Page 122 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Chronic Renal Failure: Abnormal Regulation
Reduced renal elimination of water and elec- When Ca 2+ forms a complex with phos-
trolytes is particularly important in the devel- phate, the concentration of Ca 2+ is lowered.
opment of symptoms of renal failure (see also The hypocalcemia stimulates the release of
p.110). The extracellular volume expands if PTH from the parathyroid gland, mobilizing
there is an excess of NaCl and water (→ A), calcium phosphate from bone (→ B). The result
and hypervolemia as well as edemas develop is demineralization of bone (osteomalacia).
(→ p.122); pulmonary edema being the most Normally PTH makes it possible, by simulta-
dangerous complication (→ p. 80). If it is pre- neous inhibition of renal reabsorption of phos-
Kidney, Salt and Water Balance ger of cerebral edema (→ p. 358). + + Ca 2+ concentration remains low. The stimulus
phate, for the plasma concentration of phos-
dominantly water which is in excess, the os-
phate to fall so that, despite mobilization of
motically driven entry of water increases the
calcium phosphate from bone, the solubility
intracellular volume (→ A) and there is a dan-
2+
product in plasma is not exceeded and Ca
The hypervolemia results in the release of
concentration can increase. In renal insuffi-
atrial natriuretic factor (ANF) and probably
ciency, however, renal excretion cannot be en-
also of ouabain. The latter inhibits Na /K -
hanced, plasma phosphate concentration in-
ATPase (→ A1). Vanadate (VNO 4 ), which is
creases, CaHPO 4 is precipitated, and plasma
largely excreted by the kidney, has a similar ef-
this persisting secretory stimulus the para-
its plasma level is markedly raised in renal fail-
ure.
thyroid glands hypertrophy and, in a vicious
+
+
circle, release ever larger amounts of PTH.
Inhibition of Na /K -ATPase causes reduced
5 fect. Its clearance is about the same as GFR and for PTH release therefore continues. Under
+
Na reabsorption in the kidney. Additionally, As the receptors for PTH are, in addition to
+
the intracellular K concentration falls in cells those in the kidney and bones, in many other
from diverse tissues and the cells depolarize. organs (nervous system, stomach, blood cells,
+
The intracellular concentration of Na rises. gonads) it is assumed that PTH plays a role in
+
This impairs 3Na /Ca 2+ exchange (→ A2), and the development of abnormalities in these or-
thus the intracellular concentration of Ca 2+ gans. In fact, removal of the parathyroid glands
also increases. The consequences of depolar- is thought to significantly improve numerous
ization are abnormal neuromuscular excitabil- symptoms of renal failure.
–
ity, cellular accumulation of Cl , and cell swel- The formation of calcitriol is reduced in re-
ling (→ A; see also p.10). The increased Ca 2+ nal failure, which also plays a part in causing
concentration causes vasoconstriction as well the abnormalities of mineral metabolism. Nor-
as increased release of hormones (e.g., gastrin, mally this hormone stimulates the absorption
insulin) and increased hormonal effects (e.g., of calcium and phosphate in the gut (→ B). Al-
epinephrine). though calcitriol deficiency reduces the intes-
Abnormalities of mineral metabolism also tinal absorption of phosphate, it aggravates
contribute greatly to the symptoms of renal the hypocalcemia. There are receptors for cal-
failure (→ B). If the GFR is reduced to less than citriol in various organs. Calcitriol substitution
20% of normal rate, less phosphate is filtered does not necessarily improve the symptoms
than is absorbed through the gut. Even if the and endangers the patient with renal failure
entire amount of filtered phosphate is elimi- by stimulating the intestinal absorption of
nated, i.e., there is no reabsorption, renal elim- phosphate.
ination cannot keep up with intestinal absorp-
tion, and the plasma concentration of phos-
phate rises. When the solubility is exceeded,
phosphate combines with Ca 2+ to form poorly
soluble calcium phosphate. The precipitated
calcium phosphate is deposited in the joints
112 and skin (causing joint pains or pruritus, re-
spectively).
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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