Unit Six. Animal Life
24.3. Respiration in Terrestrial Vertebrates
Amphibians Get Oxygen from Air with Lungs
One of the major challenges facing the first land vertebrates was obtaining oxygen from air. Fish gills, which are superb oxygen-gathering machines in water, don’t work in air. The gill’s system of delicate membranes has no means of support in air, and the membranes collapse on top of one another— that’s why a fish dies when kept out of water, literally suffocating in air for lack of oxygen.
Unlike a fish, if you lift a frog out of water and place it on dry ground, it doesn’t suffocate. Partly this is because the frog is able to respire through its moist skin, but mainly it is because the frog has lungs. A lung is a respiratory organ designed like a bag. The amphibian lung is hardly more than a sac with a convoluted internal membrane that opens up to a central cavity (the convoluted membrane is shown in figure 24.3a). The air moves into the sac through a tubular passage from the head and then back out again through the same passage. Lungs are not as efficient as gills because new air that is inhaled mixes with old air already in the lung. But air contains about 210 milliliters of oxygen per liter, over 20 times as much as seawater. So, because there is so much more oxygen in air, the lung doesn’t have to be as efficient as the gill.
Figure 24.3. Evolution of the vertebrate lung.
Reptiles and Mammals Increase the Lung Surface
Reptiles are far more active than amphibians, so they need more oxygen. But reptiles cannot rely on their skin for respiration the way amphibians can; their dry scaly skin is “watertight” to avoid water loss. Instead, the lungs of reptiles contain a larger surface area. The internal membrane is also convoluted but the central cavity has many small air chambers, shown as partitions in figure 24.3b, which greatly increase the surface area of the lung available for diffusion of oxygen.
Because mammals maintain a constant body temperature by heating their bodies metabolically, they have even greater metabolic demands for oxygen than do reptiles. The problem of harvesting more oxygen is solved by increasing the diffusion surface area within the lung even more. The lungs of mammals possess on their inner surface many small chambers called alveoli that look like clusters of grapes in figure 24.3c. Each cluster is connected to the main air sac in the lung by a short passageway called a bronchiole. Air within the lung passes through the bronchioles to the alveoli, where all oxygen uptake and carbon dioxide disposal takes place. In more active mammals, the individual alveoli are smaller and more numerous, increasing the diffusion surface area even more. Humans have about 300 million alveoli in each of their lungs, for a total surface area devoted to diffusion of about 80 square meters (about 42 times the surface area of the body)!
Birds Perfect the Lung
There is a limit to how much efficiency can be improved by increasing the surface area of the lung, a limit that has already been reached by the more active mammals. This efficiency is not enough for the metabolic needs of birds. Flying creates a respiratory demand for oxygen that exceeds the capacity of the saclike lungs of even the most active mammal. Unlike bats, whose flight involves considerable gliding, most birds beat their wings rapidly as they fly, often for quite a long time. This intensive wing beating uses up a lot of energy quickly, because the wing muscles must contract very frequently. Flying birds thus must carry out very active oxidative respiration within their cells to replenish the ATP expended by their flight muscles, and this requires a great deal of oxygen.
A novel way to improve the efficiency of the lung, one that does not involve further increases in its surface area, evolved in birds’s lungs. This higher-efficiency lung copes with the demands of flight. Can you guess what it is? In effect, birds do what fishes do! An avian lung is connected to a series of air sacs outside of the lung. When a bird inhales, the air passes by the lung and directly to posterior air sacs you can see in figure 24.4a, which act as holding tanks. When the bird exhales, the air flows from these air sacs forward into the lung and then on through another set of anterior air sacs in front of the lungs and out of the body. Figure 24.4b shows the three components of the bird’s respiratory system: the posterior air sacs, the air passageways through the lungs called the parabronchi, and the anterior air sacs. It takes two breathing cycles for air to pass through the bird’s respiratory system. What is the advantage of this complicated pathway? It creates a unidirectional flow of air through the lungs.
Figure 24.4. How a bird breathes.
(a) A bird's respiratory system is composed of the trachea, anterior air sacs, lungs, and posterior air sacs. (b) Breathing occurs in two cycles: In cycle 1, air is drawn from the trachea into the posterior air sacs and then is exhaled into a lung; in cycle 2, the air is drawn from the lung into the anterior air sacs and then is exhaled through the trachea. Passage of air through the lungs is always in the same direction, from posterior to anterior (right to left here). (c) Decreasing efficiency of respiratory systems, from fish (left), to bird (middle), to mammal (right), the least efficient.
Air flows through the lungs of birds in one direction only, from back to front. This one-way air flow results in two significant improvements: (1) There is no dead volume, as in the mammalian lung, so the air passing across the diffusion surface of the bird lung is always fully oxygenated; (2) just as in the gills of fishes, the flow of blood past the lung runs in a direction different from that of the unidirectional air flow. It is not opposite, as in fish; instead, the latticework of capillaries is arranged across the air flow, at a 90-degree angle, called a crosscurrent flow (the middle panel of figure 24.4c). This is not as efficient as the 180-degree arrangement of fishes (on the left), but the blood leaving the lung can still contain more oxygen than exhaled air, which no mammalian lung (shown on the right) can do. That is why a sparrow has no trouble flying at an altitude of 6,000 meters on an Andean mountain peak, whereas a mouse of the same body mass and with a similar high metabolic rate will stand panting, unable even to walk. The sparrow is simply getting more oxygen than the mouse.
Just as fish gills are the most efficient aquatic respiratory machines, bird lungs are the most efficient atmospheric ones. Both achieve high efficiency by using different types of current flow systems.
Key Learning Outcome 24.3. Terrestrial vertebrates employ lungs to extract oxygen from air. Bird lungs are the most efficient atmospheric respiratory machines, achieving a form of crosscurrent flow.