Deep breath

Human freedivers have to hold their breath for long periods of time. The current record for which is held by freediver Stéphane Mifsud, who held his breath for 11 minutes, 35 seconds. Professional freedivers have an enhanced “dive reflex”, slowing down their heart rate and metabolism by 50% in order to conserve oxygen (an untrained diver’s heart rate will decrease by 10-30%) (AIDA International). Being able to hold your breath for long periods of time relies on how much oxygen you have in your body to begin with and how fast you metabolise. In May 2013, Stig Severinson hyperventilated with oxygen before attempting the Guinness World Record for the longest breath hold for a human. The increased levels of oxygen in his body before the attempt help him hold his breath for a record 22 minutes (see video below).

When oxygen runs out respiration turns from aerobic to anaerobic, producing lactic acid which is harmful to the body. It is also 17 times less efficient at producing the energy giving ATP. Lactate recovery time is long and costly, it take roughly 17 times longer to clear as it is to produce it. During diving we call this point of respiratory change the ‘aerobic dive limit’. So how can beaked whales and other deep sea divers hold their breath for hours regularly without severely damaging their bodies with lactic acid build up? In diving records for beaked whales we can see that a long dive is followed by a series of short dives, well within the ADL. It is assumed this series of short dives acts as a recovery period from pushing past the ADL (Figure 6)(Madsen et al. 2013). For example, in Cuvier’s beaked whale, deeper, longer dives created more lactic acid and therefore had a longer recovery period before a deep dive could be made again, reducing the number of deep dives per day (Figure 8) (Schorr et al. 2014). However beaked whales have several adaptations that help them to store oxygen far more efficiently than we humans, allowing them to stay underwater for longer before hitting their ADL:

Fig7. a) When oxygen is not available pyruvic acid is converted into lactic acid and an oxygen debt is created. b) With oxygen available aerobic respiration takes place in the mitochondria producing 17 times more ATP than anaerobic respiration.  Image copyright of OpenStax College.
Fig 7. a) When oxygen is not available pyruvic acid is converted into lactic acid and an oxygen debt is created. b) With oxygen available aerobic respiration takes place in the mitochondria producing 17 times more ATP than anaerobic respiration.
Image copyright of OpenStax College.

 

The obvious choice for oxygen storage is in the lungs, however, in deep diving animals storing oxygen as gas makes them more buoyant and extra energy must be used to counteract this. Deep water pressure is also an issue here, this will be discussed later. This makes storage of oxygen in the lungs not valid choice. In fact, many marine mammals have a reduced lung size.

Fig 8. The deep and shallow dives of Cuvier’s beaked whale. Image copyright of Schorr et al. 2014.

Like us, marine mammal blood also contains haemoglobin (this is the pigment that makes blood red), this is the protein to which oxygen binds to be transported around the body. Once the oxygen binds to the haemoglobin it is no longer in gaseous form, making buoyancy issues not a problem. There is however a drawback to increasing the amount of haemoglobin in the blood; haemoglobin is carried by red blood cells (used in the body for clotting). An increase in haemoglobin means an increase in red blood cells. This could have potentially fatal consequences; an increase in blood viscosity would make it harder to pump blood around the body, with too much congestion in the blood significantly increasing the chances of blood clotting and heart attacks. Therefore the answer for deep diving marine mammals is not to increase the level of haemoglobin in their blood but to invest in an increase in blood volume. Meaning that, based on increased blood volume, in general larger animals can dive longer and deeper, although they may not be necessarily be carrying more oxygen per unit of blood. They may even take oxygenated red blood cells and hold them in the spleen, reserving them here when at rest. Then when needed, the spleen contracts releasing an extra burst of oxygen into the blood.

The haemoglobin protein therefore has a limit on how much can be stored without having negative consequences so marine mammals invest in other proteins with an affinity for oxygen, the main one being myoglobin. Myoglobin is stored in the muscle tissue, rather than the blood, and has a far higher affinity for oxygen than its counter part. Each myoglobin protein can go for 5 minutes without oxygen, slowly diffusing it directly to the mitochondria for the production of energy.

Other proteins include cytoglobin and neuroglobin to protect the brain and neuron tissue.

Want to know more? Click this link for a simplified explanation of marine mammal breath holding by Dr Michael Berenbrink of Liverpool University: http://www.bbc.co.uk/news/science-environment-22898457

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A look at extreme mammal divers