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