Scaling of Heart Function

Each contraction of the heart muscle entails a complex sequence of coordinated electrical and mechanical events (see Chapter 25). In outline, at the time of each heartbeat, the heart’s pacemaker initiates a wave of electrical activity that sweeps through the heart muscle. This electrical wave first causes the atrial chambers to contract, then pauses as it passes from the atrial to the ventricular chambers, and finally causes the ventricular chambers to contract. For the hearts of two species to contract at different rates, adjustments are required in all of these individual processes. The sweep of electrical activity in two hearts of very different size is shown in the figure and discussed below in Box Extension 7.5.

Views of the ventricles of a mouse heart and pig heart showing the sweep of activation of the superficial ventricular muscle layer during a single heartbeat. The wave of electrical activity responsible for activating contraction first appears in the superficial muscle layer at the points marked by the white asterisks. It then sweeps through the superficial muscle layer in the color-coded order shown in the center, taking a total of 4 ms in the mouse heart but 22 ms in the pig heart. (From Noujaim et al. 2007; images courtesy of Sami Noujaim and José Jalife.)

The figure shows the outer surfaces of the ventricles (the principal pumping chambers) in a mouse heart and a pig heart. By introducing a voltage-sensitive dye into the muscle cells, investigators were able to observe visually when the electrical wave reached various parts of the ventricular muscle. Colors symbolize the order of events as represented on the scale in the center of the figure: During each heartbeat, as the electrical wave sweeps across the surface of the ventricles, red areas receive the wave and contract first; purple-violet areas receive the wave and contract last; and areas colored yellow and green contract sequentially in between. Although the two hearts in the figure are from very different species and are very different in size, the spatial pattern of activation is similar. However, the temporal pattern is different. Whereas the last regions of the heart muscle to receive the electrical wave (and contract) do so after 4 milliseconds (ms) in the mouse heart, they do so after 22 ms in the pig heart. White lines on the two hearts show the position of the wave of electrical activity in 0.3-ms steps in the mouse heart and 1.5-ms steps in the pig heart. Studies of this sort represent just the first steps toward understanding how heart tissue function—not just heart anatomy—is scaled to meet the metabolic needs of different-sized mammals.

References

Noujaim, S. F., O. Berenfeld, J. Kalifa, M. Cerrone, K. Nanthakumar, F. Atienza, J. Moreno, S. Mironov, and J. Jalife. 2007. Universal scaling law of electrical turbulence in the mammalian heart. Proc. Natl. Acad. Sci. U.S.A. 104: 20985–20989.