Giraffe Cardiovascular Adaptations to Gravity

The physiological systems of animals have adapted to Earth’s gravity over the past hundreds of millions of years. In general, gravitational adaptations of the cardiovascular system are more pronounced in terrestrial species with greater height and thus greater gravity-dependent gradients of blood pressure from head to feet. For example, dinosaurs (1), tree-climbing snakes (2), giraffes (3), and other tall animals have evolved mechanisms to provide adequate blood flow and nutrition to their brains while restricting blood flow and tissue swelling in their legs. Terrestrial animals of short stature and marine animals probably require much less sophisticated cardiovascular adaptive mechanisms. At the other extreme, aquatic snakes have little ability to withstand gravity out of water and rapidly “faint” when placed head above tail (2). Moreover, when gravity is absent even over short periods of time, astronauts experience orthostatic intolerance upon readaptation to gravity (see Chapter 58). Because humans are relatively tall compared to other species of animals, they too have developed extensive and sophisticated regulatory mechanisms to maintain cerebral perfusion and prevent lower extremity edema while in an upright posture. In fact, most understanding of gravitational mechanisms to date relates to observations in humans. However, taller terrestrial animals, such as the giraffe, may allow better understanding of the physiological adaptations to gravity. For example, blood pressure in giraffes is high to pump blood to their brain, but high blood pressures in their feet would theoretically cause severe dependent edema. Cardiovascular systems generate and regulate blood pressure to provide flow to tissues. This blood flow nourishes tissues by supplying oxygen (O2) and other nutrients, and by removing carbon dioxide (CO2) and other waste products. Transmural pressures (Px : pressure gradients from inside to outside the circulation) dictate the wall stress (S) and degree of openness or closure (vessel radius r) experienced by circulatory structures according to the Law of Laplace: Px = S/r (4). Normally, five components determine transmural pressure in the circulation: (a) the dynamic pressure resulting from cardiac pumping against peripheral resistance to blood flow; (b) dynamic pressures due to inertial forces of blood during activity (e.g., the footward acceleration of blood in leg vessels at heel strike during locomotion); (c) pressure due to the finite compliance (volume/pressure characteristics) of the systemic vasculature, especially the veins (this pressure is commonly called the mean circulatory filling pressure); (d) extravascular pressure of tissues and interstitial fluid (e.g., total tissue pressure); and (e) an intravascular hydrostatic pressure due to gravity (gravitational pressure). Although extravascular tissues and fluid have mass, gravity affects extravascular hydrostatic pressures to a minor extent because tissue fluids are bound within cells or are discontinuous in most extravascular spaces (except for cerebrospinal fluid). Structural and contractile tissues respond dynamically, within physiological limits and over a long period of time, as adaptive structural and functional remodeling, to increased (or decreased) levels of physical stress by increasing (or decreasing) their functional capability and/or mass (5). Although physiological responses to stress occur within seconds to minutes, longer-term functional and structural compensatory adjustments to the initiation of stress are detectable within hours (6).