Underfilling of the arterial circulation triggers a cascade of physiologic responses that preserve blood flow to vital organs. Except when renal salt wasting is the cause, these responses reduce urinary sodium excretion, so that nearly all ingested salt is retained.
Vasoconstriction maintains the systemic blood pressure and reduces renal blood flow.
The small bowel and colon are the most common sources of isotonic fluid loss. Spectacular amounts of isotonic salt water can be lost in diarrhea. Small bowel obstruction causes pooling of several liters of salt water within the bowel lumen. Fluid may also be sequestrated in the abdominal cavity in patients with pancreatitis or peritonitis. Sequestration of fluid in the soft tissues may also complicate crush injuries with rhabdomyolysis or burns.