I’m getting over pneumonia so in a vain attempt to make lemonade out of lemons, I’m going to talk about the evolution of the lung.
The lung is one of the major mechanisms– if not the major mechanism– that enabled invasion of the land by animals.
The necessity of lungs is based on the vast difference between gaseous oxygen and oxygen dissolved in water. For one thing, when water passes over the gills of a fish (or crab) it is one fluid (water) passing next to another fluid (blood) across a thin membrane. This is very similar to what already happens within the body of a complex organism.
In fact, some organisms are so simple that they have no need of circulatory systems at all. It is sufficient that they are in contact with water that contains oxygen. The oxygen crosses the membrane easily.
But there are some downsides.
For one, the amount of oxygen that can be dissolved in water is less than the level of oxygen in air. At 0C, the amount of oxygen in water is 14.6 mg/l. Air weighs 1.2 g/l, of which 21% is oxygen. That works out to about 251 mg/l– more in the historical past. In addition, the solubility of oxygen in water is temperature dependent. If you bring the temperature up to 37C (human body temperature) the amount of O2 available drops to 6.71 mg/l. (See here.) Biochemical reactions like to take advantage of heat. But there’s an obvious tension between the oxygen available for metabolism and the heat loving biochemical reactions.
All through the ocean different organisms act out this tension from the smokers down at the sea bottom to the rich green soup of some lakes.
Think of the ancient oxygen atmosphere before organisms invaded the land as a wonderful resource yet to be exploited.
Given the sheer amount of available oxygen in the gaseous atmosphere, why did it take several hundred million years for animals to make it to land?
Well, the lungs were hard to evolve. Different groups of organisms such as insects, vertebrates, and scorpions used different strategies.
All lungs are a mechanism of concentration and distribution–gills serve the same purpose in water. The idea is to expose a specialized organ used to absorb the oxygen and serve as source to be distributed over the body. It’s a compromise between the necessity of oxygen being available to all cells of the organism to oxygen and the engineering problems of exposing the cells to the outside environment. A consequence of this compromise is the contents of lungs (or gills, for that matter) have enormous surface area to volume ratios. The human lung, for example, has about 70 m2 of surface area–a square about 8 meters and some change on a side. About the floor plan size of a studio apartment.
Arachnids such as spiders and scorpions have something called “book lungs“. These are flattened air sacks adjacent to one another, with the intervening space filled with hemolymph–the equivalent of blood. Air is moved through the sacks allowing gaseous exchange.
Insects do something different. They have entry orifices called spiracles, allowing the air to enter a trachea. The trachea branch smaller and smaller, finally becoming tracheoles–dead end, water filled compartments. (See here.) The tracheoles deliver oxygen directly to the tissues. In effect, the whole body of the insect–or at least where necessary–is riddled with lung like structures.
Vertebrates have more than one strategy but they all have the concept of a central concentrator space where oxygen is absorbed and then distributed via blood and the circulatory system.
If you look at the drawing above, you’ll see the evolution of the different vertebrate groups through time. We’re all familiar with the different extant vertebrate classes. We’ll neglect the fish for the moment. On land, they are birds, mammals, reptiles and amphibians.
The first lungs are thought to derive from the swim bladder of fish. This organ keeps gas under pressure–usually oxygen–in order to stabilize swimming. The swam bladder is filled from a gas gland which extracts out gas from the tissue and blood stream and pressurizes it in the bladder. The pressure can be quite high–even fish deep in the ocean have working swim bladders. Lungfish use the same structure in their air respiration.
The lungs of amphibians are composed of a few septa–separated sacks–which contain a few large alveoli–blind chambers where oxygen transfer takes place. Since amphibians also respire through their skin the lungs are not the sole means of oxygenation.
The first lungs that look familiar to us arose in reptiles. Reptiles have alveoli, as we do. They have complex branched lungs–though less branched than ours. Reptiles have no diaphragm and move air differently. However, that’s more of a description of reptiles in general. Crocodilians have a more complex story to tell. But we’ll get to that.
We have mammalian lungs: complex branched structures that terminate in highly vascularized containers called alveoli. Like most vertebrate lungs, air proceeds through the lungs one way at a time. In, then out, in a sort of bellows arrangement caused by the diaphragm.
There are a lot of problems with a bellows system. For one, it means that there’s a lot of dead air in the lungs. That makes the lungs inefficient. For another, the termination areas somewhat susceptible to infection. After all, it’s just a little cell where air comes in and goes out through a hole. It’s easy to close off.
One of the biochemical developments that had to occur for lungs to work was the evolution of pulmonary surfactants: chemicals that manage the surface tension on the wet inner surfaces. In the absence of the surfactants, when segments of the alveolar membranes touch, they stick. This is not a good thing. Surfactants allow them to unstick from one another. The development of these surfactants predates the lungs so they were right there to help early on.
Now, we come to my favorite lung system: that of the birds.
Birds do not have an alveolar system. Instead, their system is tubular. Image the bird lung extracted into a long tube. Air comes in one end of the system and comes out the other. Instead of alveoli, they have parabronchii–small tubes where gas exchange occurs. (See here.)
There are a host of advantages to this sort of system. For one thing, there’s no dead air. There are reservoirs where little exchange occurs. These appear to have a mechanical purpose and aid in moving the air along. But there is no dead air in the parabronchii while there is considerable unused air in the alveoli. Remember your CPR. You exhale into the patient because there’s a lot of oxygen left in your air.
Because the system is flow through, there’s the opportunity for counter current exchange. Imagine you have two flowing tubes of water, one hot and one cold, and you want to heat one up with the other. You lay the tubes down against one another, right? What direction should the water flow in the tubes? The same or opposite?
If you thought “opposite” you win the prize. If the flow is in the opposite direction then there is a continued gradient as the fluids flow past one another. While in co-current exchange, the fluids reach equilibrium and exchange stops. The kidney uses this mechanism. An alveolar system can’t because the flow is in and out. But the bird system, since its flow is one way, runs the blood supply in the opposite way.
This makes the bird system enormously efficient. Not only does it provide a means to reduce the size of the lungs–important in a flying animal– it also is much better at extracting oxygen at lower pressures. Climbers on Everest, wearing oxygen equipment, have reported seeing geese flying high above them.
The flow through system of birds actually may predate their dinosaurian ancestors. Crocodiles and monitor lizards have both been shown to have flow through systems. (See here.) Going back to the drawing above, crocodiles separated from the line leading to birds way back when they were both thecodonts. So for crocodiles to have flow through breathing either means they developed it independently or the rudiments were there prior to the evolution of dinosaurs or birds. Which makes sense. Pterosaurs also popped off from thecodonts and there is ample evidence that they had similar respiratory apparatus as birds. Maybe the marine reptiles did, too.
Coleen Farmer (See here.) has suggested the alveolar lung evolved in the late paleozoic, a period of relatively high oxygen. However, flow through lungs evolved in the ancestor of crocodiles and birds to support apnea–the stopping of breathing. Possibly this was an aquatic adaptation as it is in the crocodilians. Or for some other reason. Regardless, it did evolve and it was present in the archeosaurs. Which meant it was there and waiting when the dinosaurs came along. Then, came birds.
So, as I sit here and cough my lungs out, I look outside. The sun is shining. The birds are singing.
God I wish I was a bird.