Is the flow in the giraffe's jugular vein a ''free'' fall?

There is controversy as to whether or not the heart works against gravity in the arteries to the head in the upright position. One view is that the gravitational effects in the neck arteries are counterbalanced by the gravitational effects in the veins of the neck and the heart does not do extra pressure work. This concept has been challenged by others who claim that the heart works against gravity based on the notion that the jugular vein is collapsed and gravitational effects on jugular blood are inoperative, similar to the ''free'' fall of liquids. The present study supports the view that blood flow in the collapsible jugular vein of the giraffe is not a ''free'' fall and that the heart does not spend extra energy to raise the blood to the head. (C) 1997 Elsevier Science Inc.