A giraffe’s heart has to generate enough pressure to deliver blood to a brain high above its chest. Yet the heart is not extraordinarily heavy for the size of the animal. How can an ordinary allocation of heart muscle do such an unusual job?
Morten Smerup and colleagues tested an answer that depends on shape. In their 2016 study, the giraffe’s main pumping chamber had a thick muscular wall around a comparatively small cavity. That arrangement kept the calculated stress within the wall in a familiar mammalian range, even at high pressure. It also left less room for blood, helping explain why the measured amount pumped with each beat was relatively low.
The discovery is a trade-off within the pump. A design well suited to generating pressure need not be a design that moves a large volume at each contraction.
Pressure and muscle stress are different things
Blood pressure describes the force that blood exerts against a surface. Wall stress describes the load borne within the tissue holding that pressure. The two are connected, but chamber size and wall thickness matter too.
The authors began with a simple mechanical relationship: for a given pressure, a wider chamber puts greater stress on its wall, while a thicker wall distributes the load through more tissue. A smaller chamber enclosed by a thicker wall can therefore support high pressure without asking each unit of muscle to bear an equally exceptional stress.
This does not require an exceptionally massive heart. The same overall amount of tissue can be arranged around a different-sized cavity. The paper illustrates the point with two model hearts of equal muscle mass: the giraffe-like arrangement uses a thicker wall and leaves a smaller space inside.
Earlier observations of hearts after death had suggested such an arrangement. The important next step was to measure the chamber while it was beating, together with the pressure it generated.
Watching the pump work
The research was part of the Danish Cardiovascular Giraffe Research Programme’s work in South Africa. The team studied young giraffes under anaesthesia, using ultrasound to measure the left ventricle as it filled and contracted, and catheters to record pressure. The animals were supported upright during the measurements.
Ultrasound supplied the chamber’s length, internal width and wall thickness at different stages of a beat. Those dimensions allowed the team to estimate its volume using a simplified geometric shape. Subtracting the volume left after contraction from the volume before contraction gave an estimate of stroke volume: the amount of blood expelled in one beat.
That calculation depends on how well the assumed shape represents the real chamber. Smerup and colleagues therefore added a method that did not depend on ultrasound geometry. They measured cardiac output using inert-gas rebreathing, which estimates blood flow through the lungs from the clearance of a gas. Combined with heart rate, it provided a second estimate of blood pumped per beat.
The two approaches agreed closely. Ultrasound gave an average output of about 16.1 litres per minute, while rebreathing gave about 16.4 litres per minute under the study conditions. Agreement between methods with different assumptions strengthened the interpretation that the low measured output was real, rather than simply an artifact of the chamber-volume calculation.
A small chamber that empties a normal fraction
The ultrasound calculations gave an average left-ventricular volume of roughly 521 millilitres before contraction and a stroke volume of about 293 millilitres. The resulting ejection fraction was about 56%: a little over half the chamber’s filled volume was expelled per beat.
That distinction matters. A small stroke volume does not necessarily mean that a ventricle is failing to empty properly. Here, the fraction expelled was in the usual mammalian range, while the amount available to expel was comparatively small. Relative to body mass, stroke volume was around 0.59 millilitres per kilogram.
The chamber dimensions and pressure measurements also supported the proposed mechanical explanation. Calculated circumferential wall stress was about 7.8 kilopascals, which the authors compared with typical mammalian values. The high blood pressure therefore did not translate directly into extraordinarily high stress in the muscle wall.
The study’s central result is the connection between those observations. The small cavity and thick wall help with pressure generation, but the small cavity also limits the volume that can be moved with each beat. Describing only the thick wall would miss half the explanation.
What the measurements leave open
These were measurements in anaesthetised animals, not in giraffes walking, feeding or running. Anaesthesia can affect circulation. The ultrasound volumes also relied on a geometric approximation that had not been independently validated for giraffes. The second flow method provides reassurance about output, but does not remove every uncertainty from the chamber measurements.
The paper proposes that a small stroke volume could constrain oxygen delivery during intense exercise. That is a plausible consequence to investigate, rather than a result demonstrated by this experiment. Heart rate can rise during activity, and the study did not establish how stroke volume changes in a running giraffe or measure its maximum aerobic performance.
Likewise, a low resting output relative to other mammals is not automatically inadequate. The amount of oxygen delivered also depends on blood oxygen content and the amount extracted by tissues. Those wider adjustments require their own measurements.
Our broader giraffe science coverage includes several ways the body accommodates height. This study adds a particularly clear example of why adaptation is not simply enlargement. In the giraffe heart, distributing muscle around a smaller cavity offers a way to manage pressure, while leaving a measurable consequence for the volume of each beat.
Sources
Morten Smerup, Mads Damkjær, Emil Brøndum, Ulrik T. Baandrup, Steen Buus Kristiansen, Hans Nygaard, Jonas Funder, Christian Aalkjær, Cathrine Sauer, Rasmus Buchanan, Mads Frost Bertelsen, Kristine Østergaard, Carsten Grøndahl, Geoffrey Candy, J. Michael Hasenkam, Niels H. Secher, Peter Bie and Tobias Wang (2016). “The thick left ventricular wall of the giraffe heart normalises wall tension, but limits stroke volume and cardiac output.” Journal of Experimental Biology 219, 457–463. Paper · Author repository full text.
