Chapter 25
The Urinary System
973
25
Regulation of Urine Concentration
and Volume
Describe the mechanisms responsible for the medullary
osmotic gradient.
Explain formation of dilute versus concentrated urine.
From day to day, and even hour to hour, our intake and loss of
fluids can vary dramatically. For example, when you run on a hot
summer day, you dehydrate as you rapidly lose fluid as sweat. On
the other hand, if you drink a pitcher of lemonade while sitting on
the porch, you overhydrate. In response, the kidneys make adjust-
ments to keep the solute concentration of body fluids constant at
about 300 mOsm, the normal osmotic concentration of blood
plasma. Maintaining constant osmolality of extracellular fluids is
crucial for preventing cells, particularly in the brain, from shrink-
ing or swelling from the osmotic movement of water.
Recall from Chapter 3 (p. 72) that a solution’s osmolality is
the concentration of solute particles per kilogram of water. Be-
cause 1 osmol (equivalent to 1 mole of particles) is a fairly large
unit, the milliosmol (mOsm) (mil
0
e-oz
9
mōl), equal to 0.001 os-
mol, is generally used. In the discussion that follows, we use
mOsm to indicate mOsm/kg.
Te kidneys keep the solute load of body fluids constant by
regulating urine concentration and volume. When you dehy-
drate, your kidneys produce a small volume of concentrated
urine. When you overhydrate, your kidneys produce a large vol-
ume of dilute urine.
Te kidneys accomplish this feat using countercurrent mech-
anisms. In the kidneys, the term
countercurrent
means that fluid
flows in opposite directions through adjacent segments of the
same tube connected by a hairpin turn* (see Figure 25.16). Tis
arrangement makes it possible to exchange materials between
the two segments.
±wo types of countercurrent mechanisms determine urine
concentration and volume:
Te
countercurrent multiplier
is the interaction between
the flow of filtrate through the ascending and descending
limbs of the long nephron loops of juxtamedullary nephrons.
Te
countercurrent exchanger
is the flow of blood through
the ascending and descending portions of the vasa recta.
Tese countercurrent mechanisms establish and maintain
an osmotic gradient extending from the cortex through the
depths of the medulla. Tis gradient—the
medullary osmotic
gradient
—allows the kidneys to vary urine concentration
dramatically.
How do the kidneys form the osmotic gradient?
Focus on
the Medullary Osmotic Gradient
(Figure 25.16)
explores the
answer to this question.
Check Your Understanding
10.
In which part of the nephron does most reabsorption occur?
11.
How are primary and secondary active transport processes
(both shown in Figure 25.14) different?
12.
How does the movement of Na
1
drive the reabsorption of
water and solutes?
13.
List several substances that are secreted into the kidney
tubules.
For answers, see Appendix H.
Cortex
Outer
medulla
Inner
medulla
Reabsorption
Secretion
65% of filtrate volume
reabsorbed
H
2
O
• Na
+
, HCO
3
-
, and
many other ions
• Glucose
, amino acids,
and other nutrients
Regulated reabsorption
• Na
+
(by aldosterone;
Cl
-
follows)
Ca
2
+
(by parathyroid
hormone)
H
+
and NH
4
+
• Some dr
ugs
Regulated
secretion
K
+
(by
aldosterone)
Regulated
secretion
K
+
(by
aldosterone)
• R
eabsorption or secretion
to maintain blood pH
described in Chapter 26;
involves H
+
, HCO
3
-
,
and NH
4
+
Regulated
reabsorption
H
2
O (by ADH)
• Na
+
(by
aldosterone; Cl
-
follows)
• Ur
ea (increased
by ADH)
• Na
+
, K
+
, Cl
-
• Ur
ea
H
2
O
Figure 25.15
Summary of tubular reabsorption and
secretion.
The various regions of the renal tubule carry out
reabsorption and secretion and maintain a gradient of osmolality
within the medullary interstitial fluid. Color gradients represent
varying osmolality at different points in the interstitial fluid.
*Te term “countercurrent” is commonly misunderstood to mean that the
direction of fluid flow in the nephron loops is opposite that of the blood in the
vasa recta. In fact, there is no one-to-one relationship between individual nephron
loops and capillaries of the vasa recta as might be suggested by two-dimensional
diagrams such as Figure 25.16. Instead, there are many tubules and capillaries
packed together. Each tubule is surrounded by many blood vessels, whose flow is
not necessarily counter to flow in that tubule (see Figure 25.7).
(Text continues on p. 976.)
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