You will be given a definition of a task first, then some input of the task.
Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: You couldnt survive without your skin.
Sent 2: It has many important functions.
Sent 3: The main function of the skin is controlling what enters and leaves the body.
Sent 4: It prevents the loss of too much water from the body.
Sent 5: It also prevents bacteria and other microorganisms from entering the body.
Sent 6: The skin helps maintain a constant body temperature.
Sent 7: It keeps the body cool in two ways.
Sent 8: Sweat from sweat glands in the skin evaporates to cool the body.
Sent 9: Blood vessels in the skin dilate, or widen.
Sent 10: This action increases blood flow to the body surface.
Sent 11: This allows more heat to reach the surface.
Sent 12: The heat is then able to radiate off the body.
Sent 13: The opposite happens to retain body heat.
Sent 14: Blood vessels in the skin constrict, or narrow.
Sent 15: This decreases blood flow to the body surface.
Sent 16: This reduces the amount of heat that reaches the surface.
Sent 17: When this happens, less heat can be lost to the air. 
Question: What decreases blood flow to the body's surface?.
Output:
Blood vessels.