Giraffe have unusually high heart rates for their size, at 150 beats per minute. When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint. The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered. Conversely, the blood vessels
Ultrasonography of chronic venous insufficiency of the legs and show valvular function: Normal blood flow is anterograde (going to the heart), and from superficial to deep veins via perforator veins. However, there are two exceptions: firstly, the great saphenous vein (GSV) collaterals, (the veins that run parallel), drain the abdominal wall and have a flow from top to bottom so that when an examiner tests the sapheno-femoral junction, a false positive diagnosis might be made; secondly, in the flow from the sole of the foot venous network around 10% drains to the dorsal venous arch of the foot, going therefore against the norm, from deep to superficial veins.