Correct Response: D. An increase in the concentration of dissolved chemicals present in the blood, or osmolarity, signals to the brain that the body needs more water to dilute the chemicals and return the body to homeostasis. A decrease in heart rate and blood pressure (A) generally causes light• headedness or dizziness rather than thirst. An increase in internal temperature (B) can be an indication of infection and will signal the production of leukocytes to destroy pathogens. However, this does not usually cause the sensation of thirst. The first indication of less water in the body is a decrease in urinary output by the kidneys (C), which are signaled to reabsorb the water instead of moving it to the urinary bladder. Only when reabsorption of water by the kidneys is unsuccessful at balancing the osmolarity of the blood, is the sensation of thirst produced.
