Page 446 - Anatomy and Physiology of Farm Animals, 8th Edition

The Urinary System / 431

by transport from within the tubule cell. increase during dehydration or blood loss,
when it is appropriate to retain sodium
Once inside the tubule cell, the carbon
VetBooks.ir dioxide and water are reconverted to bicar- chloride and water.
Cells of the proximal tubule also actively
bonate and hydrogen ions, again under the
influence of carbonic anhydrase. The secrete organic anions and organic cations
bicarbonate ion can then exit the cell, using into the tubular fluid to be added to the
a membrane transporter, to be added to urine. It is by these secretory systems that
the blood again (Fig. 23‐9). Sodium accom- the kidneys eliminate many pharmaceuti-
panies the bicarbonate ions, so the electri- cals (that are organic compounds) in the
cal neutrality of body fluids is maintained. urine.
Sodium and chloride are the two pre-
dominant osmolytes in the initial filtrate,
and cells of the proximal tubule reabsorb Concentration and Dilution
70 to 75% of the sodium and chloride in the of Urine: Role of the Loop of Henle
initial filtrate. The percentage reabsorbed and Collecting Duct Transport
can be increased by the actions of angio-
tensin II and sympathetic nerves on tubule To maintain water balance during poten-
cells and by vasoconstriction of renal blood tially drastic changes in water intake, the
vessels. Angiotensin II concentrations and kidneys must be able to excrete urine that
sympathetic nerve activity to the kidneys is either more concentrated than plasma

Capillary Interstitial
lumen fluid Tubular epithelium Tubular lumen
Endothelium

Na + HCO 3 –

+
Na + Active transport Na + Na –H + Na +
countertransport
– – + H + HCO 3 –
HCO 3 HCO 3 + H
H 2 CO 3 H 2 CO 3
(Carbonic
anhydrase)
H 2 O
+
H 2 O
CO 2 CO 2 CO 2

Figure 23-9. Mechanisms of reabsorption of bicarbonate ions (HCO ) by proximal tubule cells. Source:
−
3
Reece, 2009. Reproduced with permission of John Wiley & Sons, Inc.