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5 Kidney, Salt and Water Balance F. Lang

Overview
Renal damage can impair renal perfusion as Normally the amount of filtered water and
well as glomerular and/or tubular functions solutes is a multiple of what is actually excret-
(→ A). In addition, abnormal urine composi- ed: all of the plasma water passes across the
tion can lead to precipitations (urolithiasis) renal epithelia within 20 minutes; the total ex-
that inhibit the free flow of urine. Abnormal tracellular volume within three hours. The ex-
renal function can be caused by reduced renal cretory capacity of the kidney is thus by no
excretion of useless or harmful substances means exhausted. For this reason GFR, i.e., the
(e.g., uric acid, urea, creatinine, vanadate volume controlled by the kidney, can be great-
[VnO 4 ], foreign substances [xenobiotics], and ly impaired without there being any harmful
so-called uremic toxins) whose plasma con- effect on the body. However, a reduction in
centration then rises correspondingly (→ A3). GFR will from the very beginning go hand in
Conversely, a defective glomerular filter can hand with a diminished regulatory range that
lead to renal loss of protein, while impaired will become apparent when there is an in-
tubular reabsorption can result in the in- creased load.
creased excretion of substances which are im- The kidney is not only the target organ for
portant for the body (electrolytes, minerals, bi- hormones, but also, by forming hormones, it
carbonate, glucose, amino acids). Reduced re- influences its own function as well as extrare-
nal excretory function affects the kidney’s de- nal elements of mineral metabolism (calci-
cisive contribution to the regulation of the me- triol) and blood pressure regulation (renin/an-
tabolism of water, electrolytes, minerals, and giotensin) (→ A2). The prostaglandins and ki-
acid–base balance (→ p.122ff.). Through its nins formed in the kidney primarily serve to
regulation of water and electrolyte metabo- regulate renal function. If the kidney is dam-
lism the kidney is also important for long- aged, the effects of abnormal renal excretory
term blood pressure regulation (→ p. 208ff.). function are added to those of abnormal renal
The capacity of the kidney to regulate the excretion of hormones. The hormone erythro-
composition of extracellular fluid is a function poietin, formed in the kidney, regulates eryth-
of volume, which, per unit time, is under the ropoiesis; its absence thus causes anemia
control of its epithelia. For substances that are (→ p. 32).
not secreted by tubular cells, the controlled Lastly, the kidney fulfills metabolic tasks
volume corresponds to the glomerular filtra- (→ A1). Thus, for example, in acidosis it splits
tion rate (GFR). All substances that are dis- ammonia from glutamate (ammonia is excret-
+
solved in the filtrate can either be reabsorbed ed as NH 4 ; → p. 86) and forms glucose from
or excreted by the tubular epithelium. For sub- the carbohydrate skeleton (gluconeogenesis).
stances that are secreted by the tubular epi- Glucose is also formed in the proximal tubules
thelium (e.g., potassium), the controlled vol- from absorbed lactate, and additionally fatty
ume is ultimately the entire blood plasma acids are broken down in the tubules. The kid-
that flows through the kidney (renal plasma ney plays an important role in the inactivation
flow [RPF]). of hormones. About 40% of insulin inactivation
Renal excretion is regulated or governed by takes place in the kidney, which also breaks
hormones (e.g., antidiuretic hormone [ADH] or down steroid hormones. Filtered oligopeptides
[arginine] vasopressin [AVP], aldosterone, atri- (e.g., hormones) are broken down in the tubu-
al natriuretic factor [ANF), parathyroid hor- lar lumen and the amino acids are reabsorbed.
mone [PTH], calcitriol [1,25(OH) 2 D 3 ], calcito- Reduction of functional renal tissue necessari-
nin, cortisol, prostaglandin E 2 , insulin, proges- ly impairs the above-mentioned metabolic
togens, estrogens, thyroxine, somatotropin) tasks.
and is thus adapted to requirements. Thus, dis-
orders of hormone release also impair renal
92 excretory functions.

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