The carotid rete of giraffes: Good for brain cooling, bad for blood rushes
This paper actually came out last month, but I sat on promoting it at the time thinking that there was a press release in the works. Although this post is about a month late, it also comes with a sweet new write up on the slick Anatomy to You website. The website is dedicated to explaining cool bits of anatomy to the general public. It is run by functional morphologist, John Hutchinson and professional science communicator, Lauren Sumner-Rooney. I and my co-author, Haley O'Brien, were happy to contribute to the blog series and hope many will be able to get a lot of cool information from it.
In short: giraffes have long necks that should pose a host of problems for getting blood to brain at a reasonable pressure. How giraffes solve their blood pressure problem is a mystery. The typical answer has been that the carotid rete, an arterial meshwork located just outside the brain, acts as a capacitor, slowing down high pressure blood prior to entering the brain. The problem with this classical answer is that it lacked any empirical backing. The lack of empirical backing was, in part, due to the difficulties of measuring blood flow in an 5.5 meter tall, 1–2 tonne animal. That's where we came in. Using a digital modeling approach we pumped blood through this arterial meshwork and found that pressure was essentially constant throughout the rete. Couple this with the fact that giraffes are not unique among artiodactyls in having a carotid rete (indeed, our work used a goat rete, which looks the same), and all of a sudden the textbook answer behind the giraffe blood pressure problem no longer makes sense.
So if it's not the rete then what is it? We're not sure, but knowing that it is not the rete opens up the possibilities for other structures to fill this role (e.g., the jugular veins). I'm sure the real answer will be far more amazing than the classical answer.
For more, check out the write up on the Anatomy to You blog here: Vascular ‘safety net’ doesn’t protect the brains of giraffes from dangerous pressure changes
O'Brien, H.D., Bourke, J. 2015. Physical and Computational Fluid Dynamics Models for the Hemodynamics of the Artiodactyl Carotid Rete. J. Theor. Biol. 386:122–131.