The Urinary System
Regulation of Urine Concentration
Describe the mechanisms responsible for the medullary
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
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
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:
is the interaction between
the ﬂow of ﬁltrate through the ascending and descending
limbs of the long nephron loops of juxtamedullary nephrons.
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
—allows the kidneys to vary urine concentration
How do the kidneys form the osmotic gradient?
the Medullary Osmotic Gradient
answer to this question.
Check Your Understanding
In which part of the nephron does most reabsorption occur?
How are primary and secondary active transport processes
(both shown in Figure 25.14) different?
How does the movement of Na
drive the reabsorption of
water and solutes?
List several substances that are secreted into the kidney
For answers, see Appendix H.
65% of filtrate volume
many other ions
, amino acids,
and other nutrients
• Some dr
eabsorption or secretion
to maintain blood pH
described in Chapter 26;
O (by ADH)
Summary of tubular reabsorption and
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.)