Page 548 - Anatomy and Physiology of Farm Animals, 8th Edition

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the tubules of the RT nephrons do not decrease in the number of filtering RT
nephrons. The release of AVT from the
include a segment that corresponds to a
VetBooks.ir loop of Henle, and the tubules of the RT posterior pituitary responds to changes in
extracellular fluid osmolality in a manner
nephrons are contained in the outer zones
of the lobules, where they connect to collect- similar to that for mammals. When water
ing ducts (Figure 30‐9). Avian kidneys also intake is excessive, AVT levels are reduced
contain mammalian‐type (MT) nephrons and a more dilute urine is excreted; when
that do have segments that correspond to water intake is restricted, AVT levels
the loops of Henle in mammalian kidneys, increase and a more concentrated urine is
and these loops extend downward into excreted. However, because the maximal
medullary cones that support the lobules osmolality of the interstitial fluid sur-
(Fig. 30‐10). These tubular loops of MT rounding collecting ducts is less in birds
nephrons in avian kidneys function simi- than in mammals, maximal urine osmo-
larly to loops of Henle in mammalian kid- lality is also less. The production of large
neys and contribute to the formation of an volumes of dilute urine (polyuria) can
osmotic gradient within the interstitial flu- occur normally preceding egg laying, but
ids of the medullary cone. However, the may also be associated with pathologic
magnitude of the gradient is much less in conditions.
birds. Tubular segments corresponding Birds also have other means to promote
to proximal tubules and distal tubules can water reabsorption from renal tubules, and
be found in both MT and RT nephrons, and some of these means are related to the
these segments in birds perform secretory excretion of nitrogenous wastes. Both
and reabsorptive functions similar to mammals and birds eliminate nitrogenous
nephrons in mammalian kidneys. wastes through urine, but the chemical
Glomerular filtration rate (GFR) form of the waste differs between mam-
remains relatively stable in normal mammals mals and birds. Mammals primarily excrete
despite mild to moderate changes in hydra- urea, whereas birds primarily excrete uric
tion status. However, in birds, moderate acid. Uric acid is predominantly produced
dehydration, such as with periods of water by the avian liver and is relatively nontoxic
restriction, is associated with significant when compared to urea or ammonia. Urea
reductions in GFR. Renal output is gener- enters tubular fluid (potential urine) of
ally 100 to 200 mL/kg/day in a normal bird, mammals by glomerular filtration. Uric
whereas a stressed or dehydrated bird may acid is minimally bound to protein in the
have renal output as low as 25 mL/kg/day. plasma and enters tubule fluid in avian
Some suggest that this is primarily due to kidneys by both glomerular filtration and
vasoconstriction of vessels supplying the tubular secretion with secretion greater in
RT nephrons while there is also significant most cases. Secretion of uric acid is pre-
evidence that the avian kidney is able to dominantly by the RT nephrons and is
selectively utilize one type of nephron over therefore independent of glomerular
another for filtering depending on the filtration rate, glomerular blood flow, or
physiologic status of the bird. hydration status. The unique renal portal
To affect water balance in response to system described previously supplies
changes in water intake, the water perme- blood to a peritubular capillary network
ability of collecting ducts in mammalian around tubules and is a source of uric acid
kidneys is regulated by antidiuretic hor- for secretion, even when glomerular blood
mone. Collecting ducts in avian kidneys flow and glomerular filtration are reduced.
respond similarly to arginine vasotocin Neither urea nor creatinine are major
(AVT), the avian antidiuretic hormone, nitrogenous waste in birds. Therefore,
and increase their water permeability. AVT measurement of creatinine or blood
has no effect on mean blood pressure, but urea nitrogen (BUN) levels cannot be
it is able to increase GFR by causing a used as indicators of renal function.