Breathe in… and breathe out

The average human can hold their breath for up to 1 minute (O’Connor, 2013) without training and professional freedivers can last for up to 20 minutes (O’Connor, 2013). That’s pretty impressive; imagine all the things you could do underwater in twenty minutes: explore a cave, get lost in a kelp forest or even try to wrestle a pearl from an oyster. But, whilst you’re heading back up to gulp some much needed air something’s still going down.

The sperm whale, like many other marine mammals, spends most of its time underwater so it’s pretty obvious that it must have a good breath hold capability. But how good? 20 minutes would hardly be enough to get down to where it hunts. So how about 30? 40? 50?! Well the sperm whale’s hunting ground lies down at great depths of up to 8,000ft (Davidson, 2002) where it catches prey like giant squid and other bottom dwellers (Bird). That’s a long way and in order to have time to get there, hunt and get back to the surface these creatures have managed to hold their breath for up to an hour and a half (O’Connor, 2013) but generally dive for up to an hour (Davidson, 2002:Whitehead, 2003).

A mature female with the remains f a giant squid in her mouth returns from a successful hunt
A mature female with the remains f a giant squid in her mouth returns from a successful hunt (Figure 1)

So how do they do this? Well surprisingly it’s not that much different to how humans hold their breath only the whales have a few super modifications. Firstly, although their lungs are only about half the size of ours relative to body size they are immensely more efficient.

Tidal volume is the volume of air organism inhales in a single breath when relaxed. In humans the average tidal volume is 500ml (Cliffsnotes) with the total lung capacity being a huge 6000ml (Cliffsnotes) that’s about 8% of our lungs that we fill with “new” air per breath, not a lot. If we change these figures into a percentage we can compare a human’s tidal capacity to a sperm whale’s. According to research conducted in 1940, a human has a mean total lung capacity of 7.4% of its overall body weight whereas a sperm whale has a miniscule 1.6% (Lockyer, 1981). So by these results shouldn’t humans be able to wait between breaths for longer? Well no because, like I mentioned before, the whales are incredibly efficient when it comes to dealing with oxygen.

We’ll start with the inhalation of air before a dive. Just like in humans the oxygen needed is taken through three main systems in the body, the respiratory, cardiovascular and then the cellular systems, where it is utilised by the skeletal muscle. In humans each system is of equal importance but in whales and other diving mammals the importance increases along the line (Williams and Worthy, 2002). Because terrestrial mammals have a constant supply of air they are able to afford to waste oxygen meaning that much of the oxygen inhaled in each breath will be exhaled out again, this is not the case for sperm whales who have a closed system in which breathable air is not always readily available. To combat this they have evolved multiple methods of increasing the efficiency of the oxygen in their systems. Firstly, the sperm whales have hepatocytes (red blood cells) much larger than ours which contain a higher number of haemoglobin molecules meaning that more oxygen can be transported on one cell at a time than on a human hepatocyte.

A sperm whale takes a gulp of air from the surface before diving down to the depths
A sperm whale takes a gulp of air from the surface before diving down to the depths (Figure 2)

Secondly, when the oxygen reaches the skeletal muscle tissues it is stored in proteins called myoglobin (Williams and Worthy, 2002). These are oxygen storing molecules which are similar to haemoglobin but have a higher affinity for oxygen (Mirceta et. Al., 2013). In sperm whales there are massive concentrations of these molecules which enable a vast amount of oxygen to be stored in preparation for each dive so that the whale’s tissues are able to respire aerobically for longer which, in turn, leads to a decreased amount of time the cells spend respiring anaerobically.

In extreme cases where the whale is submerged for unusually long periods of time the tissues do begin to conduct respiration anaerobically but they do not, as we do, get cramps in their muscles, this is because they have an extremely high tolerance to lactic acid (Brylske), the compound formed as the final product of anaerobic respiration. This is extremely beneficial as a cramped muscle could lead to the whale sinking down further than it is safe to do so or hinder the whale’s ascent to the surface prolonging the time between breaths.

A third great adaptation the whales have come up with is an increased response to diving which consists of three main nervous responses and is collectively referred to as the Mammalian Diving Reflex (Panneton, 2013). Many years of experimentation and careful analysis have shown that all mammals, even terrestrial mammals like humans, pigs and rats, possess this reflex (Panneton, 2013) but it is clear that the true masters of the MDR are cetaceans like the sperm whale.

The three main neural reflexes in MDR are the regulation of:

1) Respiration

2) Heart rate (bradycardia)

3) Arterial blood pressure (peripheral vasoconstriction)

In the first reflex, regulation of respiration, different types of cells respond differently. In the active diving muscles the cells increase their rate of respiration in order to produce the required levels of energy needed to function. In less vital cells the cellular respiration rate drops thereby conserving oxygen. In the second reflex the heart rate is dramatically decreased, known as bradycardia, and serves to lessen the amount of oxygen being transported in the bloodstream leaving more free for utilisation in muscles and important organs. The final response serves to redirect the blood flow away from the outermost (peripheral) tissues and organs by the vasoconstriction of minor blood vessels at and close to the extremities. This response allows oxygenated blood to be transported directly to the tissues where it is most needed.

A sperm whale inspects the camera at the surface
A sperm whale inspects the camera at the surface (Figure 3)

Although it may take more than a couple of years of evolution, if ever, for humans to be able to achieve such long periods of dynamic apnea, by the continued study of breath hold techniques in marine mammals we gain more and more insight into the fantastic world of these awesome creatures.

 

References:

Bird, J. Sperm Whales: The Deep Divers of the Oceans, (updated 06/05/2007) http://www.oceanicresearch.org/education/wonders/spermwhales.htm date accessed 22nd October 2014

Brylske, A. Humans and Whales: an Intimate Connection, http://www.dtmag.com/Stories/Dive%20Physiology/01-04-Feature.htm date accessed 21st October 2014

Cliffnotes, http://www.cliffsnotes.com/sciences/anatomy-and-physiology/the-respiratory-system/lung-volumes-and-capacities date accessed 22nd October 2014

Davidson, S. (2003) Whales and Dolphins, Usborne, page. 17

Lockyer, C. (1981) Baleen Whales from the Southern Hemisphere. Mammals in the seas: General Papers, page. 441

Mirceta, S., Signore, A. V., Burns, J. M., Cossins, A. R., Campbell, K. L. & Berenbrink, M. (2013) Evolution of Mammalian Diving Capacity Traced by Myoglobin Net Surface Charge. Science. VOL 340, page 1234192-1

O’Connor, R. (2013) How Long Can You Hold Your Breath Underwater? http://tvblogs.nationalgeographic.com/2013/03/10/how-long-can-you-hold-your-breath-underwater/ date accessed 21st October 2013

Panneton, W. M. (2013) The Mammalian Diving Response: an Enigmatic Reflex to Preserve Life?, http://physiologyonline.physiology.org/content/28/5/284 date accessed 23rd October 2014

Whitehead, H. (2003) Sperm Whales, University of Chicago, page. 167

Williams, T. M. & Worthy, G. A. J. (2002) Marine mammal biology an evolutionary approach, Oxford, page. 79

Figure 1 – Alamy

Figure 2 – albany.asn.au ( http://www.albany.asn.au/albanywhalewatching.html )

Figure 3 – Brandon Cole

3 years ago

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