Heart of an Athlete
Even the most well-oxygenated blood does a shark's cells little good if it just sits there. Compared with other fishes, sharks have an enormous blood volume. Moving all that blood from where it is oxygenated to where it is needed is the primary job of the heart.
As in other sharks, the Great White's heart is a muscular organ encased in a tough, membranous sac located behind the base of the gill basket. The shark heart is composed of four compartments, arranged in an S-shape and separated by valves. The first and second compartments (sinus venosus and auricle, respectively) are bulbous and thin-walled, the first containing a biological pace-maker which dictates the heart's contraction rate, the second acting as a blood-collecting chamber. The third compartment (ventricle) is ovoid, thick-walled and highly muscular, its rhythmic contractions squeezing blood through the heart and the rest of the circulatory system. The fourth compartment (conus arteriosus) is conical, has thin, relatively inflexible walls and three rows of one-way valves. The main function of this fourth compartment is not yet clear, but it may homogenize blood pressure leaving the heart and/or prevent blood washing back into the heart between successive contractions.
The rate of heart contraction varies considerably among shark species. In small, relatively inactive sharks such as the Spiny Dogfish, Lesser and Greater Spotted (Scyliorhinus stellaris) Catsharks. and the Leopard Shark (Triakis semifasciata), heart rate measures about 19 to 48 beats per minute. No one has yet managed to measure the heart rate of a White Shark. But in a 1997 paper, physiologists N. Chin Lai and his co-workers measured the heart rate of the White Shark's close relative, the Shortfin Mako (Isurus oxyrinchus). Lai et alii found that the Shortfin's heart beat from 28 to 78 times per minute. The upper end of this spectrum is comparable to that of humans, in which heart rate averages about 72 beats per minute. Under conditions of high activity, human heart rate may increase up to 122 beats per minute (70%) or more. Through sustained training, human athletes are able to increase stroke volume with minimal increase in heart rate. Unlike humans, however, the heart rate of most sharks does not increase significantly with exercise (by perhaps 10%). Instead, a shark's stroke volume (the amount of blood moved by the heart at each contraction) increases dramatically (by 30% or more). Increasing blood flow not only makes more oxygen available to hard-working swimming muscles, but also carries away metabolic waste products (such as carbon dioxide and lactic acid). But, since a heart is finite in size, most shark species are rather limited in their scope for activity. But this is apparently not the case with the White Shark.
In a 1985 paper, Scott Emery reported that, for a 220-pound White Shark, the heart weighs about 6.3 ounces (178 grams). In comparison, the heart of a typical teleost, tuna, and human being of the same approximate body mass is roughly 2.6 ounces (72 grams), 6.6 ounces (186 grams), and 10 ounces (284 grams), respectively. Thus, at a given body weight, the heart weight of a White Shark is proportionally much (nearly 2.5 times) greater than a teleost, slightly (4.3%) less than that of a tuna, and only moderately (38%) less than a human. Surprisingly, in a 1985 paper by Emery and his co-workers, reported that all seven species of shark studied have similar ventricle weights, but that the ventricular wall of the Great White is nearly twice as muscular (measured in terms of the relative thickness of the ventricle's two main layers) as that of all but two of the other species studied. The two exceptions are the Shortfin Mako and the Common Thresher (Alopias vulpinus), both of which are active, pelagic species. From this, Emery and his co-workers concluded that the White Shark may respond to the increased oxygen demand of exercise in a very mammal-like way, by enhancing aerobic scope through significantly increasing heart rate.
Only recently have the tools of physiological research enabled scientists to accurately measure the stroke volume and blood pressure of free-swimming sharks. Stroke volume and blood pressure are inversely related. This is because - at a given pressure - the smaller the internal space of a contracting body, the relatively greater the amount of force per unit volume of liquid being squeezed out. This principle explains why a squirt gun can achieve relatively greater range from a small force - applied by the trigger finger of the operator - than could an un-nozzled garden hose at the same internal pressure (The addition of a nozzle increases the range of a hose through a different principle of fluid dynamics, namely the Venturi effect, which need not concern us here). In a 1997 paper, N. Chin Lai and his co-workers compared the stroke volume of a Shortfin Mako to that of a White Shark. They found that the Great White has a stroke volume per unit body mass that is more than 2.5 times that of the Shortfin Mako. This suggests that the White Shark may not be capable of the feats of hyperkinetic activity as its close relative but - like a marathon runner - is able to efficiently pump huge quantities of blood with each heartbeat, maintaining moderately high activity for prolonged periods.
Lai and his co-workers also measured the blood pressure of several Shortfin Makos. They found that the average systolic (due to the heart's contraction phase) pressure of a Shortfin is about 70 millimetres of mercury - or roughly 9% of normal atmospheric pressure at sea level. By comparison, human systolic blood pressure is about 100 millimetres of mercury, or about 13% atmospheric pressure. Unfortunately, no one has yet measured the White Shark's systolic pressure, so it is unclear how the large stroke volume and unusually muscular ventricle shape this species' overall scope for activity.
It is clear, however, that the White Shark's heart is a high-performance organ, efficiently moving large quantities of blood with each contraction and responding to the increased oxygen demands of exercise in ways that have far less in common with most fishes than they do with the finest human athletes. Deep in the Great White's chest, thumping away over a lifetime punctuated by energetic chases and escapes, beats the heart of a champion among sharks.