Ask A Scientist© Biology Archive

 
Question:  In order for a giraffe brain to get blood the pressure must be high.  What 
happens then when it lowers his/hers head to drink.  One would almost think that they 
would blow the top of their heads off.
Derick C Mather

Answer:  Really the same thing that happens to you when you raise or lower your head.  
The arteries serving your heart (carotids) constrict when you lower your head, and that 
lowers the blood pressure reaching the head.  When you raise it, they dilate, raising 
the pressure. Sometimes this doesn't work very well, and you feel dizzy when you suddenly 
raise your head.  If this is severe, you may faint, which is a very efficient way of 
putting your head back below your heart.  --ProfBill