Like birds, bats have hearts that are about three times bigger than those of comparably sized mammals. The heart muscle fibers (or cells) in bats possess higher concentrations of ATP (the molecule that is utilized for energy by cells) than observed in any other mammal. These adaptations enables bats to pump more blood during a flight, a period of peak demand for oxygen.
Resting bats may have heart rates as low as 20 beats per minute. Within minutes of initiating flight, the heart rate may rise to between 400 and 1,000 beats per minute. Bats also have relatively larger lungs than most mammals, providing a larger respiratory membrane for gas exchange. This is in response to the demands for oxygen required for muscle metabolism during flying.
Bats have highly vascularized wings (i.e., rich in blood vessels) that supply the wing membrane with oxygen and other nutrients. Because of this circulation, damage to the wing membrane can heal very quickly. An unusual feature of the bat wing circulation is sphincters (muscular valves) that can close off blood flow to the capillaries and shunt blood directly from the arteries to the veins. It is not exactly known when and why this is done. Some biologists believe that the sphincters are closed and blood flows through the shunts during flight. The sphincters may open during rest to allow blood to flow into the capillaries and nourish the wing membrane.
A problem that exists for wing circulation is that the flapping of the wings creates a centrifugal force that impedes the flow of blood back to the heart, causing pooling in the extreme ends of the wings. To compensate for this, the veins of the wings have regions in between venous valves that contract rhythmically. These have been referred to as “venous hearts.” When venous hearts contract, the vein is constricted and pushes venous blood back towards the heart. The valves in mammalian veins prevent back flow, ensuring that blood will only travel in one direction. Bat blood is capable of carrying more oxygen per fl oz (ml) than other mammals.
In fact, it carries more oxygen than bird blood. It appears that this is accomplished by increasing the concentration of red blood cells (RBC), which contain the iron pigment heme that binds to oxygen. Bat blood has smaller individual RBCs than normally found in mammals and a larger number of RBCs within the same circulating blood volume. These smaller cells also provide a relatively larger surface area for gas exchange to occur. The actual mechanism of bat circulation is still not completely understood. For budding bat biologists, the bat circulatory system offers many research possibilities because little experimentation has been done on many aspects of this system.
Bat lungs are larger than the lungs of terrestrial mammals, but they do not contain the respiratory volume found in birds. The alveoli, the tiny sacs that help form the respiratory membrane, are smaller in the bat lungs than in the lungs of other mammals. The smaller the alveoli are, the greater the functional surface area for gas exchange. In addition, the alveoli are richly endowed with capillaries that bring a rich flow of blood for gas exchange. Bats are superior to other mammals at extracting oxygen from the environment, approaching the capability of birds. Bats do not have the lung volume of birds, but they have high respiratory rates that facilitate aeration. The high respiratory rates are also believed to be associated with heat removal via water vapor.