You'd have thought that maintaining a decent blood supply to your brain when your head is almost 2 m above your heart could be a serious headache, but giraffe's have evidently overcome any difficulties they might encounter. However, the mechanism that maintains sufficient blood flow to the animal's head has remained a topic of hot debate for several decades. One suggestion had been that the major blood vessels in the mammal's graceful neck form a siphon that draws blood up to the brain. In this issue of The Journal of Experimental Biology, Graham Mitchell, Shane Maloney, Duncan Mitchell and James Keegan add another contribution to the argument, suggesting that hydrostatic pressure generated by the column of blood in the carotid artery,and not a siphon, could account for the astounding blood pressures need to keep the giraffe's brain supplied(p. 2515).

The team built a mechanical model of a giraffe's neck and head, consisting of a 1660 mm long carotid artery' tube, a 1638 mm long collapsible jugular'tube, a linking tube to simulate blood flow through the brain, and a pump that simulated the heart. Adjusting the relative positions of the bottoms of the carotid and jugular tubes to create a siphon, the team tested several permutations of rigid and flexible blood vessels, and found that the siphon failed to deliver sufficient pressure. However, when the team activated the pump to simulate the heart's intrinsic hydrostatic pressure, they successfully generated pressures similar to those measured in the giraffe's carotid artery. And when the team constricted the lowest portion of the jugular tube, they found that the blood pressure rose dramatically. They suggest that by constricting the jugular at this point, giraffes could maintain sufficient blood pressure, when they raise their heads after drinking, to prevent themselves from passing out.

Mitchell, G., Maloney, S. K., Mitchell, D. and Keegan, D. J.(
2006
). The origin of mean arterial and jugular venous blood pressures in giraffes.
J. Exp. Biol.
209
,
2515
-2524.