Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38

Renal Function & Micturition 641

into the medullary pyramids. In humans, only 15% of the
nephrons have long loops.
The thick end of the ascending limb of the loop of Henle
reaches the glomerulus of the nephron from which the tubule
arose and nestles between its afferent and efferent arterioles.
Specialized cells at the end form the
macula densa,
which is
close to the efferent and particularly the afferent arteriole
(Figure 38–2). The macula, the neighboring
lacis cells,
and
the renin-secreting
juxtaglomerular cells
in the afferent arte-
riole form the
juxtaglomerular apparatus
(see Figure 39–9).
The
distal convoluted tubule,
which starts at the macula
densa, is about 5 mm long. Its epithelium is lower than that of

the proximal tubule, and although a few microvilli are

present, there is no distinct brush border. The distal tubules

coalesce to form

collecting ducts

that are about 20 mm long

and pass through the renal cortex and medulla to empty into

the pelvis of the kidney at the apexes of the medullary pyra-

mids. The epithelium of the collecting ducts is made up of

principal cells (P cells)

and

intercalated cells (I cells).

The P

cells, which predominate, are relatively tall and have few

organelles. They are involved in Na

+

reabsorption and vaso-

pressin-stimulated water reabsorption. The I cells, which are

present in smaller numbers and are also found in the distal

tubules, have more microvilli, cytoplasmic vesicles, and

FIGURE 38–2
Structural details of glomerulus. A)
Section through vascular pole, showing capillary loops.
B)
Relation of mesangial cells
and podocytes to glomerular capillaries.
C)
Detail of the way podocytes form filtration slits on the basal lamina, and the relation of the lamina to
the capillary endothelium.
D)
Enlargement of the rectangle in
C
to show the podocyte processes. The fuzzy material on their surfaces is glomerular
polyanion.

A Proximal tubule

Capsule

Red blood cells

Glomerular basal

lamina

Bowman's space

Juxtaglomerular cells

Nerve fibers

Smooth muscle

Macula densa

Distal tubule

Mesangial

cell

Podocyte

processes

Afferent

arteriole

Efferent

arteriole

B

Capillary

Capillary

Capillary Capillary

Podocyte

Podocyte

process

Basal

lamina

Cytoplasm of

endothelial

cell

Mesangial cell

C

Podocyte

Endothelium

Endothelium

Basal lamina

Basal lamina D

Foot processes

of podocytes

Filtration slit

Bowman's

space

Basal lamina

Fenestrations

Capillary lumen