On Friday we looked at the amazing abilities of some humans and some animals to hold their breath. (See, “In Breath-Holding, Kate Winslet and a Croc Are Champions.”) The actress Kate Winslet can hold her breath for seven and a quarter minutes. A crocodile, though, can hold his breath for hours. How does he do it?

When it comes to the respiratory system, the croc, like a bird, has one-way airflow, like a racetrack, where the air enters the trachea and only moves in one direction through different bronchi to exit. Miss Winslet uses tidal breathing where the air enters the trachea and the bronchi and exits through the same pathway. Also, the croc, like most birds, seems to be able to store some oxygen in sac-like structures. In addition, when it dives underwater the croc is able to use some of its muscles to move the lungs around to help. It’s hard to pin down whether the croc has a larger lung capacity than a human does. But even though some of this could give it a bit of a breath-holding advantage, when we consider the croc’s cardiovascular system it becomes apparent that this is unlikely to factor much into it.

The croc has a unique cardiovascular system. Unlike other reptiles which have a three-chambered heart, the croc has a four-chambered heart, like ours, but with a twist. Instead of having just one aorta that sends oxygenated blood from the left ventricle to the systemic arteries, the croc has two aortas. 

It has a right aorta, the outflow tract from the left ventricle, which sends oxygenated blood to the systemic arteries, mostly the brain and limbs. But its right ventricle has two outflow tracts. One is the common pulmonary trunk that sends deoxygenated blood to the lungs just like Kate’s. And the other sends deoxygenated blood through the left aorta which goes to its gut and liver. However just outside the heart, beyond the right and left aortic valves, there is a channel, called the foramen of Panizza, that allows the right and left aortas to communicate. 

When the croc is breathing in air, most of the deoxygenated blood exiting the right ventricle enters the common pulmonary trunk, while a small amount enters the left aorta. In this setting, when the heart is resting between beats, oxygenated blood from the right aorta (having come from the left ventricle) moves through the foramen of Panizza to the left aorta and onto the gut and liver. In this way, these tissues receive mostly oxygenated blood.

When the croc is underwater (not breathing), this triggers a cog-toothed valve to close off the common pulmonary trunk thereby sending most of the deoxygenated blood through the left aorta. The deoxygenated blood from the left aorta goes through the foramen of Panizza to the right aorta, feeding blood to the brain and the other tissues. 

In other words, when the croc is holding its breath underwater, because its lungs aren’t doing much work, the cog-toothed valve closes off most of the blood flow to them which increases the circulating blood volume to the rest of its body, thereby maintaining adequate blood flow. But we have to keep in mind that this diverted blood is a mixture of oxygenated and deoxygenated blood.  

Pause for Analysis

When comparing the croc to Kate, the differences in their respiratory and cardiovascular systems are unlikely to explain how the croc can hold its breath so much longer. Maybe larger lungs, being able to store some air, having one-way airflow, and muscles that enhance lung function could help. But, since the cog-toothed valve automatically closes off most of the blood flow to the croc’s lungs when it is underwater, these possible respiratory advantages would not have a chance to play a significant role.

And what about this recycled deoxygenated blood? By closing off most of the blood flow to the lungs to keep more blood within the systemic circulation, while the croc is underwater, this extra blood being shunted from the left to the right aorta has been continuously providing more and more oxygen to these same tissues. That means that as the croc stays underwater longer and its blood keeps recycling through the same tissues, it is giving off more oxygen and its blood level of oxygen is continuing to drop. How low can it go before the croc dies? 

One answer to this problem, as noted, is that with the croc being in deeper and colder water and being cold-blooded, its metabolic rate (need for oxygen) is much lower and so the less efficient anaerobic process of glycolysis can provide some energy. But there’s one more trick that the croc has up its sleeve and it is a remarkable one!  It has something to do with the animal’s unique hemoglobin.

Croc Hemoglobin

Besides being cold-blooded, and being able to use glycolysis, the other main factor that seems to allow the croc to stay underwater so much longer than Kate Winslet is its hemoglobin. No, it’s not that the croc’s hemoglobin can carry more oxygen than ours. Nor is it that the croc has a higher concentration of hemoglobin in its blood. Actually, the normal level of hemoglobin for a croc is a bit lower than it is for a human female. 

Those were two good ideas and kudos if you thought of them. So, what aspect of hemoglobin function could help explain the croc’s ability to stay underwater so much longer?

Hint

If you drove an eighteen-wheeler with a load of material from the docks of Tampa to a factory in Dallas, once you reached your destination what would be the final thing you would need to do to finish the job? 

Answer

Unload the material.

Precisely! Although Kate’s and the croc’s hemoglobin are very efficient in picking up lots of oxygen in the capillaries of their lungs, they haven’t done their job completely until they’ve unloaded the oxygen in the tissues. For survival, having enough hemoglobin loaded with enough oxygen in your red blood cells, being pumped by your cardiovascular system, is useless until that oxygen is unloaded into the tissues so your cells can grab it for their energy needs. 

It’s not as if your hemoglobin “knows” what it’s doing. It’s not as if when the hemoglobin is in the lungs it knows to pick up oxygen and then when it’s in the tissues it knows that is has to release it. Let’s see how hemoglobin does its job. 

The ability of hemoglobin to bind oxygen is related to its shape (configuration). Certain chemicals, like hydrogen ion (acid), carbon dioxide (CO₂), and 2,3 BPG (2,3 biphosphoglycerate), an organophosphate byproduct of glycolysis, affect hemoglobin’s configuration and therefore its affinity for oxygen. 

Compared to the tissues, in the lungs, where oxygen is readily available and CO₂ is released from the body, there is a lot less acidity, CO₂ and 2,3 BPG. So, in the lungs, hemoglobin can readily grab and fill up with oxygen. However, in the tissues, where there’s a lot more acidity, CO₂, and 2,3 BPG, the changes in hemoglobin configuration renders it less able to hold onto oxygen and so it releases oxygen to the cells. If you’re interested, you can read this Wikipedia article on the “Oxygen–hemoglobin dissociation curve.” 

While Kate Winslet was holding her breath, 2,3 BPG was the most important factor helping her hemoglobin release oxygen to her tissues. In fact, without 2,3 BPG, her hemoglobin would only be able to release about 10 percent of its oxygen whereas because of it, with heavy exercise it can release about 60 percent. Since Kate was not exercising heavily (just holding her breath) this means that her hemoglobin’s ability to release oxygen was much less, probably around 30 percent. (Note: the saturation of arterial hemoglobin is 95-100 percent whereas for venous hemoglobin it is 65-75 percent. This means that the tissues extract about 25-35 percent  of the available oxygen in the blood.)

With the crocodile, things are very different. As noted in an article I referenced on Friday, researchers have determined that “21 interconnected mutations” of one of the croc’s ancient ancestor’s hemoglobin resulted in its reduced affinity for oxygen, no longer being due to organophosphates like 2,3 BPG but to bicarbonate instead. In other words, rather than 2,3 BPG making hemoglobin release oxygen into the tissues, it is bicarbonate. CO₂ readily dissolves in water to form bicarbonate. So, the more the croc’s metabolism works, the more CO₂ and bicarbonate it forms in its tissues, and the more the croc’s hemoglobin can release oxygen to the cells. So much so, that it is estimated that the croc can release almost 100 percent of the oxygen on its hemoglobin to its cells! But of course, there’s a limit. When he actually runs out of oxygen and anaerobic glycolysis can’t provide enough energy to keep him alive, he dies. 

This answers the question of how the croc can keep releasing more and more oxygen from the more and more deoxygenated blood being continuously recycled through his heart while he is underwater. As the oxygen level drops further and more CO₂ and bicarbonate build up in the tissues, his hemoglobin is able to release more oxygen.

Many Factors at Work

Thus, the croc’s ability to hold his breath so much longer than Kate is multifactorial. Being cold-blooded means that for a similar ambient temperature and activity the croc uses a lot less energy, meaning it needs a lot less oxygen, and being colder just improves this ability. Being cold-blooded also means that the croc can sometimes get by for a while on the less efficient energy production of glycolysis. The croc’s hemoglobin being able to unload almost 100 percent of its oxygen to its cells, compared to only about 30 percent for Kate, is a game changer! Also, when the croc is under water, its heart is able to bypass the blood flow to the lungs when they are not bringing in new supplies of oxygen, thereby sending more blood to the rest of its organs. There the hemoglobin can somehow release even more oxygen due its reduced affinity from bicarbonate. That is just amazing. Finally, the differences in the croc’s respiratory function compared to Kate’s doesn’t seem to play a significant role here.  

The Question of Causation

One has to wonder where all of the information came from to make and assemble all of the parts of the respiratory, cardiovascular, and hematological systems to let Kate and the croc survive while holding their breath (or not holding it). Then you have to wonder what makes up each of their control mechanisms and how they know what they’re doing to keep Kate and the croc alive, especially the cog-toothed valve that automatically closes off the blood flow to the croc’s lungs when it goes underwater.

And what about the set-point for each of their respiration centers which tells them when to breathe? Obviously, since a human uses up so much more energy than a croc and our hemoglobin can’t release as much oxygen, Kate’s respiratory center must be set to tell her to breathe a lot sooner than the croc’s. In other words, the urge to breathe is not just a nuisance. It’s there to keep you alive. If Kate’s hemoglobin normally releases only 30 percent of its oxygen, then her respiratory center has to be set to tell her to breathe in a new supply of oxygen before her hemoglobin saturation drops below that level. Then she literally has no gas in the tank. This of course would be very different for the croc, since its hemoglobin can release almost 100 percent of its oxygen. So, while Kate’s respiratory center is having her come up for a breath within a few minutes, the croc’s respiratory center is taking a long nap. 

Where did the information come from to make sure the set-point for each of their respiratory centers matches their metabolism and hemoglobin’s affinity for oxygen in the tissues? And what about while the croc’s ancient ancestors were evolving this ability?  

The standard response, that evolution brought these things about because they were needed for survival, comes up again and again in the articles on the croc’s respiratory, cardiovascular, and hematological systems. But this standard response falls short, to say the least.

In our book Your Designed Body, Steve Laufmann and I discuss the concept of solving hard problems. Each of the above-mentioned organ systems and control mechanisms represents an incredibly hard problem which needs a solution that is “complete and coherent” otherwise survival is impossible. We pose the question, “How can these hurdles be overcome, and at the same time?” “At the same time” because the organism has to stay alive and reproduce while each of these hard problems is being solved! This leads to Laufmann’s First and Second Laws of Hard Problems, which “will strike experienced engineers as unarguable truisms.”

1. No amount of wishful thinking will make a hard problem go away.
2. Hard problems require ingenious solutions.

Consider the complexity of the systems within Kate and the croc and how they work to keep each of them alive. There are only two classes of causal forces; those are material causes and intelligent causes. 

Which one of the two — a Theory of Billions of Innovative Accidents, or a Theory of Biological Design — scientifically makes more sense? For more information, read our book Your Designed Body and be amazed!