30. Fluid Compartments Flashcards
- Fluid compartments
varies
m v female
what are components
Describe the major fluid compartments
of the body in the adult.
> Total body water (TBW) varies depending on age, size, gender and fat content.
It is approximately 60% of body weight (BW)
in the average adult male (i.e. 42 L)
and 50% in the average adult female.
The remainder of the weight
is made up of protein,
minerals and fat.
The main components of TBW
are the extracellular and intracellular compartments.
> Intracellular fluid (ICF) makes up two-thirds of TBW (i.e. 28 L), and is contained within the phospholipid bilayer of the cell membrane.
> Extracellular fluid (ECF) makes up
one-third of TBW (i.e. 14 L).
This is divided into:
• interstitial fluid (ISF), which makes up 75% of the ECF (i.e. 9.5 L) and lies between cells, but outside the cell membrane
• plasma, making up 25%
of the ECF (i.e. 3.5 L),
contained within the vasculature
• transcellular fluids (TCF) (i.e.1 L), which are secreted fluids that are separated from the plasma by an epithelial layer (pleural, peritoneal, gastrointestinal fluids, CSF, intra-ocular fluids, sweat, saliva and bile),
the so-called ‘third space’
> Total blood volume (TBV) consists of plasma
and red cell volume, and is 5–6 L.
Table 30.1 Fluid compartments for a 70 kg
Table 30.1 Fluid compartments for a 70 kg male
Compartment % BW % TBW % ECF Volume (L)
TBW ±60 42
ICF 40 67 28
ECF 20 33 14
- ISF 15 10.5 75 9.5
- plasma 5 3.5 25 3.5
- TCF <1 1.0
Compare the adult with the
neonate fluid compartments.
Table 30.2 Comparison of neonatal and adult fluid compartments
Compartment Adult Neonate
TBW (% BW) 60 75–85
Fat (% BW) 20–25 5–15
ECF (% BW) 20 30–45
ICF (% BW) 40 <40
Plasma (% BW) 5 5
Note that in premature babies, ECF exceeds ICF.
Describe the cell membrane and capillary barriers and the movement of molecules across them.
Cell membrane:
this is a selectively permeable membrane
that separates the intracellular contents from the extracellular environment.
It consists
of a phospholipid bilayer
- with hydrophobic heads
on either side of the membrane
and hydrophilic tails facing inwards.
This arrangement allows
fat soluble molecules
to diffuse easily across the membrane,
but prevents the movement of polar molecules (amino acids, nucleic acids, carbohydrates, proteins and ions),
which is enabled by transmembrane
protein complexes such as
pores,
channels
and
gates.
The movement of substances can be either
‘passive’
or
‘active’,
i.e. with or without the expenditure of energy.
The transport mechanisms involved include:
> Passive osmosis and diffusion across a concentration gradient:
Small molecules/ions such as
CO2 and O2 can move
across the plasma
membrane by diffusion.
The concentration gradient
also sets up an
osmotic flow for water.
> Transmembrane protein channels and transporters:
Molecules such as
sugars,
amino acids and
certain products of metabolism may:
• Passively diffuse through
protein channels
(such as aquaporins in the case of water)
in facilitated diffusion, or
• Actively be pumped across
the membrane by
transmembrane
transporters.
Endocytosis
Cell membrane creates a vesicle,
capturing the substance
and internalising it,
e.g. phagocytosis.
This is a form of active transport.
> Exocytosis:
The membrane of a vesicle
fuses with the plasma membrane,
expelling its contents
into the extracellular environment,
e.g. hormones and enzymes.
Capillary wall:
Capillary wall:
consists of a single layer of
simple squamous epithelium
and a basement membrane
(basal lamina).
Capillaries connect arteries and
veins within organ systems
across a branched network
called the capillary bed.
The more metabolically active an organ is,
the larger the capillary bed.
Small molecules (<3 nm)
such as water,
oxygen and
carbon dioxide
cross the capillary wall through
the space between cells
(paracellular transport),
while larger molecules
(>3 nm) such as
albumin and other large proteins pass
through transcellular
transport carried inside vesicles.
There are three main types of capillaries:
> Continuous:
uninterrupted lining with
tight junctions and
complete basal lamina.
Allow passive diffusion of
lipid-soluble molecules and
movement of small molecules such as
water and ions through intercellular clefts.
Skeletal muscle and skin
have numerous transport vesicles,
whereas CNS (blood–brain barrier) has few,
so sealing the paracellular space.
Fenestrated
> Fenestrated:
endothelial cells have
pores or windows
(60–80 nm in diameter)
and
a complete basal lamina.
Allow a limited amount of proteins to diffuse.
They are located in intestines,
pancreas,
endocrine glands
and renal glomeruli.
Sinusoidal
> Sinusoidal:
large open-pore (30–40 μm in diameter) capillaries,
large gaps between cell junctions
and a discontinuous basal lamina.
Allow red and white blood cells
(7.5–25 μm diameter) and
serum proteins to pass.
Present in bone marrow, lymph nodes, liver, spleen and adrenal glands.
How are the body compartment volumes estimated?
Dilutional techniques are
used to estimate compartment volumes.
An indicator dye is injected
into the compartment to be measured.
The dye should distribute
throughout that compartment,
but remain contained within it.
The concentration of the dye
is measured and the
mass administered is known.
Thus, using the formula for
volume of distribution
(Vd = mass of dye/ concentration),
the compartment volume can be estimated.
Some compartments are derived (ICF, ISF and TBV).
TBW
Freely diffusible substance Deuterium oxide
Antipyrine
ECF
Substances that do not enter cells
Inulin
Thiocyanate
Thiosulphate
