DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Tubular
lumen
Intercellular space
Interstitial
space
675
Na+
Zonula occludens
(tight junctions)
P
Solvent
Drag
I
H 2
O
X
Y
S
A
CH
WP
OSMOSIS (transcellular)
OSMOSIS (paracellular)
Luminal
Membrane
S,A,U
or CH
Y
X
WP
PD
+/ _
H 2 O
Simple diffusion (paracellular)
Simple diffusion (transcellular)
Mediated transport
Basolateral
Membrane
Simple diffusion
Mediated transport
CONVECTION
S,A,U
or CH
Na+
URINE
+ –
– +
Figure 25–3. Generic mechanism of renal epithelial cell transport (see text for details). S, symporter; A, antiporter; CH, ion channel;
WP, water pore; U, uniporter; ATPase, Na + ,K + -ATPase (sodium pump); X and Y, transported solutes; P, membrane-permeable (reabsorbable)
solutes; I, membrane-impermeable (nonreabsorbable) solutes; PD, potential difference across indicated membrane or cell.
Na +
S,A,U
or CH
ATP
X
K + ATPase
ADP
K +
CH
S,A
P
CHAPTER 25
REGULATION OF RENAL FUNCTION AND VASCULAR VOLUME
antiporters, uniporters, and channels) of solutes into the intercellular
and interstitial spaces.
5. Accumulation of Na + and other solutes in the intercellular
space creates a small osmotic pressure differential across the
epithelial cell. In water-permeable epithelium, water moves
into the intercellular spaces driven by the osmotic pressure differential.
Water moves through aqueous pores in both the luminal
and the basolateral cell membranes, as well as through tight
junctions (paracellular pathway). Bulk water flow carries some
solutes into the intercellular space by solvent drag.
6. Movement of water into the intercellular space concentrates
other solutes in the tubular fluid, resulting in an electrochemical
gradient for these substances across the epithelium.
Membrane-permeable solutes then move down their electrochemical
gradients into the intercellular space by both the
transcellular (e.g., simple diffusion, symporters, antiporters,
uniporters, and channels) and paracellular pathways.
Membrane-impermeable solutes remain in the tubular lumen
and are excreted in the urine with an obligatory amount of
water.