The magnitude of the changes that occur in the renal tubule vary in the few
species that have been studied sufficiently. In th e mammals, about 85% of the filter ed solutes and water are reabsorbed in the proximal tubule, while in amphibians
this figure is probably nearer 40 %. Under normal physiological conditions in mammals, more than 99 % of the material filtered across the glomerulus is retained by
the animal, though under other circumstances, necessitating substant ial excretion,
this ma y be reduced. Thus in freshwater fishes , which must excrete the large volumes of water gained osmotically from their environment, only 30 to 40 % of filter ed water is reabsorbed.
d) Regulation of Kidney Function. Factors that may affect kidney function have
been described in an excellent monograph by O'CONNOR (1962). These are:
(a) Composition of the plasma including the concentration of plasma proteins
and solutes such as, glucose, urea, potassium and certain anions.
(b) Arterial pressure.
(c) Renal nerves.
(d) Hormones; including adrenaline, adrenocortical and interrenal steroids and
neurohypophysial peptides.
The influences of (a) and (b) on the GFR have alread y been discus sed, while
the roles of adrenaline and the renal nerves are not currently considered to be ph ysiolo gically signific ant.
The concentration of the plasma solutes has a measure of autoregulatory control
within the kidney. The quantities filtered across the glomerulus will be related to
their plasma levels, while their reabsorption will be affected by the amounts
presented to the tubular epithelium. The latter process ma y involve either diffusion
or active transport. The regulation of plasma urea levels is a good example of such
autoregulation; filtered urea is subject to back-diffusion across the nephron wall ,
so that in mammals about 50 % of the amount filtered is excreted. Elevation of the
plasma concentration will increase the quantity filtered and a new equilibrium state
with the blood will ensue, resulting in a greater rate of excretion. If for some reason
the rate of filtration is reduced , an equilibrium with an elevated plasma urea level
results. In the case of glucose, the active reabsorption across the tubule wall has
a distinct maximum rate. If the rate of delivery exceeds this value, the excess filtered
glucose is excreted. Such a process is, of course, not primarily responsible for the
regulation of blood glucose levels, but in certain circumstances may contribute to
it. The levels of potassium and various anions in the plasma may also directly influence their rates of renal excretion.
The peptide hormones from the neurohypophysial region of the pituitary gland
increase osmotic water reabsorption from the distal segment of the renal tubule
and the collecting ducts in various tetrapod vertebrates; the y can also be shown to
decrease the GFR in certain tet rapods and to increase it in some fishes. The precise
mechanism for these latter effects is unknown, but the y are probably vascular.
The adrenocortical and interrenal steroid hormones increase sodium absorption
and potassium secretion in the nephron of some tetrapods, and under some circumstances can increase the GFR. The renal actions of such steroids in the amphibians
and fishes are not clear.
33
species that have been studied sufficiently. In th e mammals, about 85% of the filter ed solutes and water are reabsorbed in the proximal tubule, while in amphibians
this figure is probably nearer 40 %. Under normal physiological conditions in mammals, more than 99 % of the material filtered across the glomerulus is retained by
the animal, though under other circumstances, necessitating substant ial excretion,
this ma y be reduced. Thus in freshwater fishes , which must excrete the large volumes of water gained osmotically from their environment, only 30 to 40 % of filter ed water is reabsorbed.
d) Regulation of Kidney Function. Factors that may affect kidney function have
been described in an excellent monograph by O'CONNOR (1962). These are:
(a) Composition of the plasma including the concentration of plasma proteins
and solutes such as, glucose, urea, potassium and certain anions.
(b) Arterial pressure.
(c) Renal nerves.
(d) Hormones; including adrenaline, adrenocortical and interrenal steroids and
neurohypophysial peptides.
The influences of (a) and (b) on the GFR have alread y been discus sed, while
the roles of adrenaline and the renal nerves are not currently considered to be ph ysiolo gically signific ant.
The concentration of the plasma solutes has a measure of autoregulatory control
within the kidney. The quantities filtered across the glomerulus will be related to
their plasma levels, while their reabsorption will be affected by the amounts
presented to the tubular epithelium. The latter process ma y involve either diffusion
or active transport. The regulation of plasma urea levels is a good example of such
autoregulation; filtered urea is subject to back-diffusion across the nephron wall ,
so that in mammals about 50 % of the amount filtered is excreted. Elevation of the
plasma concentration will increase the quantity filtered and a new equilibrium state
with the blood will ensue, resulting in a greater rate of excretion. If for some reason
the rate of filtration is reduced , an equilibrium with an elevated plasma urea level
results. In the case of glucose, the active reabsorption across the tubule wall has
a distinct maximum rate. If the rate of delivery exceeds this value, the excess filtered
glucose is excreted. Such a process is, of course, not primarily responsible for the
regulation of blood glucose levels, but in certain circumstances may contribute to
it. The levels of potassium and various anions in the plasma may also directly influence their rates of renal excretion.
The peptide hormones from the neurohypophysial region of the pituitary gland
increase osmotic water reabsorption from the distal segment of the renal tubule
and the collecting ducts in various tetrapod vertebrates; the y can also be shown to
decrease the GFR in certain tet rapods and to increase it in some fishes. The precise
mechanism for these latter effects is unknown, but the y are probably vascular.
The adrenocortical and interrenal steroid hormones increase sodium absorption
and potassium secretion in the nephron of some tetrapods, and under some circumstances can increase the GFR. The renal actions of such steroids in the amphibians
and fishes are not clear.
33