Being the tallest animal, coming in at around 18 feet, can have its advantages. However, the heart of such a tall animal must be extraordinarily equipped to handle the daunting challenge of pumping blood not only to the legs, but also against gravity all the way up their long necks to the brain. They achieve this by maintaining blood pressures that are twice that of other mammals.
For a review on how the heart works, click here.
Researchers in Denmark and South Africa have been studying the hearts of these amazing animals to find out how they work. Their findings were presented at the recent Experimental Biology Conference in Washington, DC. What they found was that the left ventricle of the giraffe heart is much thicker compared to other mammals. This extra muscle power allows the heart to pump blood harder producing a higher blood pressure. However, since the relative weight of the giraffe heart was similar to other mammals, something had to be smaller. That something turned out to be the holding capacity of the left ventricle, which had a smaller radius than other mammals. This means that the left ventricle of the giraffe heart holds less blood allowing it to pump a much smaller volume with greater force (due to the increased wall thickness). This is how they achieve such high pressures without stressing out their hearts. Since the heart has to work hard at pumping the blood out into vessels with very high pressures, the ability of the animals to maintain a high cardiac output (the amount of blood the heart can pump out to the body per unit of time) may be limited.
(M Smerup, J Funder, E Sloth, S Buus, C AAlkjaer, T Wang, E Brondum, NH Secher, P Bie, M Damkjaer, H Nygaard, MF Bertelsen, C Grondahl, G Candy, JM Hasenkam. "How can a normal-sized heart generate high blood pressure in the giraffe?")
To learn more about the amazing anatomy and physiology of the giraffe, check out this series of videos on YouTube. For those with weak stomachs, I should warn you that the videos show the necropsy (an autopsy for animals) of a young giraffe that died mysteriously at a zoo.
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What impresses me about them isn't the high cardiac output pressure, it's the ability of the rest of the circulatory and lymphatic systems to withstand that kind of pressure. In particular, their ability to avoid massive edema of the lower extremities.