The giraffe experiences great cardiovascular challenges. When it lowers its head to drink, arterial pressure at the brain is expected to increase dramatically. This study investigated how blood pressure is regulated when lowering the head of anaesthetized giraffe.

Methods: 

We measured pressure by insertion of catheters at the central part of the carotid artery and jugular vein of 5 anaesthetized spontaneously breathing giraffes, suspended in upright position. Changes in jugular cross sectional area were visualized by ultrasound (US).

Results: 

When the giraffes head was lowered below heart level, the central arterial pressure (CAP) decreased from 20514 mmHg to 13918 mmHg, and central venous pressure (CVP) fell 2.91.2 mmHg. US images revealed significant increase of the cross section of the cranial part of the jugular veins from 0.120.04 cm2 to 3.160.59 cm2, and thus an accumulation of estimated 2.5L of blood.

Conclusion: 

When the giraffe lowers its head, central blood volume falls and the associated reduction in cardiac filling lowers CAP by 60 mmHg. This may contribute to protection of the brain capillaries. When the giraffe lifts the head, the vein collapses and the accumulated blood is returned to the central circulation, increasing preload. Our study shows that the Starling mechanism may be involved in protecting the giraffe's brain when lowering the head.