A normal healthy humans can maintain constant blood flow to their brain providing that the average arterial blood pressures is between 50 and 150 mmHg. Above 150 mmHg there is a risk that blood flow becomes too high, which can potentially cause a brain haemorrhage.  Below 50mmHg and there is a risk that blood flow will be inadequate to perfuse the brain.

Giraffes can grow to heights of 5m.  I think that the difference in cerebral blood pressure between standing fully erect and bending down to ground level to drink would be over 360mmHg. 

I have heard that giraffes have a system of one way valves in their neck veins to prevent pressure overload in the venous system when bending down to drink but it would be impossible to achieve this with their arteries because the blood has to travel in the direction of the head.  

How do giraffes maintain constant brain blood flow over such a huge range of blood pressures?

How do their cerebral arteries cope with such high pressures without bursting? 

I went to a zoo recently and saw a couple of toucans (in very small enclosures.. grr). They each had cracked beaks. Would they have cracked if they had been in the wild, and how did they crack them (was it due to stress?).