Within each kidney, there are approximate-
ly 1
million
functional units called nephrons
(NEF-ronz). Each nephron consists of two parts: a
collecting cup-like structure called the renal cor-
puscle and a long tube called the renal tubule
(Figure 15.3).
Blood plasma is filtered in the renal corpus-
cle. The two parts that make up a renal corpuscle
are the glomerulus and the glomerular (Bow-
man’s) capsule, a double-walled cup of epithelial
cells that surrounds the glomerular capillaries.
The filtered fluid, also called the
filtr a te ,
first en-
ters the glomerular capsule and then passes into
the renal tubule.
The renal tubule is twisted and divided into
three main segments. Taking them in the or-
der that fluid passes through them, they are the
proximal convoluted tubule, a long hairpin loop
called the nephron loop, and the distal convo-
luted tubule. The distal convoluted tubules of
several nephrons empty into the common col-
lecting duct. Each nephron extends from the
cortex into the medulla. Some nephrons have
short nephron loops that extend only minimally
into the medulla (cortical nephrons), while oth-
ers have long nephron loops that extend deep
into the medulla (juxtamedullary nephrons).
The kidney works by filtering almost every-
thing from the blood plasma into the renal corpus-
cle of the nephron, by selectively reabsorbing only
those substances that the body requires from the
filtrate as it flows along the length of the renal tu-
bule, and by secreting additional nonessential sub-
stances into the filtrate. Substances required by
the body include water, ions, and glucose; nones-
sential substances include urea, drugs, and toxins.
What remains in the lumen of the tubules becomes
urine and gets drained by the collecting ducts into
the renal pelvis, ureter, and urinary bladder.
For example, let’s look back at the exercise
example at the beginning of this chapter. Dur-
ing and after strenuous exercise, you lose some
body fluids through sweating and evaporation.
The kidney works to return fluid to the tissues
by secreting less water into the urine. The kid-
ney also reabsorbs some salts that your body may
need because they were lost along with the sweat.
However, the kidney also works to excrete the
metabolites produced by your working cells.
How do all these processes happen? Let’s ex-
amine the processes of filtration, reabsorption,
and secretion in more detail.
T h e s t r u c t u r e o f t h e n e p h r o n ,
s h o w i n g v a s c u l a r s u p p l y a n d d ir e c t i o n
o f b l o o d f l o w •
F ig u r e 1 5 . 3 _____________
Glomerular
capsule
Renal corpuscle
is where blood plasma is filtered:
• Located in cortex
• Consists of
glomerulus
(capillary network) and
glomerular
capsule
or
Bowman’s capsule
(double-walled cup of epithelial cells)
• Inner wall of glomerular capsule consists of modified simple
squamos epithelial cells called
podocytes
, which have
fingerlike projections called
pedicels
that wrap around
capillary endothelial cells
• Outer wall of glomerular capsule consists of simple
squamous epithelial cells
Podocyte
Pedicel
Glomerulus
Q ® 1380x
Renal corpuscle
442 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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