The primary role of the vertebrate cardiorespiratory system is to deliver oxygen to tissues and transport carbon dioxide and other wastes of metabolism to the body's excretion sites (i.e. the lungs, gills, skin, kidneys). To efficiently accomplish this, birds and mammals have evolved a divided circulatory system in which the left side of the heart pumps blood to the systemic circulation and the right side pumps blood to the pulmonary circulation. The divided system allows for the generation of high systemic blood pressures without high pulmonary pressures (which can damage the vasculature of the lungs) and prevents the mixing of oxygen-rich and oxygen-poor blood. In contrast, other amniotic vertebrates (i.e. lizards and turtles) do not possess a divided circulatory system. Rather, in these animals, the circulatory system is undivided, meaning that the potential exists for a mixing of oxygen-rich and oxygen-poor blood. However, amniotes with an undivided circulatory system have the ability to reduce blood flow to the lungs and divert it back to the systemic circulation; a phenomenon termed a right-to-left shunt.
The reasons underlying why some groups of amniotes possess a completely divided circulation whereas others do not, as well as the physiological function(s) of the right-to-left shunt, has garnered much scientific investigation. Nevertheless, clear answers to these questions have yet to emerge. Recently, a team of researchers from the University of Utah set out to investigate this curious cardiopulmonary feature and devised a series of experiments to test the hypothesis that the shunt is important for digestion. The team explains that the creation of the gastric acid required for digestion is dependent on the partial pressure of CO2 in the blood. Therefore, a right-to-left shunt should retain CO2 in the body,allow for its transport to the gastrointestinal system and ultimately aid in digestion.
To test this hypothesis, the team first examined whether shunting occurred during digestion. Indeed, the team found that American alligators(Alligator mississippiensis), instrumented with blood flow probes to monitor changes in cardiovascular status, shunted blood past the lungs after eating. Next, the team examined the contribution of the shunt to digestion. The team surgically disabled the ability of some alligators to shunt and then compared a variety of indices of digestion efficiency with another group of animals that were sham operated. By inserting a pH electrode orally into the alligators' stomach after feeding, the researchers discovered that the maximal rate of gastric acid secretion was significantly less in animals surgically prohibited from shunting than those able to shunt. Further, the researchers found that digestion was slower in the disabled animals. Repeated X-ray imaging over 23 days of a piece of bone introduced into the stomach revealed that the dissolution of the bone was slower in non-shunting animals.
The team argues that when combined, their data indicate that shunting of blood from the lungs to the systemic circulation facilitates digestion. The shunt retains CO2 and transports it to the gastrointestinal tissues where it allows for increased gastric acid production, which facilitates digestion. Thus, the role of the shunt in digestion may explain some of the novel and curious features of the undivided circulation of some amniotes.
