A glimpse into the secrets of the python’s heart – using ultrasound
Reptilian hearts are generally referred to as more ‘rudimentary’ than mammalian hearts, but they are perfectly adapted for their purpose. In fact, they have their own wonders – such as capabilities for self-healing – which far exceed our own. Unless you reject the theory of evolution*, their hearts give valuable insight into the evolution of human hearts.
* If you do happen to reject the theory of evolution and believe that Earth was populated over a short space of time due to the arrival of extra-terrestrial beings, you probably know all about the illuminati, in which case the study of snake hearts will further inform your understanding of the reptilian master race. So don’t feel excluded by this article.
An overview of the snake heart
Encyclopedia Britannica describes the heart of a snake as having a small left ventricle and a larger right ventricle, with communication between the two. The right ventricle gives rise to a pulmonary artery but also two aortas, with the oxygenated systemic blood being directed through them thanks to ridges within the ventricle. Deoxygenated blood flows from the right atrium into the right ventricle and out through the pulmonary artery, guided again by ridges, with remarkably little mixing.
Most other sources, however, describe a single ventricle and two atria, again with a ridged system and long atrioventricular valves for guiding oxygenated and deoxygenated blood. Effectively, all authors are describing the same pattern of circulation, with Encyclopedia Britannica recognising one division as a true intraventricular septum (albeit incomplete), whereas other sources deem this to be a third ridge.
Due to the lack of separation between the systemic and pulmonary systems, the pressure within the two systems is very similar – and necessarily low, to avoid damage to the delicate pulmonary system and lung. Such a low pressure pulmonary system is perfectly sustainable for ectotherms, who have lower metabolic demands to endotherms (animals which generate their own internal heat). For endotherms, however, it was necessary to evolve complete separation between the two systems in the form of two separate ventricles (one low pressure system to serve the pulmonary side, and one high pressure to pump blood around the body). Some mammals (humans included) are born with an incomplete intraventricular septum – a type of congenital heart disease termed a ventricular septal defect, or VSD. Communication between the two chambers, unless surgically corrected, causes damage to the lungs and pulmonary system (pulmonary hypertension), and pulmonary oedema. This is because the pulmonary system is a low pressure system, and cannot cope with high systemic pressures. Animals with a univentricular heart therefore tend not to generate high systemic pressures, but pythons have developed a remarkable way around this.
Pythons’ ingenious dual pressure system
The python’s heart, however, is different to that of other types of snake. Despite the pulmonary and arterial systems being connected to the same chamber, the python is capable of generating high arterial blood pressures and low pulmonary pressures (Jensen et al., 2010). These high blood pressures are important when coping with the high oxygen demands of digesting large meals, with the python famed for its ability to increase cardiac muscle mass by 40% within 48 hours of feeding (Owen, 2005). The ridges in the heart of the python are more developed than in other snakes and reptiles (varanid lizards being the only other exception (Millard & Johansen, 1973)), forming a complete septum from the apex of the heart to 3/4 of the length down to the base. When the atrioventricular valves open during ventricular filling, they seal off communication across this remaining gap, forming a temporary but complete septum between oxygenated blood flowing in from the left atrium and deoxygenated blood arriving from the right. During ventricular contraction – when high pressures are generated for systemic blood flow – two other ridges are pushed up against this gap again. This allows high pressures to be generated in the left side of the ventricle, without being transmitted to the pulmonary system. In other words, the python achieves a functionally biventricular system.
Scanning the python heart
Much of what is known about pressure separation in the hearts of pythons was discovered relatively recently, in large part thanks to echocardiography. I visited Maskell Reps in Thurrock to scan some pythons with the Siui Apogee 2300, a versatile, high-end cardiac and abdominal ultrasound system that has so far coped remarkably with absolutely anything I’ve thrown at it! We tried a range of probes: first linear, with which we also checked for follicles and eggs; then phased array, traditionally used for cardiac imaging but which struggled with the pythons due to the heart being in the near field of the transducer; and finally microconvex. Microconvex probes are usually used for small animal abdominal scanning, particularly for dogs and cats, but it performed remarkably well in these snakes. Even the microconvex probe of the Scan Pad – a much more basic abdominal ultrasound machine – was capable of generating relatively clear images. Here is a clip from the Apogee 2300:
Watching this clip, it is possible to appreciate the closing off of one side from the other during the contraction phase, and one of the long atrioventricular valves can be seen particularly clearly in the bottom segment of the heart during diastole. Having almost exclusively scanned mammalian hearts in the past, it was difficult to get my head around during an hour’s scanning, but – as is always the way with ultrasound – the more I watch the clips, the more the pieces begin to fall into place. Proficiency in any new type of scanning requires time and patience, as I discuss in my book on canine pregnancy scanning here. I am looking forward to returning to Maskell Pythons for some more practice, and to obtain more Colour Doppler clips like this one to further elucidate the patterns of blood flow:
Jensen, B., Neilsen, J., Axelsson, M. et al. (2010). How the python heart separates pulmonary and systemic blood pressures and blood flows. The Journal of Experimental Biology 213, 1611-1617.
Millard, R., Johansen, K. (1973). Ventricular outflow dynamics in the lizard, varanus nilocticus: responses to hypoxia, hypercarbia and diving. In Journal of Experimental Biology, 60, 871-880.
Owen, J. (2005). Pythons Grow Bigger Hearts at Mealtimes. In National Geographic.
With thanks to Scott Maskell.
July 01, 2018