Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38Renal Function & Micturition 657

RELATION OF URINE

CONCENTRATION TO GFR

The magnitude of the osmotic gradient along the medullary
pyramids is increased when the rate of flow of fluid through
the loops of Henle is decreased. A reduction in GFR such as
that caused by dehydration produces a decrease in the volume
of fluid presented to the countercurrent mechanism, so that
the rate of flow in the loops declines and the urine becomes
more concentrated. When the GFR is low, the urine can be-
come quite concentrated in the absence of vasopressin. If one
renal artery is constricted in an animal with diabetes insipidus,
the urine excreted on the side of the constriction becomes hy-
pertonic because of the reduction in GFR, whereas that excret-
ed on the opposite side remains hypotonic.

"FREE WATER CLEARANCE"

In order to quantitate the gain or loss of water by excretion of
a concentrated or dilute urine, the "free water clearance"

(CH 2 O) is sometimes calculated. This is the difference between

the urine volume and the clearance of osmoles (COsm):

CH 2 O = V

UOsm V

POsm

where V

• is the urine flow rate and UOsm and POsm the urine
  and plasma osmolality, respectively. COsm is the amount of
  water necessary to excrete the osmotic load in a urine that is
  isotonic with plasma. Therefore, CH 2 O is negative when the
  urine is hypertonic and positive when the urine is hypotonic.
  For example, using the data in Table 38–7, the values for
  CH 2 O are –1.3 mL/min (–1.9 L/d) during maximal antidiure-
  sis and 14.5 mL/min (20.9 L/d) in the absence of vasopressin.

REGULATION OF Na

+

EXCRETION

Na+ is filtered in large amounts, but it is actively transported out

of all portions of the tubule except the descending thin limb of

Henle’s loop. Normally, 96% to well over 99% of the filtered Na+

is reabsorbed. Because Na+ is the most abundant cation in ECF

and because Na+ salts account for over 90% of the osmotically

active solute in the plasma and interstitial fluid, the amount of

Na+ in the body is a prime determinant of the ECF volume.

Therefore, it is not surprising that multiple regulatory mecha-

nisms have evolved in terrestrial animals to control the excre-

tion of this ion. Through the operation of these regulatory

mechanisms, the amount of Na+ excreted is adjusted to equal

the amount ingested over a wide range of dietary intakes, and

the individual stays in Na+ balance. Thus, urinary Na+ output

ranges from less than 1 mEq/d on a low-salt diet to 400 mEq/d

or more when the dietary Na+ intake is high. In addition, there

is a natriuresis when saline is infused intravenously and a de-

crease in Na+ excretion when ECF volume is reduced.

MECHANISMS

Variations in Na+ excretion are brought about by changes in

GFR (Table 38–9) and changes in tubular reabsorption, pri-

marily in the 3% of filtered Na+ that reaches the collecting ducts.

The factors affecting the GFR, including tubuloglomerular

feedback, have been discussed previously. Factors affecting Na+

reabsorption include the circulating level of aldosterone and

other adrenocortical hormones, the circulating level of ANP

and other natriuretic hormones, and the rate of tubular secre-

tion of H+ and K+.

EFFECTS OF

ADRENOCORTICAL STEROIDS

Adrenal mineralocorticoids such as aldosterone increase tu-

bular reabsorption of Na+ in association with secretion of K+

and H+ and also Na+ reabsorption with Cl–. When these hor-

mones are injected into adrenalectomized animals, a latent

FIGURE 38–18 Approximate relationship between urine
concentration and urine flow in osmotic diuresis in humans. The
dashed line in the lower diagram indicates the concentration at which
the urine is isosmotic with plasma. (Reproduced with permission from
Berliner RW, Giebisch G in: Best and Taylor’s Physiological Basis of Medical Practice, 9th
ed. Brobeck JR [editor]. Williams & Wilkins, 1979.)

Urine flow (mL/min)

Isosmotic

Diabetes

insipidus

Solute load (mosm/min)

Maximal

vasopressin

21

18

15

12

9

6

3

0

0.9 1.8 2.7 4.5 6.3

Urine osmolality (mosm/L)

Isosmotic

Diabetes insipidus

Urine flow (mL/min)

Maximal

vasopressin

1400

1200

1000

800

600

400

200

0

3 6 9 12 15 18 21