The Blood Pressure of Giraffes
As discussed in this chapter, giraffes have, compared with any other mammal, a very high mean blood pressure of ~250 mmHg. Human blood pressure is ~90 mmHg. Its size is determined by the length of the neck, the height of the head above the heart, by hydrostatic pressure generated by gravity acting on the column of blood in the carotid artery, and contractions of the heart muscles: blood pressure must be high enough to ensure that blood reaches the brain. Uniquely in giraffes blood pressure is regulated by receptors that are located in both the carotid and occipital arteries. Once thought to be ~2.5% of body mass the heart is smaller (~0.5% of body mass) but its muscle walls, especially of the interventricular wall and left ventricle wall, are exceptionally thick (up to 8 cm). The relative cardiac output is the same as in other mammals (~5 L 100 kg–1 of body mass) through a combination of a higher than predicted heart rate (70 b min–1 vs 50 b min–1) and smaller than predicted stroke volume (~0.7 ml kg–1 body mass vs 1.2 ml kg–1). Stroke volume is small because the left ventricle muscle wall is thick. The origin of high blood pressure is the resistance to blood flow, which is about twice what it is in other mammals. The higher resistance results from a combination of the thick muscular walls and narrow lumens of a giraffe’s blood vessels and unique mechanisms that regulate blood flow to the brain.