma <285mosm/L
• Hypertonic fluids have a higher osmolality than plasma >300mosm/L
Plasma osmolality (mosm/L) = 2[Na+] + 0.055 [Glc] + 0.36 [urea](meq/L) (mg/dL) (mg/dL)
Changes in fluid balance• Dehydration: loss of fluid from the body• Isotonic contraction (dehydration)
– Loss of water and electrolytes in equal proportions e.g. severe bleeding, burns
ECFNo net
movement of water
ICFVolume Decreased No change
Osmolality No change No change
Hypertonic dehydration• Loss of water in excess of electrolytes
(especially sodium)• Fluid lost has lower tonicity compared
to plasma e.g. severe sweating
ECFWater moves
from ICF to ECF
ICFVolume Decrease
dDecrease
d
Osmolality Increased
Increased
Hypotonic dehydration • Loss of more electrolytes than water
e.g. Addison disease
ECFWater moves
from ECF to ICF
ICFVolume Decrease
d Increased
Osmolality
Decreased
Decrease
d
Fluid replacement• If the ECF compartment is reduced
(i.e. isotonic contraction) fluid replacement has to remain in the ECF
• 0.9% saline is commonly used for this purpose
• 1L of saline will be distributed according to the ECF compartment volume: 250mL to plasma
750 mL to interstitial fluid
The use of colloids• Colloid contains proteins• This solution will remain in the
circulation (i.e. plasma compartment) and will not leave it to enter the ISF or ICF compartments
• not always available
What about 5% dextrose?• After infusion, the body's cells use
the glucose to generate energy • Only the water is left behind!• Water is distributed in all the fluid
compartments• After 1L is infused
– ~ 660mL enters the ICF compartment– ~ 340mL the ECF
~ 85mL to plasma, 255 to ISF
Tissue fluid formation and reabsorption
• The capillaries are tiny blood vessels • contain an arteriolar and venular end• Tissue fluid is formed in the capillaries-
fluid comes out at the arterial end and goes back in at the venous end
• Pressure of blood forces fluid out “hydrostatic pressure”
• Osmotic pressure in the blood brings fluid in to capillary “colloid osmotic pressure”
Starling’s forces• Hydrostatic pressure
– Pressure exerted by blood in the capillary
– Increased when blood pressure is high– High at arterial end and lower at venous
end• Colloid osmotic pressure
– Pressure exerted by proteins in plasma (do not cross the capillary membrane)
Arterial end Venous end
Capillary hydrostatic pressure
Interstitial space
hydrostatic pressure
Capillary colloid osmotic
pressure
Interstitial space
colloid osmotic pressure
Starling’s forces
Q = rate of fluid formationKf = permeability coefficientS = surface areaP = hydrostatic pressure = colloid osmotic pressurec = capillaryi = interstitial fluid
Q = kf. S [(Pc+ i) – (Pi+ c)]
Starling’s forces
Pc
37mmHg
i0mmHg
c25mmH
g
Pi1mmHg
Pc
17mmHg
Pi1mmHg
i0mmHg
c25mmH
g
Arteriolarend
Venular end
Tissue fluid formation & absorption
Arteriolar endQ=kf.s[(37+0) – (25+1)]
Q= 11mmHgNet outward pressure
Venular endQ=kf.s[(17+0) – (25+1)]
Q= -9mmHgNet inward movement
15% tissue fluidDrained by
lymphatic system
Oedema formation• Accumulation of excessive
amounts of fluid in the interstitial spaces
• Exceeds the capacity of the lymphatic vessels to drain it
• Caused by altered Starling’s forces
Mechanisms of oedema formation
Factors that are increased
• Increased capillary hydrostatic pressure
• Increased capillary permeability
Factors that are reduced
• Decreased capillary colloid osmotic pressure
• Decreased lymphatic drainage
Increased capillary hydrostatic pressure
• Increase in hydrostatic pressure at the venous end
• Causes include– Venous thrombosis (blood clot
obstructing venous drainage) - Right ventricular failure: systemic oedema– Left ventricular failure: pulmonary
oedema
Oedema due to reduced colloid osmotic pressure
• Hypoalbuminaemia reduced plasma albumin
level– Reduction in formation (in liver)– Increase in loss (from kidney,
intestines, skin)
Increased capillary permeability causing oedema
• Mainly causes localised oedema– Following insect bites → histamine
release– Allergic conditions– Capillary damage
Lymphatic obstruction as a cause of oedema
• Called lymphoedema– Lymphatic vessels obstructed e.g.
by filarial parasite– After removal of lymphatic vessels
e.g. after surgery for cancer
Clinical examination• Oedema usually causes an increase in
body weight • Pit formed in affected tissue• Lymphoedema does NOT cause a pit