Human Physiology, 14th edition (2016)

598 Chapter 17

A person in a state of normal hydration excretes about
1.5 L of urine per day, indicating that 99.2% of the glomerular
ultra-filtrate volume is reabsorbed. Notice that small changes in
percent reabsorption translate into large changes in urine vol-
ume. Drinking more water—and thus decreasing ADH secretion
( fig.  17.20 and table  17.3 )—results in correspondingly larger
volumes of urine excretion. It should be noted, however, that
even in the complete absence of ADH some water is still reab-
sorbed through the collecting ducts.

CLINICAL APPLICATION

Diabetes insipidus is characterized by polyuria (a large

urine volume—from 3 to 10 L per day), thirst, and polydipsia

(drinking a lot of fluids). The urine is dilute, with a hypotonic

concentration of less than 300 mOsm. There are two major

types of diabetes insipidus: (1) central diabetes insipidus,

caused by inadequate secretion of ADH (arginine vasopres-

sin); and (2) nephrogenic diabetes insipidus, caused by

the inability of the kidneys to respond to ADH. These two

types can be distinguished by measuring plasma arginine

vasopressin, and by challenging the kidneys with a syn-

thetic ADH called desmopressin. Nephrogenic diabetes

insipidus may be caused by genetic defects in either the

aquaporin channels or the ADH receptors. More commonly,

it is acquired as a response to drug therapy (from lithium

given to treat bipolar disorder, and from certain antibiotics)

or other causes. People with central diabetes insipidus can

take desmopressin when needed, and those with nephro-

genic diabetes insipidus must drink a lot of water to prevent

dehydration.

Clinical Investigation CLUES

Lauren was on lithium for her bipolar disorder and expe-

rienced dizziness upon standing.

• What condition likely caused Lauren’s dizziness,
  and how is that related to dehydration?


• What treatment did the physician advise for this
  condition?

Table 17.3 | Antidiuretic Hormone Secretion and Action

Stimulus Receptors Secretion of ADH

Effects on

Urine Volume Effects on Blood

↑Osmolality (dehydration) Osmoreceptors in hypothalamus Increased Decreased Increased water retention;

decreased blood osmolality

↓Osmolality Osmoreceptors in hypothalamus Decreased Increased Water loss increases blood

osmolality

↑Blood volume Stretch receptors in left atrium Decreased Increased Decreased blood volume

↓Blood volume Stretch receptors in left atrium Increased Decreased Increased blood volume

LEARNING OUTCOMES

After studying this section, you should be able to:

9. Explain how renal plasma clearance is affected by
   reabsorption and secretion, and how it is used to
   measure GFR and total renal blood flow.


10. Define transport maximum and renal plasma
    threshold, and explain their significance.

17.4 RENAL PLASMA CLEARANCE

As blood passes through the kidneys, some of the constit-

uents of the plasma are removed and excreted in the urine.

The blood is thus “cleared” of particular solutes in the pro-

cess of urine formation. These solutes may be removed

from the blood by filtration through the glomerular capillar-

ies or by secretion by the tubular cells into the filtrate.

| CHECKPOINT

6a. Describe the mechanisms for salt and water

reabsorption in the proximal tubule.

6b. Compare the transport of Na^1 , Cl^2 , and water across

the walls of the proximal tubule, ascending and

descending limbs of the loop of Henle, and collecting

duct.

7. Describe the interaction between the ascending and
   descending limbs of the loop and explain how this
   interaction results in a hypertonic renal medulla.


8. Explain how ADH helps the body conserve water.
   How do variations in ADH secretion affect the volume
   and concentration of urine?

One of the major functions of the kidneys is to eliminate excess

ions and waste products from the blood. Clearing the blood of

these substances is accomplished through their excretion in the

urine. Because of renal clearance, the concentrations of these