Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38Renal Function & Micturition 649

TABLE 38–5 Renal handling of various plasma constituents in a normal adult human on an average diet.

Per 24 Hours

Substance Filtered Reabsorbed Secreted Excreted Percentage Reabsorbed

Na+ (mEq) 26,000 25,850 150 99.4

K+ (mEq) 600 560 a 502 90 93.3

Cl– (mEq) 18,000 17,850 150 99.2

HCO 3 – (mEq) 4,900 4,900 0 100

Urea (mmol) 870 460 b 410 53

Creatinine (mmol) 12 1 c 1 c 12

Uric acid (mmol) 50 49 4 5 98

Glucose (mmol) 800 800 0 100

Total solute (mOsm) 54,000 53,400 100 700 98.9

Water (mL) 180,000 179,000 1000 99.4

aK+ is both reabsorbed and secreted.
bUrea moves into as well as out of some portions of the nephron.
cVariable secretion and probable reabsorption of creatinine in humans.

TABLE 38–6 Transport proteins involved
in the movement of Na+ and Cl– across the
apical membranes of renal tubular cells.a

Site Apical Transporter Function

Proximal tubule Na/glucose CT Na+ uptake, glucose uptake

Na+/Pi CT Na+ uptake, Pi uptake

Na+ amino acid CT Na+ uptake, amino acid

uptake

Na/lactate CT Na+ uptake, lactate uptake

Na/H exchanger Na+ uptake, H+ extrusion

Cl/base exchanger Cl– uptake

Thick ascending

limb

Na–K–2Cl CT Na+ uptake, Cl– uptake, K+

uptake

Na/H exchanger Na+ uptake, H+ extrusion

K+ channels K+ extrusion (recycling)

Distal convolut-

ed tubule

NaCl CT Na+ uptake, Cl– uptake

Collecting duct Na+ channel

(ENaC)

Na+ uptake

aUptake indicates movement from tubular lumen to cell interior, extrusion is move-
ment from cell interior to tubular lumen. CT, cotransporter; Pi, inorganic phosphate.
Modified with permission from Schnermann JB, Sayegh EI: Kidney Physiology. Lip-
pincott-Raven, 1998.

FIGURE 38–8 Mechanism of Na+ reabsorption in the

proximal tubule. Na+ moves out of the tubular lumen by cotransport

and exchange mechanism through the apical membrane of the tubule

(dashed line). The Na+ is then actively transported into the interstitial

fluid by Na, K^ ATPase in the basolateral membrane (solid lines). K+ en-

ters the interstitial fluid via K+ channels. A small amount of Na+, other

solutes, and H 2 O re-enter the tubular lumen by passive transport

through the tight junctions (dotted lines).

Na+

Na+

Na+, etc

Na+

Na+

K+

K+

K+

Tubular

lumen

Tight

junction

Lateral

intercellular

space

Interstitial

fluid

Na+