Page 445 - Anatomy and Physiology of Farm Animals, 8th Edition

430 / Anatomy and Physiology of Farm Animals

the blood (hydrostatic) pressure in the cap- with a luminal border that is modified
with microvilli (brush border). The length
illary and the osmotic pressure generated
VetBooks.ir by the plasma proteins of the blood in the and brush border provide for a large
amount of cell membrane surface area,
capillaries. The hydrostatic pressure in the
urinary space of Bowman’s capsule and the and the proximal tubule does more tubu-
osmotic pressure generated by proteins in lar transport than any other nephron
the fluid in the space can also be factors, segment. The cellular junctions between
and these become important in disease proximal tubule cells are also permeable to
states (e.g., blockage of the urinary tract or some substances in the filtrate (e.g. chlo-
renal tubules). ride ions) so that some transport can occur
In mammals glomerular filtration rate between the cells.
(GFR) and renal blood flow remain rela- Glucose and amino acids are examples
tively stable in normally hydrated animals of essential nutrients that are reabsorbed
in spite of minor short‐term fluctuations in from the filtrate by cells of the proximal
arterial blood pressure (20 to 30 mmHg). tubule. Normally, 100% of the glucose
This stability is maintained by mechanisms and amino acids in the initial filtrate are
intrinsic to the kidney, and this phenome- reabsorbed by the proximal tubule. This
non is termed renal autoregulation. Severe reabsorption involves secondary active
dehydration or severe blood loss results transport using a sodium‐linked cotrans-
in lowering of blood pressure out of the porter in a manner similar to glucose
autoregulatory range, and this leads to absorption in the small intestine (see
vasoconstriction of preglomerular Chapter 21, Fig. 21‐7). Substances such as
vessels, including afferent arterioles. glucose that require membrane transport-
This vasoconstriction is produced by ers for reabsorption have limits to the
increases in sympathetic nerve activity amount that can be reabsorbed as the fluid
to the kidneys and increases in vasocon- flows through the tubules. This limit is the
strictors such as angiotensin II. The low tubular maximum, or transport maxi-
blood pressure and renal vasoconstric- mum. The blood level at which the amount
tion can reduce glomerular filtration to of a substance presented to the tubules by
the point of renal failure. This type of glomerular filtration exceeds the transport
renal failure is termed prerenal. maximum is the renal threshold. Animals
The GFR of mammals is normally about or people with uncontrolled diabetes
100 times that of urine flow rate (typical mellitus often have blood glucose levels
values for GFR are 3 to 5 mL/kg body that exceed their renal thresholds for
weight per minute). The high GFR relative glucose. In these cases, the increased
to urine flow allows for a continuous filtra- amounts of glucose in the filtrate cannot
tion of the plasma and the rapid removal of be completely reabsorbed by the proxi-
unwanted or toxic substances from the mal tubules, and glucose is present in
body. If such substances can readily pass voided urine (glucosuria).
through the glomerular filtration barrier Bicarbonate ions are the predominant
and are not reabsorbed from the renal base in the plasma and other extracellular
tubules, they are rapidly eliminated via the fluids throughout the body. Normally, the
urine. proximal tubule reabsorbs almost 85 to
90% of the bicarbonate ions in the initial
filtrate to maintain this ready supply of
Proximal Tubule Transport base. The transport of bicarbonate ions
from the tubular lumen into proximal
The proximal convoluted tubule is the tubule cells entails their conversion to car-
longest of the tubules, and proximal bon dioxide and water under the influence
tubules make up most of the renal cortex. of the enzyme carbonic anhydrase. This
Typical proximal tubule cells are cuboidal, reaction requires a hydrogen ion supplied