An upright giraffe, by analogy with humans, ought to suffer massive oedema in its feet; moreover, when it lowers its head to drink, the blood should rush down into it and be unable to flow up again. New pressure measurements reported on page 59 of this issue by Hargens et al. Show why neither of these things happens, and also contain some surprising observations of highly variable venous pressure (P.) in the leg and of a counter-gravitational gradient of P. in the neck, which greatly improve our understanding of the circulation of blood in the giraffe. The latter result, in particular demonstrates conclusively that the jugular vein is normally highly collapsed and therefore the circulation in the head and neck does not resemble a siphon, despite recent opinion to the contrary.
