Human Physiology, 14th edition (2016)

Physiology of the Kidneys 609

acidification of the urine and reabsorption of bicarbonate are

summarized in figure 17.29.

Reabsorption of Bicarbonate

and Secretion of H^1

The cells of the proximal tubule use Na^1 /H^1 pumps to trans-

port H^1 into the filtrate in exchange for Na^1 from the filtrate

( fig. 17.29 ). This exchange is “antiport” cotransport (chapter 6),

because the Na^1 and H^1 move in opposite directions across the

apical portion of the plasma membrane (facing the tubule lumen).

Antiport cotransport is a form of secondary active transport,

because Na^1 diffuses down the concentration gradient main-

tained by primary active transport Na^1 /K^1 pumps in the baso-

lateral portion of the plasma membrane. Most of the H^1 secreted

into the filtrate from the proximal tubule is used for the reabsorp-

tion of bicarbonate.

The apical membranes of the tubule cells (facing the lumen)

are impermeable to bicarbonate. The reabsorption of bicarbon-

ate must therefore occur indirectly. When the urine is acidic,

H C O 32 combines with H^1 to form carbonic acid. Carbonic acid

in the filtrate is then converted to CO 2 and H 2 O in a reaction

catalyzed by carbonic anhydrase. This enzyme is located in the

apical plasma membrane of the proximal tubule in contact with

the filtrate. Notice that the reaction that occurs in the filtrate is

the same one that occurs within red blood cells in pulmonary

capillaries (chapter 16, section 16.7).

The tubule cell cytoplasm also contains carbonic anhydrase.

As CO 2 concentrations increase in the filtrate, the CO 2 diffuses

Figure 17.28 The reabsorption of Na^1 and secretion
of K^1 and H^1. In the distal tubule, K^1 and H^1 are secreted
in response to the potential difference produced by the
reabsorption of Na^1. High concentrations of H^1 may therefore
decrease K^1 secretion, and vice versa.

Peritubular capillaries

Blood

Na+

Na+

Na+

Na+

Na+

K+ or H+

K+/H+ K

H+ +

H+

K+

K+

K+

Distal tubule

Cortical collecting duct

Na+

Na+

Na+

Na+

Na+

Na+

Medullary

collecting

duct

Ascending limb

of Henle’s loop

Na+

Blood

HCO 3

Proximal tubule

Distal tubule

HCO 3

Na+

Na+

Filtration

H+

Na+

H+

H 2 CO 3 H 2 O + CO 2

H 2 O + CO 2 H+

H+

+ H+ H 2 CO 3

CA

CA

HPO 4 –^2 H 2 PO 4 –

NH 3 NH 4 +

ATPase K+

Lumen

Proximal tubule cell

Na+

Na+

HCO–

3

Figure 17.29 Acidification of the
urine. This diagram summarizes how the urine
becomes acidified and how bicarbonate is reabsorbed
from the filtrate. It also depicts the buffering of
the urine by phosphate and ammonium buffers.
(CA  5  carbonic anhydrase.) The inset depicts an
expanded view of proximal tubule cells. Note: The
cells of the proximal tubule can also produce extra
bicarbonate and ammonium ions from the metabolism
of glutamine (not shown in this figure).