An allometric analysis of the giraffe cardiovascular system

Abstract

There has been co-evolution of a long neck and high blood pressure in giraffes. How the cardiovascular system (CVS) has adapted to produce a high blood pressure, and how it compares with other similar sized mammals largely is unknown. We have measured body mass and heart structure in 56 giraffes of both genders ranging in body mass from 18kg to 1500kg, and developed allometric equations that relate changes in heart dimensions to growth and to cardiovascular function. Predictions made from these equations match measurements made in giraffes. We have found that heart mass increases as body mass increases but it has a relative mass of 0.51±0.7% of body mass which is the same as that in other mammals. The left ventricular and interventricular walls are hypertrophied and their thicknesses are linearly related to neck length.Systemic blood pressure increases as body mass and neck length increase and is twice that of mammals of the same body mass. Cardiac output is the same as, but peripheral resistance double that predicted for similar sized mammals. We have concluded that increasing hydrostatic pressure of the column of blood during neck elongation results in cardiac hypertrophy and concurrent hypertrophy of arteriole walls raising peripheral resistance, with an increase in blood pressure following.

The thick left ventricular wall of the giraffe heart normalises wall tension, but limits stroke volume and cardiac output

“…Our in vivo echocardiography revealed that the giraffe heart is characterised by a small left ventricular cavity and hence a small ventricular radius as well as a comparatively large left ventricular myocardial wall thickness, which has only been inferred previously from post mortem observations (Mitchell and Skinner, 2009;Østergaard et al, 2013). Since we also confirmed the conspicuously high mean Pa and the associated high intraventricular pressures in the giraffe, our findings support the hypothesis that the combination of thick left ventricular wall and a low radius of the left ventricular cavity allow for a normal mammalian myocardial wall stress in the giraffe heart.…”

“…Thus, although the thick ventricular wall resembles the pathophysiological changes during acquired ventricular hypertrophy in response to aortic stenosis or hypertension in humans (Grossman et al, 1975;Hood et al, 1968;Sandler and Dodge, 1963), there is no indication of the secondary myocardial fibrosis in giraffes that inevitably accompanies the acquired left ventricular hypertrophy observed in human disease. It is also noteworthy that newborn giraffes have a myocardial wall thickness relative to ventricular diameter that is similar to other mammals, such that the ventricular wall thickening seems to arise as mean Pa increases with neck length (Mitchell and Skinner, 2009). Because the ventricular remodelling develops in response to increased afterload as the giraffe grows taller and because total myocardial volume of adult giraffes resembles that of similar-sized mammals, we suggest that the term 'concentric eutrophy' is used for the normal physiological state of the adult giraffe myocardium.…”

“…However, several recent and independent investigations, based on much larger sample sizes than the original studies by Goetz, now reveal that giraffes are merely endowed with the same relative cardiac mass as all other mammals, i.e. 0.5-0.6% of body mass (M b ; Brøndum et al, 2009;Mitchell and Skinner, 2009;Østergaard et al, 2013;Perez et al, 2008). Incidentally, Edwards Crisp (1864a,b) had already reported normal mammalian heart size in giraffes by the middle of the nineteenth century and Goetz's original report therefore appears erroneous (Mitchell and Skinner, 2009).…”

“…Thus, an attractive possibility is that the giraffe heart has normalized wall stress in response to the high intraventricular blood pressure by having a lower left ventricular radius and a thicker ventricular wall (Mitchell and Skinner, 2009). This can be achieved by a typical mammalian heart mass relative to M b , but must entail a lower stroke volume (V S ) per unit of cardiac mass than in similar-sized mammals (see Fig.…”

Scaling of left ventricle cardiomyocyte ultrastructure across development in the kangaroo Macropus fuliginosus

“…For instance, human heart mass increases with body mass during prenatal development with an exponent of 1.19, but following parturition, heart mass increases with body mass with an exponent of 0.89 (Hirokawa, 1972). Similarly, prenatal heart mass in the giraffe Giraffa camelopardalis scales with an exponent of 1.03, whereas postnatal heart mass scales with a shallower exponent of 0.90 (Mitchell and Skinner, 2009). A broader ontogenetic body mass range can also be obtained from marsupial mammal species, where neonates are highly altricial, and much of development occurs within the maternal pouch (Renfree, 2006;Tyndale-Biscoe and Janssens, 1988).…”