(13) Explain the differences in gas exchange in birds compared to mammals. Which one is more efficient in gas exchange and why must this be so?

A. Both birds and mammals have lungs.
In addition, birds have accessory air sacs to keep air flowing thru the lungs as they inhale of exhale. Air flows one way only, forming a continuous circuit thru the interconnected system regardless of whether the bird is inhaling or exhaling.
Birds’ gas exchange method is more efficient since they always have air flows anytime. This is crucial for them since flying needs more oxygen.

B.In mammals the air travels from the trachea to the bronchi. Then the bronchi branch off into narrower tubes called the bronchioles. The bronchiole brings the gas to tiny sacs called alveoli. The gas then diffuses across the alveoli wall into the blood or out of the blood (depending on the conc. of O2 and CO2). Mammal’s lung expands while inhaling and compress when exhaling. In birds the air travels from the trachea to the posterior sacs then to the parabronchi within the lungs. In the parabronchi gas exchange occurs along the length of the parabronchi. After the gas leaves the parabronchi it goes into the anterior air sacs then leaves from the trachea. Birds have a one-way airflow lung. (Simple description of bird lungs – Inhalation – posterior air sacs fills with outside air while lungs empty and anterior air sacs fill with air from lungs. Exhalation – posterior air sacs empty, lungs fill with air from posterior sacs, and anterior air sacs empty.)
Birds gas exchange is more efficient because when flying over mountain the partial pressure of oxygen is s low that if birds had lungs like humans they would black out or die because human lungs does not have a continuous flow of oxygen. Bird lungs maximize the amount of oxygen taken in because they have a continuous flow of oxygen in their lung.

C. Both birds and mammals use lungs. Birds have two chambers to get the most oxygen. The first breath goes into one chamber. On the second breath the remaining air goes into another chamber where the rest of O2 is absorbed. Only then is the breath released. Birds need to be able to get a lot of oxygen because it flies in altitudes with low oxygen. Mammals just have one chamber to breath the air is only used once before its released.

D.the avian respiratory system is heterogeneously partitioned and completely separates the functions of ventilation and gas exchange (2). In birds, nine air sacs act as bellows to ventilate the small, constant volume. The air sacs occupy ~90% of the total respiratory system volume in a bird; the remaining 10% is comprised of the lung, containing hundred of gas exchange units called parabronchi.
These structural differences between alveolar and parabronchial lungs result in different models of gas exchange in birds and mammals. The gas-exchanging parabronchi in avian lungs are arranged in parallel and are connected at both ends to secondary bronchi, which act as conducting airways that ventilate the parabronchi with air from the trachea or air sacs. The parabronchi are perfused along their entire length by pulmonary mixed-venous blood, so ventilation and perfusion can be thought of as occurring at right angles to one another and gas exchange in a bird lung is described by a cross-current model (14). The theoretical efficiency of cross-current gas exchange is greater than alveolar exchange, and under ideal conditions arterial PO2 (PaO2) exceeds end-parabronchial (or expired) PO2. In contrast, ideal alveolar gas exchange in mammals can only result in PaO2 equaling expired (i.e., alveolar) values.

E.Birds have air sacs in their lungs to store oxygen even when it is exhaling. During first inhalation, most of the air flows past the lungs into a posterior air sac. That air passes through the lungs upon exhalation and the next inhalation ends up in the anterior air sac. At the same time, the posterior sacs draw in more air. This flow pattern allows oxygenated blood to leave the lungs with the In mammals, the lungs have innumerable small sacs that increase surface area across which oxygen readily diffuses into the bloodstream.