Mammalian Physiology BIOAP 4580
RENAL TURNOVER OF THE
EXTRACELLULAR FLUID COMPARTMENT
A. Functional résumé: the kidney, the organ of extracellular fluid homeostasis.
volume and composition of the extracellular fluid compartment is the primary concern of the
kidneys; the formation of urine is only the consequence of this regulation.
What is regulated
to remain „normal‟ is the extracellular fluid compartment, not the urine.
volume and composition of the urine can vary widely.
However, it is considered abnormal if
the urine contains protein and/or glucose.
The presence of these two solutes in the urine
signals problems that must be followed up.
1. Extracellular fluid homeostasis: the selective excretion or conservation of:
a) Extracellular ions K
, etc., glucose, amino acids, urea, etc., all of which are
kept essentially at constant plasma concentrations over the lifetime of the animal,
O; a change by only 1 mOsm (from 295 to 296 mOsm) is sufficient stimulus for
the release of the antidiuretic hormone and the initiation of antidiuresis,
c) Nitrogenous wastes of protein metabolism (urea, uric acid, creatinine, ammonia),
; pH is regulated within narrow limits,
e) Removal of natural (endogenous) organic acids and bases and foreign organic
compounds (including drugs) from the circulation.
2. Endocrine functions:
a) Synthesis of erythropoeitin (EPO) by the renal erythropoeitic factor (REF) which
b) Renin (regulation of extracellular fluid volume and hence blood pressure),
c) Synthesis of the active form of vitamin D: 1, 25 dihydroxycholecalciferol.
B. Renal regulation of the extracellular fluid compartment: Matching output to input.
1. Regulation of ion balance in the extracellular fluid
a) Intake of ions Na
The intake is
unpredictable and at the whim of the diet and social habits,
b) Output of ions.
The output is also unpredictable since the loss of salts (ions) in feces,
sweat, lactation from the extracellular fluid compartment serves other physiological
c) Constant extracellular fluid concentrations are maintained through the precise
regulation of conservation/excretion via the kidney. The regulation is mostly under
endocrine control involving the hormones aldosterone, calcitonin, parathyroid
hormone, 1,25-dihydroxycholecalciferol, the renin angiotensin system, and natriuretic
There is also some neural innervation/regulation of the kidney.
kidneys function quite well without neural input, which is one of the reasons why
they were the first organ to be successfully transplanted.
2. Regulation of water balance in the extracellular fluid