Page 448 - Anatomy and Physiology of Farm Animals, 8th Edition

The Urinary System / 433

Descending ISOTONIC 300
VetBooks.ir vasa rectae mOsm/L

Loop
Ascending
of Henle vasa rectae
HYPOTONIC
Flow
Blood Flow Blood Cortex
flow flow Medulla

400
mOsm/L
NaCl
H O
2
H O
2
H 2 O

NaCl H O 800
2
mOsm/L
H O
2
NaCl
Collecting
duct

HYPERTONIC
loop of Henle HYPERTONIC 1200
vasa rectae mOsm/L

Figure 23-10. Transport of sodium chloride and water by loop of Henle and vasa rectae. Arrows indi-
cate direction of net transport in the different tubular segments and vessels. The osmolarity of the inter-
stitial fluids in the cortex is 300 mOsm/L; the osmolarity of the interstitial fluids in the medulla is shown
as a gradient that increases from outer to inner medulla. Hypertonic, isotonic, and hypotonic refer to
fluid and plasma in tubules and vessels, respectively.

medulla without disrupting the gradient. Collecting Duct Transport
There is a small net gain of both water and Antidiuretic Hormone
and particles by the vasa rectae, and some
of these are the water and particles that Principal cells in the collecting ducts are
were reabsorbed from the loop of Henle the target cells for ADH. If ADH is not
(Fig. 23‐10). present, the luminal cell membrane of
The maximal osmolality of the osmotic these cells is relatively water impermeable.
gradient in the renal medulla differs among ADH stimulates the insertion of water
species, and maximal urine concentration channels into these cell membranes to
ability is determined by the maximal osmo- increase the overall water permeability of
lality of the gradient (Fig. 23‐10). collecting ducts.