Question 215

Question:

A 25-year-old woman has a 3-day history of vomiting and diarrhea. She has postural hypotension and poor tissue turgor. Her serum sodium concentration is 130 mEq/L. Which of the following findings is most likely?

Correct Answer: D.  Increased serum ADH (vasopressin) concentration

ADH (vasopressin) is secreted by the posterior pituitary gland. Its secretion is in response to either activation of baroreceptors in the veins, atria and carotid bodies in response to reduced plasma volume, or by activation of osmoreceptors in the hypothalamus in response to increased plasma oncotic pressure. The patient in this question has orthostatic hypotension and poor skin turgor, clinical signs suggesting volume depletion. Likewise, plasma volume is low and ADH levels are responsively increased. Because plasma sodium concentration is lower than normal, the renin-angiotensin-aldosterone system will be activated and the concentrations of all three will be increased. ANP is secreted by atrial myocetes in response to increased blood pressure, the opposite of this patient. In dehydrated states, urine osmolality is higher than plasma/serum osmolality due to the action of ADH increasing permeability of the collecting duct to reabsorb water.