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THE OSMOTIC GRADIENT IN KIDNEY MEDULLA:
A RETOLD STORY
S. Kurbel,1 K. Dodig,1 and R. Radić2
Departments of 1Physiology and 2Anatomy, Osijek Medical Faculty, 31000 Osijek, Croatia
T
ADV PHYSIOL EDUC 26: 278 –281, 2002;
10.1152/advan.00037.2001.
Key words: kidney; countercurrent mechanisms; education
stitial hydrostatic pressure and compromise kidney
architecture and function. Solute and water traffic
in described horizons is governed by osmotic forces
and by the positive interstitial pressure. Increases
in urea availability (diet, etc.), sodium or water
depletion, and different circulatory phenomena are
all able to modulate this traffic and thus change the
resulting osmotic concentration gradients in kidney
medulla.
The building up of an osmotic gradient in the kidney
medulla is among the most complex mechanisms presented to students of physiology. During lectures,
everything seems simple, but reinterpretation is often
difficult. Students use different textbooks, but this
practice is of limited value, because they often reproduce similar images taken from a common source.
This article is an attempt to make the story easier by
describing it from another perspective.
The conventional approach is to describe separately
the countercurrent multiplier in nephron tubules, the
countercurrent exchanger in vasa recta, and the role
of collecting ducts (1, 2, 3). Schematic presentations
of consecutive steps are often used to explain the
development of the osmotic gradient.
We will try to describe the levels (or horizons) of
different osmolarity (as shown in Fig. 1 and cross
sections in Fig. 2). Horizon 0 is the cortical level
above the medulla with interstitial osmolarity of 300
mOsm/l, containing cortical nephrons with short
loops and ordinary peritubular capillaries. In this
level, blood flow through the peritubular capillaries
washes out any accumulation of interstitial solutes.
In the presented concept, kidney tissue space is
considered to be limited, with a positive interstitial
hydrostatic pressure. In other words, all of the fluid
that goes into the interstitial space must leave it.
Any accumulation of fluid would increase the inter-
Horizons 1–3 contain collecting ducts, tubules, and
vasa recta of juxtamedullary nephrons. We can begin with Horizon 1 at the corticomedullary border
(interstitial osmolarity of 300 –500 mOsm/l). Horizon 2 is deeper in the medulla, with osmolarity of
DESCRIPTION
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his article is an attempt to simplify lecturing about the osmotic gradient in the
kidney medulla. In the model presented, the kidneys are described as a
limited space with a positive interstitial hydrostatic pressure. Traffic of water,
sodium, and urea is described in levels (or horizons) of different osmolarity, governed
by osmotic forces and positive interstitial pressure. In this way, actions of the
countercurrent multiplier in nephron tubules and of the countercurrent exchanger
in vasa recta are integrated in each horizon. We hope that this approach can help
students to better accept conventional presentations in their textbooks.
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interstitial space is maintained, because sodium entering from the ascending tubule and blood flow in the
ascending vasa recta (from deeper medullary horizons) is balanced by sodium removal by blood leaving
Horizon 1 via the descending vasa recta and the ascending vasa recta.
500 – 800 mOsm/l, whereas Horizon 3 in the inner
medulla is the place of urea circulation (800 –1,200
mOsm/l).
HORIZON 0
In the cortical level, blood flows through the net of
peritubular capillaries, in which blood is near isosmotic and in equilibrium with the interstitial space,
taking out water or solute surplus. Hyperosmolarity
coming by interstitial diffusion from the juxtamedullary Horizon 1 is being continuously washed
away.
Interstitial hyperosmolarity pulls water from descending vasa recta and descending tubule. Both
water-permeable structures come from the upper
level of less osmolarity. Interstitial water is drawn
into the more hyperosmolar content in the ascending vasa recta that comes from the lower and more
osmolar levels. Water from the hypotonic content
of the collecting duct can enter the interstitium
only in the presence of antidiuretic hormone
(ADH).
HORIZON 1
The interstitial osmotic pressure is 300 –500 mOsm/l,
caused by sodium pumped from the ascending tubule
to the interstitial space. The ascending tubule content
is of decreasing osmolarity, caused by the pumping
process that simultaneously increases osmolarity in
the interstitial space. Sodium balance in the Horizon 1
The interstitial hyperosmolarity generated in Horizon
1 spreads to the neighboring levels, up (Horizon 0)
and down (Horizon 2).
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FIG. 1.
Horizons of different osmolarity in kidney tissue (Td, descending tubule, Ta, ascending
tubule, Pc, peritubular capillary, Vd, descending vasa recta, Va, ascending vasa recta, CD,
collective duct).
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HORIZON 2
The situation from Horizon 1 is just accentuated in
this more osmotic horizon. Sodium is pumped from
the ascending tubule filtrate. Interstitial sodium balances with sodium concentrations in both vasa
recta. Water from the descending tubule and from
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FIG. 2.
Water and solute traffic in horizons of different osmolarity. Horizon 0: in the cortical level, blood flows through the
network of peritubular capillaries, in which blood is near isosmotic and in equilibrium with the interstitial space, taking
out water or solute surplus. Horizon 1: interstitial osmotic pressure is 300 –500 mOsm/l, caused by sodium pumping out
from the ascending tubule to the interstitial space. The ascending tubule content is of decreasing osmolarity, caused by
the pumping process that increases osmolarity in the interstitial space. Some sodium enters the descending vasa recta,
and some leaves the ascending vasa recta along its concentration gradients. Interstitial hyperosmolarity pulls water from
descending vasa recta and descending tubule. Both water-permeable structures come from the upper level of less
osmolarity. Interstitial water is taken out by the more hyperosmolar content in the ascending vasa recta that comes from
the lower and more osmolar levels. Water from the hypotonic content of the collecting duct can enter the interstitium
only in the presence of antidiuretic hormone ADH). Horizon 2: sodium is pumped out from the ascending tubule.
Interstitial sodium balances with sodium concentrations in both vasa recta. Water from the descending tubule and from
descending vasa recta enters the interstitial space. It is forced to leave by the even more hyperosmotic content in the
ascending vasa recta. ADH is needed to allow water from collecting ducts to enter interstitial space. Horizon 3: urea
recirculates in the inner medulla, building a stronger osmotic gradient. It enters interstitial space from the collecting duct
forced by its concentration gradient, if ADH is present and water is taken out from the collecting duct, building up the
urea concentration. Increased interstitial osmolarity pulls the remaining water from descending structures. Interstitial
water is taken away by the even more osmolar content in the ascending vasa recta.
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We thank Drs. C. E. Ott and D. F. Speck from the Department of
Physiology, University of Kentucky, Lexington, Kentucky.
descending vasa recta enters the interstitial space. It is
forced to leave by the even more hyperosmotic content in
the ascending vasa recta. ADH is needed to allow water
from collecting ducts to enter the Horizon 2 interstitial
space.
The work on this paper was supported by the Croatian Ministry of
Science (Grant 127051).
Address for reprint requests and other correspondence: S. Kurbel,
Dept. of Physiology Osijek Medical Faculty, J Huttlera 4, 31000
Osijek, Croatia (E-mail: [email protected]).
HORIZON 3
Received 28 September 2001; accepted in final form 20 August
2002
References
1. Ganong WF. Review of medical physiology. New York: Lange
Medical Books/McGraw-Hill, 1999, p. 691– 693.
2. Guyton AC and Hall JE. Human Physiology and Mechanisms of Disease. Philadelphia, PA: Saunders, 1997, p. 236 –
241.
3. Jackson BA and Ott CE. Renal System. Madison, CT: Fence
Creek Publishers 1999, p. 84 –90.
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Urea recirculates in the inner medulla, building a
stronger osmotic gradient. It enters the interstitial
space from the collecting duct, following the concentration gradient created by the ADH-sensitive
water reabsorption. Increased interstitial osmolarity pulls the remaining water from the descending
structures. Interstitial water is taken away by the
even more osmolar content in the ascending vasa
recta.