Page 177 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Reabsorption of Water, Formation of pertonic towards the papillae (see below) and
Concentrated Urine if the vasa recta are permeable to water. Part of
the water diffuses by osmosis from the de-
The glomeruli filter around 180 L of plasma scending vasa recta to the ascending ones,
water each day (= GFR; ! p. 152). By compari- thereby “bypassing” the inner medulla (! A4).
.
son, the normal urine output (VU) is relatively Due to the extraction of water, the concentra-
small (0.5 to 2 L/day). Normal fluctuations are tion of all other blood components increases as
.
called antidiuresis (low VU) and diuresis (high the blood approaches the papilla. The plasma
.
VU; ! p. 172). Urine output above the range of osmolality in the vasa recta is therefore con-
Kidneys, Salt, and Water Balance plasma and glomerular filtrate is about rises. Conversely, substances entering the
normal is called polyuria. Below normal output
tinuously adjusted to the osmolality of the sur-
is defined as oliguria (! 0.5 L/day) or anuria
rounding interstitium, which rises towards the
papilla. The hematocrit in the vasa recta also
(! 0.1 L/day). The osmolality (! p. 377) of
290 mOsm/kg H 2O (= P osm); that of the final
blood in the renal medulla diffuse from the as-
urine (U osm) ranges from 50 (hypotonic urine in
cending to the descending vasa recta, provided
extreme water diuresis) to about 1200 mOsm/
the walls of both vessels are permeable to
kg H 2O (hypertonic urine in maximally con-
them (e.g., urea; ! C). The countercurrent ex-
change in the vasa recta permits the necessary
centrated urine). Since water diuresis permits
supply of blood to the renal medulla without
the excretion of large volumes of H 2O without
the simultaneous loss of NaCl and other sol-
renal medulla and hence impairing the urine
“free water clearance” (C H 2 O). This allows the
concentration capacity of the kidney.
7 utes, this is known as “free water excretion”, or significantly altering the high osmolality of the
kidney to normalize decreases in plasma
In a countercurrent multiplier such as the
loop of Henle, a concentration gradient be-
osmolality, for example (! p. 170). The C H 2 O
represents to the volume of water that could be tween the two limbs is maintained by the ex-
theoretically extracted in order for the urine to penditure of energy (! A5). The countercur-
reach the same osmolality as the plasma: rent flow amplifies the relatively small
.
C H 2 O " VU (1–[U osm/P osm]). [7.11] gradient at all points between the limbs (local
gradient of about 200 mOsm/kg H 2O) to a rela-
Countercurrent Systems tively large gradient along the limb of the loop
A simple exchange system (! A1) can consist of (about 1000 mOsm/kg H 2O). The longer the
two tubes in which parallel streams of water flow, one loop and the higher the one-step gradient, the
cold (0 #C) and one hot (100 #C). Due to the exchange steeper the multiplied gradient. In addition, it
of heat between them, the water leaving the ends of is inversely proportional to (the square of) the
both tubes will be about 50 #C, that is, the initially flow rate in the loop.
steep temperature gradient of 100 #C will be offset.
In countercurrent exchange of heat (! A2), the
fluid within the tubes flows in opposite directions. Reabsorption of Water
Since a temperature gradient is present in all parts of Approximately 65% of the GFR is reabsorbed at
the tube, heat is exchanged along the entire length. the proximal convoluted tubule, PCT (! B and
Molecules can also be exchanged, provided the wall p. 157 D). The driving “force” for this is the re-
of the tube is permeable to them and that a concen- absorption of solutes, especially Na and Cl . –
+
tration gradient exists for the substance. This slightly dilutes the urine in the tubule, but
If the countercurrent exchange of heat occurs in a
hairpin-shaped loop, the bend of which is in contact H 2O immediately follows this small osmotic
with an environment with a temperature different gradient because the PCT is “leaky” (! p. 154).
from that inside the tube (ice, ! A3), the fluid exit- The reabsorption of water can occur by a para-
ing the loop will be only slightly colder than that cellular route (through leaky tight junctions)
entering it, because heat always passes from the or transcellular route, i.e., through water chan-
warmer limb of the loop to the colder limb. nels (aquaporin type 1 = AQP1) in the two cell
Countercurrent exchange of water in the vasa membranes. The urine in PCT therefore re-
recta of the renal medulla (! A6 and p. 150) oc- mains (virtually) isotonic. Oncotic pressure
164
curs if the medulla becomes increasingly hy- (! p. 378) in the peritubular capillaries pro-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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