A whale of a tale

[joewr]

Dr. Deco,

Why don't whales (or any other sea mammals--or sea snakes, for that matter) get the bends? Can we humans ever do anything like that?

Joewr

 

[Dr Deco]

Dear joewr:

This is a very interesting question, and the answer is not known - - - but there is much speculation. The following possibilities exist:

• The animals are air breathers and are essentially breath-hold divers. There is therefore only a limited amount of nitrogen that they can dissolve in their tissues.
• The circulation to the tissues slows down because much of the oxygen necessary for swimming is held in myoglobin in tissues rather than hemoglobin in blood.
• The animals do not produce many tissue micronuclei because they are not weight bearing on arms and legs (this would possibly be offset by swimming motions).

My guess is that they simply do not pick up enough inert gas from a single lungful of air.

Dr Deco

 

[Dr Deco]

Dear joewr;

This reader has asked that I expand on my earlier answer to this question, and I thought that I would share the reply with others in the FORUM.

As far as I know, the answer to this question is not settled.

• 1. Since these animals do not continue to breathe underwater (as a scuba diver does) they will only have a limited supply of nitrogen that is in their lungs and what is in the blood stream. Naturally, the lungs will be compressed and nitrogen at pressure will enter the blood and be distributed to the tissues. This is gas supply is limited, however to the volume of gas in the lungs - - a few quarts at best to be distributed over the whole of the body tissue volume. Thus, these animals will not really be taking up much nitrogen even if the dive is very deep. It will be limited to the volume of gas in the lungs, and that is very limited.
  
  2. Much of the oxygen in the muscles of these animals is stored in myoglobin, a molecule similar to hemoglobin, but found only in muscle tissue. Since there is essentially (chemically) stored oxygen in this tissue, it is not as important to have all of the oxygen in the blood and lungs as with humans. These creatures could shut down their circulatory system and still have oxygen for muscle activity since the oxygen is stored in the muscle itself. This would be equivalent to us being able to breath-hold dive with the oxygen stored in our own muscles; this is not something we can do for much longer than a few seconds. We can naturally function aerobically (without oxygen), as a sprinter does, but this can only last for a fraction of a minute. For example, a hundred yard dash only requires a few seconds and it is over. This would not be the case with diving mammals.
  
  This reduction in the blood supply is termed the diving reflex and is present even in humans. We can easily not that our heart rate slows if we hold our breath. Some animals such as giraffes have adaptations in the circulation (one way valves in the neck veins, for example) that help them maintain the blood supply as they raise and lower their necks. Diving birds also slow their heart rate (I believe). The diving birds (cormorants, for example) are also an unsolved question. They are likewise breath-hold divers.
  
  3. There is then the question of micronuclei formation in the tissue, since the nuclei (seeds) must be present for decompression bubble formation, possible these form only in very limited quantities or maybe even not at all. No one has performed a Doppler bubble monitoring so it is an open question. Possibly the surface tension of the body’s fluids is high and bubbles simply can not form with the gas supplied by the breathe hold type of dive.
  
  4. We know that air-breathing animals differ in their susceptibility to DCS (mice are more resistant than dogs which are more resistant that humans). Generally, the larger the animal, the more susceptible it is to DCS (if the dissolved tissue gas is the same). The large sea mammals should be susceptible, but I suspect that the dissolved tissue gas is less, because they are breath hold divers. Conversely, sea snakes would be much less susceptible because they are small.

I hope that this is a somewhat more complete answer. To the best of my knowledge, this is basically all that is known.
________________
Dr Deco

 

[rcohn]

I've always heard that the lungs collapse in diving mammals, forcing the air out of the alveoli into trachea where it can no longer be absorbed. This prevents further N2 uptake limiting the N2 supply to that which is already in the bloodstream and tissues

American Scientist had a very good article diving mammals and birds in 1997. Unfortunately only the abstract and a couple of pictures with captions are available on the web. Fortunately one figure, http://www.amsci.org/amsci/articles/97articles/kooymancap10.html illustrates this concept. The magazine should be available in any university library and hopefully most public libraries.

Ralph


http://www.amsci.org/amsci/articles/97articles/Kooyman.html
American Scientist
November-December 1997, Volume 85, No. 6
--------------------------------------------------------------------------------
The Challenges of Diving to Depth

Gerald L. Kooyman and Paul J. Ponganis
--------------------------------------------------------------------------------

Abstract

People have always measured their physical abilities against those of other animals. Ancient mythologies are filled with tales of people who fly like birds or submerge themselves to great depths beneath the oceans like seals and whales. The physical reality, of course, is quite different, and people require complex equipment to do either -- which makes the question of how animals achieve these feats all the more interesting. Marine mammals and birds can dive to depths greater than one kilometer, versus 130 meters for the record-setting human dive. The immediate problem for deep-sea divers is getting enough oxygen and avoiding pressure-related decompression sickness, commonly known as the "bends". Deep-diving marine animals, on the other hand, possess the physiological equipment to deal with these problems. In this article, the authors describe the adaptations that allow animals to achieve what people can still only dream about.

 

[joewr]

Dr. Deco and RCohn,

So, the current understanding is that ocean-going mammals limit the amount of N2 in the bloodstream by chemically bonding O2 in muscle tissue--since "hemo" means "blood", I presume "myo" means "muscle" or "tissue"? Presumably that storage process happens near the ocean's surface so the N2 is expelled as the O2 is retained. And, I guess this means that blood vessels and veins that are a long way from the heart of a whale--around the tail, say--do not have large amounts of dissolved N2 in them due to this process. Further, the residual N2 in the lungs is prevented from entering the blood stream by a "squeezing" processes that halts mass transfer across the lung/vein interface.

Okay, our lungs should go through the same process as the ocean-going mammals--so that leaves the myoglobin part of the process to consider. Now, this probably sounds like Isaac Azimov or Arthur Clark (Jules Verne?), but could humans find a way to "inject" myoglobin into our system? This may be akin to athletes injecting their own O2-rich blood in order to enhance performance. Imagine how different diving would be if you could eliminate tanks, etc. and did not have to worry about DCS?

Thanks for the information--it is fascinating stuff to me.

Joewr

 

[John Reinertson]

I'm just a country family doc, but let me take a stab at some of this.

The lung collapse referred to is just that. The small air sacs of the lungs collapse at depth. They do so because the air within the mammal gets increasingly compressed, and takes up less space.(Boyle's Law)
The trachea and larger bronchi are rigid and can't easily collapse, so the alveoli collapse and the remaining air remains pressurized in the bronchi and trachea.
Since the bronchi and trachea have much less circulation and much smaller net surface area than the small air sacs of the lungs, The nitrogen in the trachea and bronchi get only partly absorbed.

The whale at the surface has a large amount of oxygen stored in his muscles, bound to myoglobin.
He has a modest amount of total nitrogen available at the surface to dissolve at depth, so the whale's tissues don't saturate as fast or as completely as we do.
With less nitrogen dissolved, and less chance of supersaturation to a critical degree there is less chance of it bubbling out of the bloodstream or joints or other tissues.

When we poor humans dive, we keep breathing a nearly unlimited supply of nitrogen from our tanks, and with rising partial pressures of nitrogen at rising depth, we saturate our tissues much more thoroughly than our Cetacean cousins.

Our Cetacean cousins start out with a much larger body amount of oxygen dissolved/bound to myoglobin.. so they can stay down longer before running out of 02.

They also tolerate higher levels of CO2 than we do, so they get further on one breath and have less nitrogen to absorb.

There are some other esoteric issues involved, but they're beyond my ability to explain.

John

 

[turnerjd]

Originally posted by joewr
--since "hemo" means "blood",
Joewr

Haemo (UK + europe + Japan) hemo (US) actually means Iron.

It is the iron in the haemoglobin that gives blood its characteristic red colour. - it follows that activities like haemolysis are then the loss of the Iron bearing molecule, and the subsequent inability to carry oxygen.

Jon T

 

[Dr Deco]

Dear Readers:

These are all wonderful answers to help explain the “whale of a tale. My thanks to all of you who have sent in material on this question.
____________
Dr Deco