Extracellular Fluid Composition
and Volume
Learning Objectives
• Know the distribution of bodily fluids and
composition of intracellular and extracellular
fluid.
• Know how to calculate the volume of the
different fluid compartments.
• Know how to calculate the shifts in osmolarity.
• Know what causes the 2 types of edema.
• Know what mechanisms the body uses to
prevent edema.
Water Balance
The body must maintain a relatively constant volume and composition of the bodily
fluids under a wide variety of conditions.
Distribution of Bodily Fluids
• Most of the H2O is
intracellular.
• The total body H2O is ~
60% of the body weight
(~ 42 L).
• The % depends on age,
gender, and degree of
obesity.
Composition of Intracellular and Extracellular
Fluid
Note: The intracellular compartment of Ca2+ is very low compared to the extracellular.
Ca2+ is a very important signaling molecule for electrical signals and protein activation.
Calculating Compartment fluid Volume
This works only if the indicator uniformly diffused and only into the measured fluid
compartment, and if the indicator itself is not metabolized or excreted.
Calculating Total Body Water
• Use radioactive water (3H2O) or heavy water
(2H2O).
• This will mix with the total body water in just a
few hours and the dilution method (previous
slide) can be used.
Determine Extracellular and Intracellular fluid
Volume
• For extracellular volume, use a substance that
does not cross cell membranes, but does pass
through capillary pores.
- Radioactive insulin is used for this purpose.
• Then, calculate intracellular volume from:
Intracellular volume = total body water –
extracellular volume
Plasma and Interstitial Fluid Volume
• For plasma volume, use an indicator that does
not diffuse across plasma membranes or
capillary pores.
- 125I-albumin is used for this purpose.
• Interstitial fluid volume can then be calculated
from: interstitial fluid volume = extracellular
fluid volume – plasma volume
Measure Blood Volume
• Remember, hematocrit can be measured by
centrifuging
Osmotic Pressure and Osmosis
• Chapter 25 reviews some basic concepts of
osmosis and osmotic pressure.
• You should already know this material, plus,
we used it to cover osmotic forces that
determine the fluid flow at capillaries.
• For cells, remember that H2O passively
diffuses across plasma membranes, but not
most ions and solutes.
Adding Different Osmotic Solutions to the
Extracellular Fluid
Calculating Shifts in Osmolarity
• What happens when 2L of 3% solution of NaCl is added to the
extracellular fluid of a 70-kg person, whose initial plasma
osmolarity is 280 mOsm/L?
• Initially, the total body fluid is 42L, the extracellular fluid is 14L at
280 mOsm/L (3,920 mOsm) and the intracellular fluid is 28L at 280
mOsm/L (7,840 mOsm).
• The NaCl added was 2L x 3g/100ml = 60g or 1,026 mol (MW of NaCl
= 58.5 g/mol). 1.026 mol NaCl = 2,051 mOsm (1.026 x 2 x 1,000)
• Now, the total body fluid is 44L and the extracelular fluid is 16L with
5,971 mOsm at 373 mOsm/L.
• Then, the intracellular and extracellular concentrations will
equilibrate to 313.9 mOsm/L (13,811 mOsm/44L).
• Thus, the extracellular fluid becomes 19.02L (5,971 mOsm/313.9
mOsm/L) and the intracellular fluid becomes 27.98L (7,840
mOsm/313.9 mOsm/L).
Edema
• Edema is the presence of excess fluid in the
body tissues. Broadly, there are 2 types of
edema:
- Intracellular edema (less common)
- Extracellular edema
Intracellular Edema
• Can be caused by depressed metabolism or
inadequate nutrition, such as decreased blood
flow.
• The membrane pumps become depressed and
the cells are unable to pump out the Na+ that
leaks in, causing the edema.
Extracellular Edema
Caused by:
Increases in capillary
filtration
Increase capillary filtration coefficient
Increase capillary hydrostatic pressure
Decrease plasma osmotic pressure
Blockage of lymph flow
The net rate of capillary filtration in
the entire body is ~ 2ml/min, which is
returned to the circulation by the
lymphatic system
Changes in Capillary Pressures
• You should already be familiar with these factors.
• Factors that increase capillary hydrostatic pressure:
- Excessive retention of salt and H2O
- High venous pressure
- Decreased arteriolar resistance
• Factors that decrease capillary osmotic pressure:
- Loss of plasma proteins (in urine, from deamaged skin,
failure to produce proteins as occurs in cirrhosis or
malnutrition).
• Factors that increase capillary permeability:
- Immune reactions (histamine release)
- Toxins
- Burns
Lymphatic Blockage
• In the body, the mean capillary forces show a net
filtration pressure of 0.3 mm Hg, tending to move fluid
outward through the capillary pores.
• The net rate of filtration in the entire body is ~ 2 ml/min,
which is returned to the circulation by the lymphatic
system.
• Edema that occurs by blockage of the lymphatic can be
especially severe, because the lymphatic system returns
proteins to the blood that pass through the capillaries.
• Thus, in addition to the back-up of fluid, the osmotic
pressure of the interstitial fluid increases.
Factors that Cause Blockage of Lymph Return
• Cancer
• Infection (elephatiasis is caused by a
mosquito-born filaria worm)
• Congenital absence or abnormality of the
lymph vessels.
Factors that Prevent Edema
• Low compliance of the interstitium under
normal conditions.
• Increased lymph flow.
• Washdown of interstitial fluid protein.
Interstitium
Low Compliance of Interstitium
• This is established by the interstitial gel, which is
the proteoglycan polymers in the interstitium.
• This prevents fluid from flowing easily.
• As a result, the addition of extra fluid causes a
large incease in interstitial hydrostatic pressure
(decreasing capillary filtration rate).
• However, too much interstitial fluid creates
channels in the gel, increasing the compliance of
the interstitium.
• Note – the interstitial gel also helps to prevent
excessive fluid from flowing to your feet when
standing.
Low Compliance of Interstitium
Increased Lymph Flow
• When fluid begins to accumulate in the
interstitium, lymph flow can increase 10- to
50-fold.
• The increased lymph flow also helps decrease
the [interstitial proteins], called “washdown”
(see text).
• “Washdown” occurs because with increased
flow, proteins are removed faster than they
filter acrosss the capillaries.