785
CHAPTER TWENTY
Urinary System
20.3
URINE FORMATION
The main function of the nephrons and collecting ducts is to
control the composition of body fl uids and remove wastes from
the blood. The product is
urine,
which is excreted from the
body. Urine contains wastes, excess water, and electrolytes.
Urine formation begins when the glomerular capillaries
F
lter plasma, a process called
glomerular f
ltration
(glo-
mer
u-lar F l-tra
shun). Recall from chapter 15 (pp. 578–579)
that the force of blood pressure drives F
ltration at capillaries
throughout the body. Most of this fl uid is reabsorbed into the
bloodstream by the colloid osmotic pressure of the plasma,
leaving only a small volume of interstitial fl
uid
(f g. 20.16
a
)
.
Nephrons take F ltration to another level, using two cap-
illaries working in series. The F rst capillary bed is special-
ized only to F lter. However, instead of forming interstitial
fl uid, the F ltered fl uid (F ltrate) moves into the renal tubule,
where some of it is destined to become urine (F
g. 20.16
b
).
Glomerular F
ltration produces 180 liters of fl
uid, more
than four times the total body water, every 24 hours.
Obviously this could not continue for long unless most of
this F ltered fl uid were returned to the internal environment.
is under relatively low pressure (see F
g. 20.10). Branches
of this system that primarily receive blood from the effer-
ent arterioles of the juxtamedullary nephrons form capil-
lary loops called
vasa recta.
These loops dip into the renal
medulla and are closely associated with the loops of the
juxtamedullary nephrons
(f
g. 20.14)
. Blood fl
ows through
the vasa recta and returns to the renal cortex, where it
joins blood from other branches of the peritubular capil-
lary system and enters the venous system of the kidney.
Figure 20.15
summarizes the pathway that blood follows
as it passes through the blood vessels of the kidney and
nephron.
PRACTICE
4
Explain why nephrons are considered functional units.
5
Describe the system of vessels that supplies blood to the kidney.
6
Name the parts of a nephron.
7
Which structures comprise the juxtaglomerular apparatus?
8
Distinguish a cortical nephron from a juxtamedullary nephron.
9
Describe the blood supply of a nephron.
FIGURE 20.13
Cortical nephrons are close to the surface of a kidney; juxtamedullary nephrons are near the renal medulla.
Cortical nephron
Juxtamedullary
nephron
Renal
cortex
Renal
medulla
Collecting
duct
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