Almost all organisms require oxygen. Oxidation of carbohydrates, proteins and fats releases chemical energy in a step-by-step process that occurs in cells — energy for all the activities of life. Carbon dioxide is a byproduct of that process and needs to be removed. In almost all vertebrates, oxygen is carried through the body on hemoglobin molecules (containing iron, which can bond to oxygen) in red blood cells, and the air-breathing organs are closely associated with the vascular or circulatory system.
Vertebrates have many ways to get oxygen from the air (and get rid of carbon dioxide). Most terrestrial vertebrates have lungs, which are basically a collection of small sacs with moist linings through which oxygen can pass into adjacent capillaries, where blood cells with hemoglobin take it up and circulate it to the rest of the body. Carbon dioxide goes the other way, from the blood into the lungs and back out to the air. Air comes in, used air goes out, the whole process known as gas exchange.
Mammalian lungs are bags of those small sacs in the chest cavity, which expands and contracts by muscular activity of the diaphragm and ribs. The gas-exchange sacs are dead-ends, and muscle contractions do not empty them completely — some residual oxygen is left inside after a mammal exhales. One breath, in then out, completes the respiratory cycle. In humans, a very small amount of gas exchange also occurs through the skin.
Birds have lungs, too, but theirs are small and denser, compressed against the spine; they are also more complex, arranged to allow air to move just in one direction over the gas-exchange surfaces, so little or no residual oxygen is left behind. In this system, the lungs work together with air sacs in the body cavity, and it takes two breaths to complete the cycle. The lung has two sets of passageways: one set is equipped with blood-filled capillaries, and gas exchange occurs there. The other set passes the used air back out. So the whole cycle works like this: Inhale — incoming air goes mostly to posterior air sacs; exhale — air goes through gas-exchange parts of lung; inhale again — used air goes to anterior air sacs; exhale again — used air goes out. That makes for continuous airflow through the lungs — a more efficient system than in mammals. Birds don’t have a muscular diaphragm; they expand and contract the chest cavity by chiefly lowering and raising the breast bone.
Mammals and birds that regularly dive for a living (think seals and penguins, for instance) have a well-developed dive reflex. Many other (maybe all) mammals, including humans, have the same reflex but less well-developed; some sources suggest that all terrestrial vertebrates might have the reflex to some degree. The reflex is an automatic physiological response to immersion, particularly in cold water. The response is triggered by covering the face with cold water while the animal is holding its breath. Nerves that control the reflex quickly slow the heart rate, shunt blood from peripheral vessels to vital organs, and release more red blood cells from the spleen. As a result, more oxygen can be delivered to the parts that matter during the dive.
The blue whale, some of them weighing in at over 200 hundred tons, is said to be the largest animal ever to live on earth. When it dives, its dive reflex slows its heart rate way down, to about four or even two beats per minute. Blood is kept flowing from the heart by a very elastic major artery that regulates the flow out to the body parts. When a diving whale resurfaces, it has to breathe quickly, to restore oxygen balance. Blue whales speed up their heart beats to 30-37 beats per minute, which is extremely fast for a heart as big as this whale’s. Researchers suspect that it cannot beat any faster than that — and this restriction might place a limit on how big an air-breathing animal can be.
What about the other vertebrates?
In the past decade or so, physiologists have discovered that some reptiles have a very complex respiratory system that somehow combines the mammalian two-way flow system with the avian one-way flow system. I won’t even try to explain how that might work! This arrangement has been found in alligators, monitor lizards, and iguanas, which have very different life styles, so it is not clear how – or if — the respiratory system is adaptive to the ways of life. It will be interesting to see what further research turns up about the respiratory systems of other lizards and snakes, as well as amphibians.
Whatever amphibian lungs are like, amphibians have another important way to get oxygen. Lots of gas exchange occurs through the skin, which is typically moist, allowing gases to diffuse across the membranes. Some amphibians do without lungs altogether. There’s a whole group of lungless terrestrial salamanders that are lungless as adults. They breathe through their skin and mouth linings, which have a good blood supply.
Among the fishes, the best known air-breathers are the several species of lungfish, living in the southern hemisphere. Modern lungfish live in fresh water but ancient forms also lived in salty water. The lung is a modified air bladder, which in most fishes is an organ of buoyancy — but it can also serve as a primitive means of breathing air.
In lungfishes, there is a well-developed blood supply to the lungs, allowing for good gas exchange. Lungfishes can survive drought by burrowing into mud, at which times they depend on air-breathing, even if they also have gills.
Fish known as bichirs are reported to be obligate air-breathers, with well-vascularized lungs; they live in poorly oxygenated waters in Africa. A variety of other fishes, including mud skippers and some eels that spend a lot of time out of water, can breathe air through the skin and mouth linings. Some fish have a special “labyrinth” organ above the gill chamber that is used for breathing air when the fish is out of water; one of these species is said to be an obligate air-breather.
Lungs, skin, mouth lining, air bladders, labyrinth organs … the vertebrates breathe in many ways. But then there are the insects and other invertebrates, which have additional means of getting oxygen from the air. Perhaps I should save that for another time…
• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column.