786
UNIT FIVE
glomerular capillary plasma and into the glomerular capsules.
Large molecules, such as proteins, are restricted primarily
because of their size. The F ltration of these materials through
the capillary walls is much like the F ltration at the arteriole
ends of other capillaries throughout the body. The glomerular
capillaries, however, are many times more permeable to small
molecules than are the capillaries in other tissues, due to the
many tiny openings (fenestrae) in their walls
(f g. 20.17)
.
RECONNECT
To Chapter 15, Exchanges in the Capillaries,
pages 578–579.
The glomerular capsule receives the resulting
glom-
erular f
ltrate,
which has about the same composition as
The kidney accomplishes this by the process of
tubular reab-
sorption
(too
bu-lar re-ab-sorp
shun), selectively reclaiming
just the right amounts of substances, such as water, electro-
lytes, and glucose, that the body requires. Waste products
and substances in excess are allowed out of the body. Some
substances that the body must eliminate, such as hydro-
gen ions and certain toxins, are removed even faster than
through F
ltration alone by the process of
tubular secretion
(too
bu-lar se-kre
shun).
Overall, the following relationship determines the vol-
ume of substances excreted in the urine:
urinary excretion = glomerular f
ltration
+ tubular secretion
– tubular reabsorption
The F
nal product of these processes is
urine.
Glomerular Filtration
Urine formation begins when
glomerular filtration
filters
water and other small dissolved molecules and ions out of the
FIGURE 20.14
The capillary loop of the vasa recta is closely
associated with the nephron loop of a juxtamedullary nephron.
Proximal
convoluted
tubule
Glomerulus
Efferent
arteriole
Peritubular
capillaries
Vasa recta
Nephron loop
Collecting duct
Distal
convoluted
tubule
Afferent arteriole
Cortical radiate
artery and vein
Glomerular
capsule
Renal artery
Interlobar artery
Arcuate artery
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Cortical radiate artery
Peritubular capillaries
Cortical radiate vein
Interlobar vein
Renal vein
Arcuate vein
FIGURE 20.15
Pathway of blood through the blood vessels of the
kidney and nephron.
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