end to end in a straight line, they would cover
more than 50 miles.
RENAL DIALYSIS: MACHINES TO CLEANSE
THE BODY WHEN THE KIDNEYS CANNOT
Researchers began working in the 1920s to
develop a safe, effective substitute to cleanse
metabolic wastes from the BLOOD when the
KIDNEYSfailed. By the early 1950s such a substi-
tute—the hemodialysis machine—entered clini-
cal use. And by the 1970s hemodialysis was the
standard treatment for END-STAGE RENAL DISEASE
(ESRD). Today nearly 300,000 Americans rely on
hemodialysis.The kidneys and blood pressure The kidneys
regulate blood pressure by controlling the volume
of the blood and through the production of the
hormone renin, which is the cornerstone of the
body’s RENIN–ALDOSTERONE–angiotensin (RAA) sys-
tem for regulating blood pressure. Renin and
aldosterone initiate chemical actions that result in
constricting blood vessels and increasing blood
volume to raise blood pressure.
The tubules in the nephrons continuously
adjust the amounts of sodium, potassium, and
chloride they reabsorb from the filtrate. Where
goes the electrolytes, so goes the water. The more
of electrolytes the tubules draw back into the
blood, the higher the amount of water that fol-
lows. Increased reabsorption increases blood vol-
ume and raises blood pressure. Decreased
reabsorption sends the electrolytes in the filtrate,
along with the water that they draw, out of the
body in the urine to drop both blood volume and
blood pressure.
Within each nephron, where the distal tubule
and the afferent arteriole (the blood vessel that
brings blood into the glomerulus) nearly touch,
are two clusters of specialized sensory cells. The
macula densa resides within the walls of the distal
tubule; its cells sense the concentration of elec-
trolytes, primarily sodium, in the filtrate. In the
interstitial space between the distal tubule and the
afferent arteriole are the juxtaglomerular cells,
which sense the pressure of the blood as it courses
through the afferent arteriole.
The clusters are in constant communication
with one another, using chemical signals to regu-
late how much electrolytes and water the tubules
reabsorb from the filtrate. As well, these cell clus-
ters send a continuous barrage of NERVEsignals to
the brainstem, which just as continuously deter-
mines the adjustments in renin release necessary
to maintain the blood pressure at the level the
body needs. Renin sets in motion the cascade of
chemical events that converts the inactive protein
angiotensinogen (also called angiotensin I) into
the very potent vasoconstrictor (chemical that
causes the blood vessels to narrow and stiffen,
raising blood pressure) angiotensin II. Angiotensin
I causes the peripheral arterioles to constrict.
Angiotensin II also signals the adrenal cortex of
the ADRENAL GLANDSto release ALDOSTERONE, a hor-
mone that stimulates the tubule to pull even more
sodium (and, of course, water) back into the blood
from the filtrate. The result is a rise in blood pres-
sure. The brainstem also instructs the HYPOTHALA-
MUSto release ANTIDIURETIC HORMONE(ADH) when
blood volume and pressure fall below a certain
threshold and to withhold ADH when blood vol-
ume is above that threshold. The threshold varies
with the body’s activities, and the cascade of
actions is a process of perpetual adjustment.
Doctors take advantage of these mechanisms to
treat HYPERTENSION(high blood pressure). Diuretic
medications—“water pills”—act on the tubules to
block them from reabsorbing sodium and chloride.
This increases the amount of water in the filtrate,
preventing the tubules from increasing blood
volume. Various antihypertensive medications,
such as angiotensin-converting enzyme (ACE)
inhibitors, target different stages of the angio-
tensin conversion process.
The kidneys and fluid balanceThe processes of
the kidney that regulate blood pressure also main-
tain the body’s fluid and electrolyte balances. The
hypothalamus monitors the amount of water in
the body, using ADH as the chemical messenger
that tells the kidneys the body needs more water
or less water.
The kidneys and erythropoiesis It seems a bit
odd, at first, that the kidneys produce the hor-
mone that stimulates the bone marrow to produce
new erythrocytes (red blood cells). But no other
organs have such intimate exposure to the blood
that they can literally “examine” each cell. With
every heartbeat 20 percent of the body’s bloodThe Urinary System 171