volume surges through the kidneys. This blood
disperses among the million or so glomeruli, the
microscopic capillaries within the nephrons. Spe-
cialized sensors in the walls of the glomeruli detect
the levels of oxygen in the erythrocytes as they
pass by. Low levels of oxygen stimulate the kid-
neys to synthesize (produce and release) EPO.
EPO travels through the blood circulation to the
bone marrow, where it stimulates the production
of new erythrocytes (erythropoiesis).
Erythropoiesis suffers in RENAL FAILUREbecause
the blood cannot circulate through the glomeruli,
resulting in ANEMIA. This is why people who have
renal failure feel so fatigued. And erythropoiesis
comes to a near-halt in ESRD, when the kidneys
no longer function at all. Not only do toxins accu-
mulate in the blood when the kidneys fail but also
the blood cannot deliver enough oxygen because
it does have enough red blood cells to carry the
load. Many people who have renal failure take
EPO supplement, a product of RECOMBINANT DNA
technology, to maintain adequate erythropoiesis.
The kidneys and bone health Mention strong
bonesand the first association is likely to be “cal-
cium.” It might just as well be calcitriol, the hor-
mone form of vitamin D, because without
calcitriol, the body cannot use the calcium it
receives. The kidneys convert dietary vitamin D, a
fat-soluble vitamin inert within the body in its
dietary form, to calcitriol. The kidneys further par-
ticipate in the body’s calcium balance because they
determine how much calcium to reabsorb from
the filtrate and return to the blood circulation.
Calcium is essential for numerous body activi-
ties ranging from HEALINGand cell repair to nerve
and MUSCLEcell communication. Calcium makes it
possible to walk across the room, from the SKELE-
TONthat supports the body to the nerve impulses
that instruct muscle fibers from heart to soles to
contract. People who have chronic renal failure
and other forms of chronic renal disease often
take vitamin D supplement (calciferol). Adequate
vitamin D is essential for appropriate growth in
children; without it there is no growth. PARATHY-
ROID HORMONEacts on the distal tubule to increase
the amount of calcium the tubule reabsorbs.
The kidneys and urine production This most
familiar and seemingly simple function of the kid-
neys is, of course, its most important. Without the
urine to carry metabolic wastes from the blood,
none of the kidney’s other functions would be
necessary for very long. Urine is a mixture of the
water, electrolytes, and metabolic wastes (prima-
rily urea) the kidneys extract from the blood. As
blood passes through the glomerulus pressure
squeezes much of the blood’s water, along with
electrolytes and wastes, through the glomerular
walls into Bowman’s capsule. This mixture, the fil-
trate, collects in the capsule and drains into the
tubule. The tubule reabsorbs about two thirds of
the water and electrolytes, passing on the remain-
der to become urine. Collecting tubules carry the
urine into the renal pelvis, where it drains into the
ureters that then channel it to the BLADDER.
Holding the urine: the bladder Suspended in
the lower pelvis is the bladder, an expandable
muscular sac that collects the urine that drains
from the kidneys. When empty the bladder is
about the size of a lemon; when filled to its capac-
ity of about 500 milliliters the bladder is about the
size of a cantaloupe. Its three-layer wall consists of
a mucous inner layer, middle layer of smooth
muscle, and fibrous membrane outer layer. The
middle layer, called the detrusor muscle, relaxes to
allow the bladder to distend when filling with
urine and contracts to push urine from the blad-
der into the URETHRAfor passage from the body.
The bladder holds the urine in a more collo-
quial sense as well, allowing conscious override of
the micturition REFLEX, an involuntary function of
the sympathetic NERVOUS SYSTEM, that initiates URI-
NATION. At about two or three years of age the
brain, muscles, and nerves have matured enough
for conscious control to take over certain involun-
tary functions. Voluntary urination—toilet train-
ing—is the hallmark of this effort and marks the
rite of passage from baby to child. Voluntary con-
trol of urination uses certain centers in the brain
in coordination with voluntary muscles such as
the pubococcygeal muscle to manage the timing of
urination, though if the bladder becomes too full
the micturition reflex becomes too intense for
conscious control to overcome.
Tubes of urine transport: the ureters and ure-
thra From each kidney a thin muscular tube
drops about 12 inches to join with the bladder.
Urine trickles from the kidney’s collecting tubules
into the renal pelvis, a deltalike structure that172 The Urinary System