nephrons undergo alterations in function—ﬁ ltration, reabsorp-
tion, and secretion—to compensate for the missing nephrons.
For example, each remaining nephron increases its rate of potas-
sium secretion so that the total amount of potassium the kidneys
excrete is maintained at normal levels. The limits of regulation
are restricted, however. To use potassium as our example again,
if someone with severe renal disease were to go on a diet high in
potassium, the remaining nephrons might not be able to secrete
enough potassium to prevent potassium retention.
Other problems arise in uremia because of abnormal
secretion of the hormones the kidneys produce. Thus, decreased
secretion of erythropo
ietin results in anemia (Chapter 12).
Decreased ability to form 1,25-(OH)
D results in deﬁ cient
absorption of calcium from the gastrointestinal tract, with
a resulting decrease in plasma calcium, increase in PTH, and
inadequate bone calciﬁ cation (secondary hyperparathyroidism).
Erythropoietin and 1,25-(OH)
D (calcitriol) can be adminis-
tered to patients with uremia.
In the case of the secreted enzyme renin, there is rarely
too little secretion, but rather, too much secretion by the juxta-
glomerular cells of the damaged kidneys. The main reasons for
the increase in renin are decreased sodium delivery to the mac-
ula densa and decreased perfusion of affected nephrons (intrare-
nal baroreceptor). The result is increased plasma angiotensin II
concentration and the development of
inhibitors and angiotensin II receptor blockers can be used to
lower blood pressure and improve sodium and water balance.
Hemodialysis, Peritoneal Dialysis,
Failing kidneys may reach a point when they can no longer
excrete water and ions at rates that maintain body balances of
these substances, nor can they excrete waste products as fast as
they are produced. Dietary alterations can help minimize but
not eliminate these problems. For example, lowering potassium
intake reduces the amount of potassium to be excreted. The clin-
ical techniques used to perform the kidneys’ excretory functions
are hemodialysis and peritoneal dialysis. The general term
means to separate substances using a permeable membrane.
The artiﬁ cial kidney is an apparatus that utilizes a process
to remove wastes and excess substances
from the blood (
). During hemodialysis, blood is
and purified water)
Dialysis fluid and ultrafiltrate
of plasma output
Simpliﬁ ed diagram of hemodialysis. Note that blood and dialysis ﬂ
uid ﬂ ow in opposite directions through the dialyzer (countercurrent).
The blood ﬂ ow can be 400 ml/min, and the dialysis ﬂ
uid ﬂ ow rate can be 1000 ml/min! During a three- to four-hour dialysis session,
approximately 72 to 96 L of blood and 3000 to 4000 L of dialysis ﬂ uid passes through the dialyzer. The dialyzer is composed of many strands
of very thin dialysis tubing. Blood ﬂ ows inside each tube, and dialysis ﬂ uid bathes the outside of the dialysis tubing. This provides a large
surface area for diffusion of waste products out of the blood and into the dialysis ﬂ