When animals need more oxygen, they typically breathe harder. However, this is not the case for most reptiles, because their hearts are structured differently from ours. Rather than having two atria and two ventricles like mammals, their hearts have two atria and one ventricle that is only partially divided. This allows them to direct blood away from the lungs, reducing the amount of oxygen in the blood, or towards the lungs, increasing the blood oxygen levels. This means that reptiles can change either how much air they breathe in or where they send their blood when faced with low oxygen. But which strategy is more important and where do the snakes sense that their oxygen is too low? And how do they decide which strategy to employ and when? These questions led Catalina Reyes (now at the University of California, San Diego, USA), Angelina Fong (now at the University of Melbourne, Australia) and Bill Milsom of the University of British Columbia, Canada, to team up with Cleo Leite of the Federal University of São Carlos, Brazil, and Augusto Abe of the Universidade Estadual Paulista, Brazil, to determine where the South American rattlesnake (Crotalus durissus) sensed blood oxygen levels and if the sites played different roles.
After arriving in Brazil, Reyes and colleagues began the difficult and potentially dangerous task of locating the oxygen sensors in the circulatory system of the rattlesnakes. The team selected three locations in the circulatory system where they had previously found oxygen sensors in turtles: the aorta (which supplies blood to the body), the carotid artery (which supplies blood to the brain) and the pulmonary artery (which delivers blood to the lungs to load with oxygen). The researchers then injected sodium cyanide – which mimics having low levels of oxygen – into these locations to see how the rattlesnakes would respond. When the low-oxygen mimic was injected into each location, the rattlesnakes; heart rates went up, their breathing increased and the snakes directed more blood to the lungs. As a higher heart rate, increased breathing and pumping blood to the lungs instead of the body are all ways of delivering more oxygen into the snake's tissues, the researchers concluded that these areas certainly had oxygen sensors.
Having found these receptors, the team wanted to know whether the ways in which the rattlesnakes deliver oxygen change depending on which sensor detects the low oxygen levels. When the sodium cyanide was injected into the aorta and carotid artery, the rattlesnakes started to breathe faster and more deeply than they had before. However, when the scientists injected the chemical into the pulmonary artery, the snakes only increased how deeply they were breathing, but not how quickly. This suggests that each site responds slightly differently and that the snakes’ ability to shunt blood to the lungs is as important as changing how they breathe when facing low levels of oxygen.
Reyes and colleagues then wanted to determine which chemicals in these cells were sending messages to the brain, telling it that there was not enough oxygen. So, they took tissue samples from all three locations and found cells containing serotonin and acetylcholine – chemicals known to be involved in sensing oxygen. The researchers found serotonin in cells at each site, but cells containing acetylcholine only occurred in the aorta and pulmonary artery, also suggesting that each site responds slightly differently.
So, not only do rattlesnakes sense oxygen in different locations from humans but also their heart allows them to control where to send their blood when they sense their oxygen levels are low. This means that, for snakes, shunting blood might be just as important as breathing when the oxygen levels drop.
