The high pressure in giraffe leg arteries renders them vulnerable to edema. We investigated whether large and small arteries in the legs and the tight fascia protect leg capillaries. Ultrasound imaging of foreleg arteries in anesthetized giraffes and ex vivo examination revealed abrupt thickening of the arterial wall and a reduction of its internal diameter just below the elbow. At and distal to this narrowing, the artery constricted spontaneously and in response to norepinephrine and intravascular pressure recordings revealed a dynamic, viscous pressure drop along the artery. Histology of the isolated median artery confirmed dense sympathetic innervation at the narrowing. Structure and contractility of small arteries from muscular beds in the leg and neck were compared. The arteries from the legs demonstrated an increased media thickness to lumen diameter ratio, increased media volume, and increased numbers of smooth muscle cells per segment length and they furthermore contracted more strongly than arteries from the neck (500 ± 49 vs. 318 ± 43 mmHg, n=6 legs and neck, respectively). Finally the transient increase in interstitial pressure following injection of saline was 5.5 ± 1.7 times larger (n=8) in the leg than in the neck. We conclude that 1) tissue compliance in the legs is low; 2) large arteries of the legs function as resistance arteries; and 3) structural adaptation of small muscle arteries allow them to develop an extraordinary tension. All three findings can contribute to protection of the capillaries in giraffe legs from a high arterial pressure.
- smooth muscle
- sympathetic innvervation
- resistance artey structure
- Copyright © 2013, American Journal of Physiology - Regulatory, Integrative and Comparative Physiology