HUMAN BIOLOGY

230 Chapter 12

aDH Antidiuretic hormone;
it stimulates nephrons to
reabsorb water.

aldosterone A hormone
that concentrates urine by
reducing the amount of
water leaving distal tubules.

juxtaglomerular apparatus
The region where efferent
arterioles lie close to a
nephron’s distal tubules.

sodium is actively transported out of the ascending limb—

but water can’t move with it.

The filtrate now moves into the distal tubule. Its cells

continue to remove salt but don’t also let water escape.

Hence, a dilute urine moves on into the collecting duct.

Naturally, as salt leaves the filtrate moving through a

nephron tubule, the concentration of solutes rises outside

the tubule and falls inside it. This steep gradient helps

drive the reabsorption of valuable solutes, which move

into peritubular capillaries. It also draws water out of the

descending limb by osmosis.

Urea boosts the gradient. As water is reabsorbed, urea

left in the filtrate becomes concentrated. Some of it will be

excreted in urine, but when filtrate enters the final portion

of the collecting duct, some urea also will diffuse out—

so the concentration of solutes in the inner medulla rises

even more.

Drink a large glass of water and the next time you “go,”

your urine may be pale and dilute. If you sleep 8 hours

without a break, your urine will be concentrated and

darker yellow. As described next, hormones control how

much water the kidneys add to urine. These controls also

adjust blood pressure.

Hormones control whether kidneys make

urine that is concentrated or dilute

When you don’t take in as much water as your body

loses, the salt concentration in your blood rises. In the

brain, receptors sense this change and trigger the release

of antidiuretic hormone, or aDH. It acts on cells in distal

tubules and collecting ducts so that more water moves out

of them and is reabsorbed into the blood (Figure 12.8). As

a result, the urine becomes more concentrated. Gradually

the additional water in blood reduces the salt concentra-

tion there. It also increases the blood volume and blood

How Kidneys Help manage Fluid Balance

and Blood pressure

Figure 12.8 aDH and aldosterone act in

different parts of kidney nephrons.

ADH targets

aldosterone

KIDNEY target

CORTEX

KIDNEY

MEDULLA

Figure 12.7 Water and salt are reabsorbed in the

loop of Henle.

H 2 O

Na+

Cl–

kidney loop of

medulla Henle

saltiest

near turn

cortex

medulla

n The kidneys concentrate urine before it flows to the

bladder. These concentration mechanisms help regulate

blood volume and blood pressure.

n Links to Chemical bonds 2.3, Liver functions 11.5

Overall, the total volume of your body fluids, including

blood plasma, doesn’t vary much. This is because during

reabsorption, the kidneys adjust how much water and salt

(sodium 1 chloride ions) the body conserves or excretes

in urine. As you know, blood and tissue fluid are mostly

water. In general, when the volume of blood increases or

decreases, so does blood pressure. The kidneys help ensure

that the volume of extracellular fluid, and blood in particu-

lar, stays within a normal range.

Water follows salt as urine forms

Although about two-thirds of filtered salt and water is

reabsorbed in the proximal tubule, the filtrate usually still

contains more of both than the body can afford to lose in

urine. This situation is addressed as the filtrate enters the

loop of Henle, which descends into

the kidney medulla (Figure 12.7).

There the loop is surrounded by

extremely salty tissue fluid. Water

can pass through the thin wall of

the loop’s descending limb, so more

water moves out by osmosis and

is reabsorbed. As the water leaves,

the salt concentration in the fluid

still inside the descending limb

increases until it matches that in the

fluid outside.

Now the filtrate “rounds the turn” of the loop and enters

the ascending limb. The wall of this part of the nephron

tubule doesn’t allow water to pass through. This is an

important variation in the tubule’s structure, because here

12.4

© Cengage Learning © Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).