Giraffe are known by most people around the world as creatures with extraordinarily long necks. This has many advantages as it allows them to reach areas for food which is inaccessible to most other competitor species. It also allows them an elevated view of their surroundings. However this long appendage which has seven cervical vertebrae also poses some challenges for the giraffe for, whereas it makes it easy for them to reach things high up it makes things equally difficult for them when they require something at ground level. Now what could a giraffe be needing at ground level? Only one thing – water! To drink water giraffe have to adopt a very ungainly position with legs widely spread as shown in the images. This places them in a position of extreme vulnerability to predators for it requires much effort and precious time to get into this position and regain an upright posture when they have finished drinking. Giraffe seem to be aware of this because they are very circumspect before they actually commit themselves to taking up the drinking position and will stand for a long time at a waterhole carefully observing the surroundings before they will finally move to the waters edge and drink. Having to lower the head to a position much lower than the heart also presents unique physiological challenges.
Medical doctors during the mid-1950's, concerned with high blood pressure in humans, conducted some physiological experiments on giraffe. The giraffe's long neck piqued their interest. Changes in blood pressure, occurring when the giraffe leans down to drink, would create problems that had to have been solved by some physiological means. Unless there was some mechanism, lowering the head would increase the blood pressure to such an extent that rupture of blood vessels in the brain would be highly likely. The heart must pump blood up 2.5 meters to the brain when the giraffe is erect, and down 2.5 meters when the giraffe stoops to drink. The circulatory system must have some way of preventing the blood from rushing too quickly back to the heart from the brain when the animal is erect or down to the brain when the animal's head is lowered. The giraffe raises and lowers it’s head quickly.
Tests showed that the blood pressure at the base of the brain was 200 mm Hg (millimeters of mercury) when the giraffe is upright and, instead of being higher as expected, dropped to 175 mm Hg when the head was lowered. The viscosity of giraffe blood and its protein content was expected to be high - thicker things flowing more slowly. Instead, the viscosity was found to be the same as man, and the protein content, which might have caused high osmotic pressure, was found to be lower than that of man.
As in most ruminants, the blood reaches the brain from the heart by way of the common and external carotid arteries. The two external carotids divide, just before each reaches the brain, into many small vessels forming a tight network that is called the rete mirabile. The vessels of the giraffe rete have elastic walls which can accommodate excess blood when the head is lowered so that the brain is not flooded. As a further safeguard for the brain while the giraffe is in this position, a connection between the carotid artery and the vertebral artery drains off a portion of the blood even before it reaches this network. The walls of the rete mirabile vessels are also elastic enough to retain sufficient blood when the head is raised so that the brain's supply is not depleted momentarily during the system's pressure changes.