Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38Renal Function & Micturition 651

out of the cells by Na, K ATPase and the amino acids leave by
passive or facilitated diffusion to the interstitial fluid.
Some Cl– is reabsorbed with Na+ and K+ in the thick
ascending limb of the loop of Henle. In addition, two mem-
bers of the family of Cl channels have been identified in the
kidney. Mutations in the gene for one of the renal channels is
associated with Ca2+-containing kidney stones and hypercal-
ciuria (Dent disease), but how tubular transport of Ca2+ and
Cl– are linked is still unsettled.

PAH TRANSPORT

The dynamics of PAH transport illustrate the operation of the
active transport mechanisms that secrete substances into the
tubular fluid (see Clinical Box 38–1). The filtered load of PAH
is a linear function of the plasma level, but PAH secretion in-
creases as PPAH rises only until a maximal secretion rate
(TmPAH) is reached (Figure 38–11). When PPAH is low, CPAH
is high; but as PPAH rises above TmPAH, CPAH falls progres-
sively. It eventually approaches the clearance of inulin (CIn)
(Figure 38–12), because the amount of PAH secreted becomes
a smaller and smaller fraction of the total amount excreted.

Conversely, the clearance of glucose is essentially zero at PG le-

vels below the renal threshold; but above the threshold, CG ris-

es to approach CIn as PG is raised.

The use of CPA H to measure ERPF is discussed above.

TUBULOGLOMERULAR FEEDBACK &

GLOMERULOTUBULAR BALANCE

Signals from the renal tubule in each nephron feed back to af-

fect filtration in its glomerulus. As the rate of flow through

the ascending limb of the loop of Henle and first part of the

distal tubule increases, glomerular filtration in the same

nephron decreases, and, conversely, a decrease in flow in-

creases the GFR (Figure 38–13). This process, which is called

FIGURE 38–11 Relation between plasma levels (P) and
excretion (UV) of PAH and inulin.

FIGURE 38–12 Clearance of inulin, glucose, and PAH at
various plasma levels of each substance in humans.

UV

. Inulin

P

PAH

Splay

Clearance (mL/min)Inulin

Glucose

Plasma level (P)

20 40 60 80

200 400 600

Glucose, mg/dL

PAH, mg/dL

PAH

600

500

400

300

200

100

0

CLINICAL BOX 38–1

Other Substances Secreted by the Tubules

Derivatives of hippuric acid in addition to PAH, phenol red

and other sulfonphthalein dyes, penicillin, and a variety of

iodinated dyes are actively secreted into the tubular fluid.

Substances that are normally produced in the body and se-

creted by the tubules include various ethereal sulfates, ster-

oid and other glucuronides, and 5-hydroxyindoleacetic

acid, the principal metabolite of serotonin.

FIGURE 38–13 Mechanisms of glomerulotubular balance

and tubuloglomerular feedback.

Renal arteriolar

pressure

Glomerular capillary

pressure

GFR

Solute reabsorption

in proximal tubule

Solute reabsorption

in thick ascending limb

Salt and fluid

delivery to

the distal tubule

Tubulo-

glomerular

Glomerulo- feedback

tubular

balance