Following an intense workout your clothes are drenched in sweat. You estimate you lost at least 0.5 L of sweat. Your (similarly sized) friend skipped her workout to donate blood and you found out she donated 0.5 L of plasma. After these activities, which of you will have a greater increase in vasopressin secretion and why?

I will have a greater increase in vasopressin secretion than my friend who donated blood.

Explanation:

Vasopressin, also known as Arginine vasopressin (AVP), Argipressin, or Antidiuretic hormone (ADH), is synthesized in the hypothalamus, and is released from the posterior pituitary gland, into the genreral circulation in response to a hypertonic extracellular fluid concentration. When a person looses a large volume of water from the body, from whatever cause, the blood becomes hypertonic, and the total volume of body fluids reduces (hypertonic, hypovolemic), in response to this change, ADH acts on the cells of the Distal Convoluted Tubule and Collecting Duct of the kidneys to reabsorb water into the blood, hence resulting in the excretion of a more concentrated urine with a reduced volume, this tends to return the tonicity of the blood towards the normal.

In the instances given in this example, when I lost 0.5L of sweat, which contains about 99% water, I loose more body water than my friend who donated blood with a total plasma volume of 0.5L, because plasma contains 92%, which is less volume of water than what I lost.