dehydration is due to either the loss of sodium or water. water is lost through the skin and lungs (insensible loss) and the kidneys. concentrated, or dark urine is one of the symptoms of water deficit dehydration.
i am pulling this information from http://www.emedicine.com/ped/topic556.htm
(an article entitled dehydration
) because it describes part of the pathophysiology better than i can." the negative fluid balance causing dehydration results from decreased intake, increased output (renal, gastrointestinal, or insensible losses), or fluid shift (ascites, effusions, and capillary leak states such as burns and sepsis). the decrease in total body water causes reductions in both the intracellular and extracellular fluid volumes. . . dehydration is often categorized according to serum sodium concentration as isonatremic. . .hyponatremic. . .or hypernatremic. isonatremic dehydration is the most common. . .variations in serum sodium reflect the composition of the fluids lost and have different pathophysiologic effects.
isonatremic (isotonic) dehydration occurs when the lost fluid is similar in sodium concentration to the blood. sodium and water losses are of the same relative magnitude in both the intravascular and extravascular fluid compartments.
hyponatremic (hypotonic) dehydration occurs when the lost fluid contains more sodium than the blood (loss of hypertonic fluid). relatively more sodium than water is lost. because the serum sodium is low, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss.
hypernatremic (hypertonic) dehydration occurs when the lost fluid contains less sodium than the blood (loss of hypotonic fluid). relatively less sodium than water is lost. because the serum sodium is high, extravascular water shifts to the intravascular space, minimizing intravascular volume depletion for a given amount of total body water loss. . . during hypernatremic dehydration, water is osmotically pulled from cells into the extracellular space. to compensate, cells can generate osmotically active particles (idiogenic osmoles) that pull water back into the cell and maintain cellular fluid volume."
the role of the kidneys in this is that the resulting decreased renal blood flow causes the rennin-angiotensin system to increase sodium and water reabsorption in the kidney. this causes decreased urine output as well as the urine that is produced to be dark and concentrated.
merry christmas to all of you down under from all of us up north!