Pressure profile and morphology of the arteries along the giraffe limb

Giraffes are the tallest animals on earth and the effects of gravity on their cardiovascular system have puzzled physiologists for centuries. The authors measured arterial and venous pressure in the foreleg of anesthetized giraffes, suspended in upright standing position, and determined the ratio between tunica media and lumen areas along the length of the femoral/tibial arteries in the hindleg. Volume fraction of elastin, density of vasa vasorum and innervations was estimated by stereology. Immunohistological staining with S100 was used to examine the innervation. The pressure increase in the artery and vein along the foreleg was not significantly different from what was expected on basis of gravity. The area of the arterial lumen in the hindleg decreased towards the hoof from 11.2 ± 4.2 to 0.6 ± 0.5 mm(2) (n = 10, P = 0.001), but most of this narrowing occurred within 2-4 cm immediately below the knee. This abrupt narrowing was associated with a marked increase in media to lumen area ratio (from 1.2 ± 0.5 to 7.8 ± 2.5; P = 0.001), and a decrease in mean volume fraction of elastin from 38 ± 6% proximal to the narrowing to 5.8 ± 1.1% distally (P = 0.001). The narrowing had a six-fold higher innervation density than the immediate distal and proximal regions. The sudden narrowing was also observed in the hind legs of neonates, indicating that it does not develop as an adaptation to the high transmural pressure in the standing giraffe. More likely it represents a preadaptation to the high pressures experienced by adult giraffes.