Fact of the Day .CA

The Carotid Sinus is an arterial blood vessel that is part of the circulatory system.  In humans it is found in the neck.  What is special about this particular blood vessel is that it contains blood pressure receptors, known as baroreceptors, that are stimulated by high blood pressure.  When stimulated, the baroreceptors signal the heart to slow down in order to decrease the blood pressure.

In some martial arts and seen in film, people have gone unconcious when a certain part of their neck is pressed.  This can be done because if the carotid sinus is pressed it will increase the blood pressure in only that area.  As a result the baroreceptors within the carotid sinus will send a signal to the heart to slow down in order to lower the increased blood pressure.  Since the blood pressure in the rest of the body has not increased, this will cause the blood pressure to fall too low resulting in unconciousness.  Unconsiousness occurs because there is not enough blood being transported to the brain; if the body is horizontal then the blood does not have to work against gravity in order to reach the brain.

Sources:

http://dictionary.reference.com/browse/carotid sinus
http://www.healthscout.com/ency/68/74/main.html
http://en.wikipedia.org/wiki/Carotid_sinus