active tubular secretio n into the late DCT and collecting ducts. • Controlling blood pH. When blood pH
drops toward the acidic end of its homeostatic range, the renal tubule cells actively secrete more H+
into the filtrate and retain and generate more HC03 - (a base). As a result, blood pH rises and the urine
drains off the excess H•. Conversely, when blood pH approaches the alkaline end of its range, CJ- is
reabsorbed instead of HC03 -, which is allowed to leave the body in urine. We will discuss the kidneys'
role in pH homeostasis in more detail in Chapter 26. Check Your Understanding 16. List several
substances that are secreted into the kidney tubules. For answers, see Answers Appendix. The kidneys
create and use an osmotic gradient to regulate urine concentration and volume Learning Outcomes IIDescribe t he mechanisms responsible for the medulla ry osmotic gradient. II- Explain how dilute and
concentrated urine a re formed. 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 fl uid as sweat. On the other hand, if you drink a pitcher of lemonade while sitting on the porch, you
may overhydrate. In response, the kidneys make adjustments 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, frorn
shrinking or swelling from the osmotic movement of water. Recall from Chapter 3 (pp. 71-73) that a
solution's osmolality is the concentration of solute particles per kilogram of water. Because I osmol
(equivalent to I mole of particles) is a fairly large unit, the milliosmol (mOsm) (mil"e-oz' mol), equal to
0.001 osmol, is generally used. In the discussion that follows, we use mOsm to indicate mOsm/kg. The
kidneys keep the solute load of body fluids constant by regulating urine concentration and volume. •
When you dehydrate, your kidneys produce a small volume of concentrated urine. • When you
overhydrate, your kidneys produce a large voltune of dilute urine. The kidneys accomplish this feat using
countercurrent mechanisms. 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 Focus on the
Medullary Osn,otic Gradient, Focus Figure 25.1, pp. 996-997). This arrangement makes it possible to
exchange materials between the t\vo segments. Chapt er 25 The Urinary System 995 1\vo types of
countercurrent mechanisms determine urine concentration and volume: • The countercurrent
multiplier is the interaction bet\veen the flow of filtrate through the ascending and descending limbs of
the long nephron loops of juxtamedullary nephrons. • The countercurrent exchanger is the flow of blood
through the ascending and descending portions of the vasa recta. These countercurrent mechanisms
establish and mainta in an osrnotic gradient extending frorn the cortex through the depths of the
medulla (Figure 25.18). This 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
Osnrotic Gradient (Focus Figure 25.1 on pp. 996-997) explores the answer to this question. This greatly
enlarged nephron and collecting duct show you the arrangement of structures that pass through the
osmotic gradient in the renal medulla. If you keep t his anatomy in mind, it will help you understand the
process of making either very dilute or very concentrated urine. The osmolality of the interstitial fluid in
the renal cortex is isotonic at 300 mOsm. The osmolality of the interstitial fluid of the renal medulla
increases progressively from 300 mOsm at the cortex-medulla junction to 1200 mOsm at the medullapelvis junction. Figure 25.18 Osmotic gradient in t he renal medulla. *The term "countercurrent" is
commonJy 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 twodimensional diagrams such as Focus
Figure 25. I. Instead, there are many tubules and capillaries packed together like a bundle of straws.
Each tubule is surrounded by many blood vessels, whose flow is not ne.cessariJy counter to flow in that
tubule (see Figure 25.8}. (