For most creatures, sitting down to digest a good meal isn't a time of pulse-racing physical exertion. But for pythons, digestion sends their pulse rate soaring as the reptile's three-chambered heart rises to the metabolic challenge. All reptile hearts have a single ventricle where oxygenated and deoxygenated bloods mix, reducing the amount of oxygen that can circulate around the reptile's body and limiting the creature's maximum metabolic rate. A muscular ridge partially divides all reptiles' ventricles, but never to the extent of developing two independent ventricle chambers. But Tobias Wang,Jordi Altimiras and Michael Axelsson knew that the muscular ridge in a python's heart is much larger than in many less active species. They wondered whether the size of the ventricle's ridge was in any way correlated with a reptile's metabolic demands. Working in Sweden, they tested how blood flows through a beating python heart, and were amazed when they found that the python's muscular ridge effectively divides its ventricle in two(p. 2715)!
Reptiles of all sizes and life styles have the same basic cardiac plan: two atria delivering blood to a single ventricle. On leaving the ventricle, the blood either returns to the lungs, where it is reoxygenated, or is shunted into the body when a sphincter muscle constricts around the pulmonary artery,temporarily isolating the heart from the lungs. Oxygenated blood in the ventricle is continually diluted by the deoxygenated blood returning from the reptile's body, naturally limiting the amount of oxygen that the reptile can deliver to meet its body's metabolic demands. For most reptiles, this state of affairs is not troubling, as their metabolic rates never rise too high. But some reptiles have active lifestyles. Would they need more oxygen than a single ventricle could deliver, or have they overcome the dilution problem to fuel their active lifestyles?
Although python's lives are not hectic, their heart and metabolic rates rocket during digestion, and they also have a well-developed ridge in their ventricle. What effect would that have on the way the ventricle delivers blood to the lungs and body?
The team fed saline solution into either the pulmonary or systemic veins. If the ventricle was undivided, saline would pump out of both of the ventricle's arteries, but if the ridge divided the ventricle in two, the heart would only pump saline out of the same side as the vein delivering saline to the heart. Wang explains that they were astonished when saline only pumped out of one artery. The python's ventricle was divided in two and behaving like a four-chambered mammalian heart!
Wang admits that this is a very controversial result for reptile cardiology, but he believes that the metabolic advantage the python gets from maximising oxygen delivery to its body could account for the creature's high metabolic rate during digestion. Of course, one data point `does not a correlation make', but Wang is now looking to other families of Brazilian snakes to see if any other snakes have such clearly divided hearts.