Oxygen and carbon dioxide pass to and from blood respectively in tiny air sacs, called alveoli, at the tips of the smallest airway branches. In bird lungs, air loops in one direction through a series of tubes lined with blood vessels for gas exchange.
Aerodynamic forces act like valves to sustain the one-way flow through cycles of inhalation and exhalation. That is all wrong. Iguanas do not fly," Farmer added. The revelations make clear that scientists have much to learn about the physiology of lungs in species other than mammals.
Many people believe turtles can breathe underwater, as they can remain submerged for such long periods. This is not the case. Turtles must surface to take gulps of air. It is not uncommon for female turtles to drown during mating, and every year, thousands of turtles drown when they get caught up in fishing nets. There are some major differences between birds and reptiles. Find out more here.
Crocodiles and alligators have the most advanced respiratory system within the class Reptilia. Their respiration is most similar to humans. They have an epiglottis that separates their airway and esophagus, a trachea connecting the mouth and lungs, and chest muscles that expand and contract to control breathing.
To take a breath, the diaphragm muscle contracts. This pulls air through the nostrils, through the epiglottis, and down the trachea. Upon reaching the lungs, gas exchange occurs via the alveoli.
Oxygen is passed via the alveoli into the capillaries, and carbon dioxide is then expired from the body when the diaphragm relaxes. Unlike humans, reptiles have a unidirectional respiratory system. Instead of the air being separated equally between the two lungs, air enters one lung, then flows along a bypassed airway into the second lung before being expelled from the body.
This form of respiration is more efficient as it increases the amount of oxygen in each breathe. Additionally, when crocodiles and alligators dive, their heart changes how oxygen is delivered. These animals have a valve called the foramen of Panizza.
At rest, this valve is open, allowing free flow of oxygenated blood to the stomach and intestines. During a dive, the pulmonary artery is also contracting, which reduces blood flow to the lungs. Most reptiles have a slow metabolism, so reduced oxygen supply during dives will not affect them. On the surface, their lungs look like they would use breathing, Schachner says. To test this possibility, the researchers dissected the lizards' lungs and filled them with water containing suspended spheres, to better track how the water flowed.
The water flowed tidally through the large chamber, but unidirectionally in the smaller brachial tubes. The discovery of unidirectional breathing in monitor lizards could either mean that the trait evolved in the common ancestor of birds, crocodiles and lizards—an animal that lived roughly million years ago and resembled an iguana—or that the feature evolved independently in each evolutionary branch, Schachner says.
Crocodiles can hold their breath for upwards of 20 minutes, and ancient marine reptiles may have found the trait useful for long dives, Schachner says. The trait could have also been an adaptation to lower oxygen levels on Earth, she says. During the early Triassic era million years ago, oxygen made up 12 percent of air, compared with 21 percent today. These lizards get more oxygen from air than any other reptile and they live in environments ranging from parched desert to tropical forest.
Schachner believes that no one found unidirectional breathing in lizards because the trait is so hard to measure, especially in wild animals.
Wedel hopes the discovery by Schachner's team will inspire others to eschew conventional wisdom. This article is reproduced with permission from the magazine Nature. The article was first published on December 11, Already a subscriber?
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