Aims: 

In both human and experimental hypertension the resistance arteries are structurally altered i.e. the media is thickened while the lumen size is reduced. This provides an amplification of the resistance increase in response to vasoconstriction and may thus be important for the increased resistance seen in most forms of hypertension. The amplified vasoconstrictor response may also serve though to protect the capillary bed against a high transmural pressure. The vascular structure might also be important in the giraffe, where due to the height of the animal, the capillary bed in the legs might be exposed to a very high transmural pressure.

Methods: 

To test this possibility we obtained biopsies of leg muscles and from muscles on lower and upper neck neck1 and neck2, respectively from six male giraffes (Giraffa camleopardalis). We also used ultrasound to follow the conduit arteries from the leg and measured the pressure along the leg arteries. From the biopsies small arteries were dissected and mounted in a myograph.

Results: 

The normalized diameters of the arteries from the 3 sites did not differ, whereas the media thickness decreased significantly with the height and was leg: 24.7±2.4 mm, neck1: 16.4±1.1 mm and neck 2: 12.3±0.4 mm. With ultrasound a noradrenaline sensitive sphincter like structure was apparent in the main artery at the leg immediately below the elbow. Constriction of this structure provided a resistance which results in a viscous pressure drop of up till 100 mmHg.

Conclusion: 

The study strongly suggests that structurale features of both conduit and resistance arteries play a role in protecting capillaries in the leg against a high transmural pressure.