Page 37 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Osmosis, Filtration and Convection
In filtration (! B),
Water flow or volume flow (J V) across a mem- J V ! K f ! ∆P–∆π [1.13]
brane, in living organisms is achieved through Filtration occurs through capillary walls,
osmosis (diffusion of water) or filtration. They which allow the passage of small ions and
can occur only if the membrane is water-per- molecules (σ = 0; see below), but not of plasma
meable. This allows osmotic and hydrostatic proteins (! B, molecule x). Their concentra-
pressure differences (∆π and ∆P) across the tion difference leads to an oncotic pressure
membrane to drive the fluids through it. difference (∆π) that opposes ∆P. Therefore, fil-
Osmotic flow equals the hydraulic conduc-
tration can occur only if ∆P " ∆π (! B, p. 152,
Fundamentals and Cell Physiology calculated using van’t Hoff’s law, as modified are carried along with the water flow of osmo-
tivity (K f) times the osmotic pressure differ-
p. 208).
Solvent drag occurs when solute particles
ence (∆π) (! A):
[1.11]
J V ! K f ! ∆π
The osmotic pressure difference (∆π) can be
sis or filtration. The amount of solvent drag for
solute X (J X) depends mainly on osmotic flow
by Staverman:
(J V) and the mean solute activity a x (! p. 376)
∆π ! σ ! R ! T ! ∆C osm,
[1.12]
at the site of penetration, but also on the
where σ is the reflection coefficient of the par-
degree of particle reflection from the mem-
ticles (see below), R is the universal gas con-
brane, which is described using the reflection
stant (! p. 20), T is the absolute temperature,
coefficient (σ). Solvent drag for solute X (J X) is
–1
tween the lower and higher particle concen-
J x ! J V (1 – σ) a x [mol ! s ]
trations, C osm –C osm (! A). Since ∆C osm, the
a
b
Larger molecules such as proteins are entirely
1 and ∆C osm [osm ! kgH 2O ] is the difference be- therefore calculated as –1 [1.14]
driving force for osmosis, is a negative value, J V reflected, and σ = 1 (! B, molecule X). Reflec-
is also negative (Eq. 1.11). The water therefore tion of smaller molecules is lower, and σ$ 1.
flows against the concentration gradient of the When urea passes through the wall of the
solute particles. In other words, the higher proximal renal tubule, for example, σ =
concentration, C osm, attracts the water. When 0.68. The value (1–σ) is also called the sieving
b
the concentration of water is considered in os- coefficient (! p. 154).
a
mosis, the H 2O concentration in A,a, C H 2 O, is Plasma protein binding occurs when small-
b
a
b
greater than that in A,b, C H 2 O. C H 2 O –C H 2 O is molecular substances in plasma bind to pro-
therefore the driving force for H 2O diffusion teins (! C). This hinders the free penetration
(! A). Osmosis also cannot occur unless the of the substances through the endothelium or
reflection coefficient is greater than zero the glomerular filter (! p. 154 ff.). At a glo-
(σ " 0), that is, unless the membrane is less merular filtration fraction of 20%, 20% of a
permeable to the solutes than to water. freely filterable substance is filtered out. If,
Aquaporins (AQP) are water channels that however, 9/10 of the substance is bound to
permit the passage of water in many cell mem- plasma proteins, only 2% will be filtered during
branes. A chief cell in the renal collecting duct each renal pass.
contains a total of ca. 107 water channels, com- Convection functions to transport solutes
prising AQP2 (regulated) in the luminal mem- over long distances—e.g., in the circulation or
brane, and AQP3 and 4 (permanent?) in the ba- urinary tract. The solute is then carried along
solateral membrane. The permeability of the like a piece of driftwood. The quantity of solute
epithelium of the renal collecting duct to transported over time (J conv) is the product of
–1
3
water (! A, right panel) is controlled by the in- volume flow J V (in m ! s ) and the solute con-
–3
sertion and removal of AQP2, which is stored in centration C (mol ! m ):
the membrane of intracellular vesicles. In the J conv ! J V ! C [mol ! s ]. [1.15]
–1
presence of the antidiuretic hormone ADH (V 2 The flow of gases in the respiratory tract, the
receptors, cAMP; ! p. 274), water channels transmission of heat in the blood and the re-
are inserted in the luminal membrane within lease of heat in the form of warmed air occurs
minutes, thereby increasing the water perme- through convection (! p. 222).
24 ability of the membrane to around 1.5 # 10 – 17
L s – 1 per channel.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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