1. Despite the continuity of the circulatory system, blood flowing down the veins of the neck does not assist blood flowing up the arteries. Because of collapsible veins, gravitational pressure gradients are not matched in arterial and venous sides of circulatory loops above the heart as would be necessary for a siphon to operate. 2. All animals have arterial blood pressures capable of lifting the blood to the head without the assistance from a siphon. In cases of longer vertical distances, such as in giraffes and climbing snakes, the heart does more work against the effects of gravity on the arterial blood column. 3. The potential energy of the blood in the head is lost as frictional heat produced in the veins.