Page 450 - Anatomy and Physiology of Farm Animals, 8th Edition

The Urinary System / 435

(A) No antidiuretic hormone (B) Antidiuretic hormone present
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Collecting
duct
duct Collecting
Hypotonic Hypotonic

HOH
HOH
Cortex
Medulla

400
mOsm/L
HOH HOH

800
mOsm/L

HOH
HOH

1200
Large volume
Large volume mOsm/L Small volume
dilute (HYPOTONIC) concentrated (HYPERTONIC)
dilute (HYPOTONIC)

Figure 23-11. (A) Distal tubule and collecting duct when ADH is absent. Water permeability of collect-
ing is low, so tubular fluid remains hypotonic (dilute). (B) Distal tubule and collecting duct when ADH is
present. Water permeability of collecting duct is increased, so water is reabsorbed and tubular fluid
becomes hypertonic (concentrated).

reabsorption of sodium when sodium must The concentration of potassium in
be retained to maintain sodium balance. plasma and other extracellular fluids also
The regulation of aldosterone secretion regulates aldosterone secretion. Increases
from the adrenal cortex relative to sodium in potassium concentration directly stimu-
balance is via the renin–angiotensin sys- late cells of the adrenal cortex to secrete
tem. When sodium must be retained (such aldosterone. Aldosterone promotes potas-
as with a low‐salt diet or after loss of extra- sium secretion by principal cells, and this
cellular fluid with sodium), the renin– tends to increase the urinary loss of potas-
angiotensin system is activated, and sium. The increased loss of potassium in
angiotensin II stimulates cells of the adre- the urine reduces plasma potassium, and
nal cortex to secrete aldosterone. thus potassium plasma concentration and