162
U N I T 2
Integrative Body Functions
The osmotic activity that nondiffusible particles
exert in pulling water from one side of the semiper-
meable membrane to the other is measured by a unit
called an
osmole
. In the clinical setting, osmotic activ-
ity usually is expressed in milliosmoles (one thou-
sandth of an osmole) per liter. Each nondiffusible
particle, large or small, is equally effective in its abil-
ity to pull water through a semipermeable membrane.
Thus, it is the
number
, rather than the
size
, of the non-
diffusible particles that determines the osmotic activity
of a solution.
The osmotic activity of a solution may be
expressed in terms of either its osmolarity or osmo-
lality.
Osmolarity
refers to the osmolar concentration
in 1 L of solution (mOsm/L H
2
O) and
osmolality
to
the osmolar concentration in 1 kg of water (mOsm/
kg H
2
O).
2
Osmolarity is often used when referring to
fluids outside the body and osmolality for describing
fluids inside the body. Because 1 L of water weighs
1 kg, the terms
osmolarity
and
osmolality
are often
used interchangeably.
Serum osmolality, which is largely determined by
Na
+
and its attendant anions (Cl
–
and HCO
3
–
), normally
ranges between 275 and 295 mOsm/kg H
2
O. Blood urea
nitrogen (BUN) and glucose, which also are osmotically
active, usually account for less than 5% of the total
osmotic pressure in the ECF compartment. However,
this can change, such as when blood glucose levels are
elevated in persons with diabetes mellitus or when BUN
levels change rapidly in persons with chronic kidney
disease.
The term
tonicity
refers to the tension or effect
that a solution with impermeable solutes exerts
on cell size because of water movement across the
cell membrane.
3
Solutions to which body cells are
exposed can be classified as isotonic, hypotonic, or
hypertonic depending on whether they cause cells to
swell or shrink (Fig.
8-3
A). Cells placed in an
isotonic
solution
, which has the same effective osmolality as
the ICF, neither shrink nor swell. An example of an
isotonic solution is 0.9% sodium chloride. When cells
are placed in a
hypotonic solution
, which has a lower
osmolality than the ICF, they swell as water moves
into the cell (Fig.
8-3
B). When they are placed in a
hypertonic solution
, which has a greater osmolality
than the ICF, they shrink as water is pulled out of the
cell (Fig.
8-3
C).
Compartmental Distribution of Body
Fluids
Body water, which constitutes about 60% of body
weight in the adult, is distributed between the ICF and
ECF compartments.
1–4
The fluid in the ICF compartment
constitutes approximately 40% of body weight, and
that in the ECF approximately 20% of body weight.
The fluid in the ECF compartment is further divided
into two major subdivisions: the
plasma compartment
,
which constitutes approximately 5% of body weight,
and the
interstitial compartment
, which constitutes
approximately 14% of body weight (Fig. 8-4). The
fluid in the interstitial compartment acts as a transport
vehicle for gases, nutrients, wastes, and other materi-
als that move between the vascular compartment and
body cells. The interstitial fluid compartment also pro-
vides a reservoir from which vascular volume can be
maintained during periods of hemorrhage or loss of
vascular volume. An interstitial gel, which is a sponge-
like material supported by collagen fibers, fills the tissue
spaces and aids in even distribution of interstitial fluid.
Normally, most of the fluid in the interstitium is in gel
form.
2
The interstitial gel, which has a firmer consis-
tency than water, opposes the outflow of water from the
capillaries, preventing the accumulation of free water in
the interstitial spaces.
A
Isotonic solution
B
Hypotonic solution
C
Hypertonic solution
FIGURE 8-3.
Tonicity: Red cells undergo no change in size
in isotonic solutions
(A)
.They increase in size in hypotonic
solutions
(B)
and decrease in size in hypertonic solutions
(C)
.
300
200
100
0
Osmolarity - mOsm/L
Total body water = 60% body weight
Intracellular water
40% body weight
Extracellular water
20% body weight
14% 5% 1%
28 liters
Interstitial
10 liters
Plasma 3.5 liters
Transcellular 1 liter
FIGURE 8-4.
Approximate size of body compartments in a
70-kg adult.
