question: If it is more efficient to exhale then hold your breath, why do no cetaceans (whales, dolphins, etc.) exhale while under water?
comment: Cetaceans will hold their breath as long as possible to get the most oxygen from it. They will exhale just before they breach the surface so when they do, they can quickly inhale and spend less time on the surface. Swimming on the surface has more drag than swimming completely underwater, so they try spend the minimal amount of time there.
passages: The structure of the respiratory and circulatory systems is of particular importance for the life of marine mammals. The oxygen balance is effective. Each breath can replace up to 90% of the total lung volume. For land mammals, in comparison, this value is usually about 15%. During inhalation, about twice as much oxygen is absorbed by the lung tissue as in a land mammal. As with all mammals, the oxygen is stored in the blood and the lungs, but in cetaceans, it is also stored in various tissues, mainly in the muscles. The muscle pigment, myoglobin, provides an effective bond. This additional oxygen storage is vital for deep diving, since beyond a depth around , the lung tissue is almost completely compressed by the water pressure.
   Breath-hold diving depth is limited in animals when the volume of rigid walled internal air spaces is occupied by all of the compressed gas of the breath and the soft spaces have collapsed under external pressure. Animals that can dive deeply have internal air spaces that can extensively collapse without harm, and may actively exhale before diving to avoid absorption of inert gas during the dive.
   While diving, the animals reduce their oxygen consumption by lowering the heart activity and blood circulation; individual organs receive no oxygen during this time. Some rorquals can dive for up to 40 minutes, sperm whales between 60 and 90 minutes and bottlenose whales for two hours. Diving depths average about . Species such as sperm whales can dive to , although more commonly .
  
  Section::::Behaviour.:Social relations.
   polar bear. All are air-breathing, and while some such as the sperm whale can dive for prolonged periods, all must return to the surface to breathe.
  
  Section::::Marine habitats.
   Conscious breathing cetaceans sleep but cannot afford to be unconscious for long, because they may drown. While knowledge of sleep in wild cetaceans is limited, toothed cetaceans in captivity have been recorded to exhibit unihemispheric slow-wave sleep (USWS), which means they sleep with one side of their brain at a time, so that they may swim, breathe consciously and avoid both predators and social contact during their period of rest.
   Aerobic diving capacity is limited by available oxygen and the rate at which it is consumed. Diving mammals and birds have a considerably greater blood volume than terrestrial animals of similar size, and in addition have a far greater concentration of haemoglobin and myoglobin, and this haemoglobin and myoglobin is also capable of carrying a higher oxygen load. During diving, the hematocrit and hemoglobin are temporarily increased by reflex splenic contraction, which discharges a large additional amount of red blood cells. The brain tissue of diving mammals also contains higher levels of neuroglobin and cytoglobin than terrestrial animals.
   In certain active pelagic sharks, water passes through the mouth and over the gills while they are moving, in a process known as "ram ventilation". While at rest, most sharks pump water over their gills, as most bony fish do, to ensure that oxygenated water continues to flow over their gills. But a small number of species have lost the ability to pump water through their gills and must swim without rest. These species are "obligate ram ventilators" and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.
   Seals, whales and porpoises have slower respiratory rates and larger tidal volume to total lung capacity ratio than land animals which gives them a large exchange of gas during each breath and compensates for low respiratory rate. This allows greater utilisation of available oxygen and reduce energy expenditure.
  
  In seals, bradycardia of the diving reflex reduces heart rate to about 10% of resting level at the start of a dive.
  
  Section::::References.
  
  Section::::References.:Sources.
  
  BULLET::::- CD-ROM prepared and distributed by the National Technical Information Service (NTIS)in partnership with NOAA and Best Publishing Company/ref
   Weddell seals' metabolism is relatively constant during deep-water dives, so another way to compensate for functioning with a lack of oxygen over an extended period of time must exist. Seals, unlike other terrestrial mammals such as humans, can undergo anaerobic metabolism for these extended dives, which causes a build-up of lactic acid in the muscles. The seals can also release oxygenated blood from their spleens into the rest of their bodies, acting as an oxygen reserve.
  
  Section::::Behavior.:Vocalizing.
   Through a principal components analysis demonstrated that "Rodhocetus" had trunk and limb proportions similar to the Russian desman, a foot-powered swimmer using its tail mainly as a rudder. From this Gingerich concluded that "Rodhocetus" was swimming mostly at the surface by alternate strokes of its hind feet, and that it was insulated by fur rather than blubber, as are "Dorudon" and modern cetaceans, which made it buoyant and incapable of deep diving.
  
  Section::::Description.:"R. kasrani".
   brainstem of fish are responsible for the genesis of the respiratory rhythm. The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the homology of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see Involuntary control of respiration).
   Marine mammals are able to dive for long periods. Both pinnipeds and cetaceans have large and complex blood vessel systems which serve to store oxygen to support deep diving. Other important reservoirs include muscles, blood, and the spleen which all have the capacity to hold a high concentration of oxygen. They are also capable of bradycardia (reduced heart rate), and vasoconstriction (shunting most of the oxygen to vital organs such as the brain and heart) to allow extended diving times and cope with oxygen deprivation. If oxygen is depleted, marine mammals can access substantial reservoirs of glycogen that support anaerobic glycolysis of the cells involved during conditions of systemic hypoxia associated with prolonged submersion.
   In order for the lungs to allow gas exchange, the lungs first need to have gas in them. In modern tetrapods, three important breathing mechanisms are conserved from early ancestors, the first being a CO/H+ detection system. In modern tetrapod breathing, the impulse to take a breath is triggered by a buildup of CO in the bloodstream and not a lack of O. A similar CO/H+ detection system is found in all Osteichthyes, which implies that the last common ancestor of all Osteichthyes had a need of this sort of detection system. The second mechanism for a breath is a surfactant system in the lungs to facilitate gas exchange. This is also found in all Osteichthyes, even those that are almost entirely aquatic. The highly conserved nature of this system suggests that even aquatic Osteichthyes have some need for a surfactant system, which may seem strange as there is no gas underwater. The third mechanism for a breath is the actual motion of the breath. This mechanism predates the last common ancestor of Osteichthyes, as it can be observed in "Lampetra camtshatica," the sister clade to Osteichthyes. In Lampreys, this mechanism takes the form of a "cough", where the lamprey shakes its body to allow water flow across its gills. When CO levels in the lamprey's blood climb too high, a signal is sent to a central pattern generator that causes the lamprey to "cough" and allow CO to leave its body. This linkage between the CO detection system and the central pattern generator is extremely similar to the linkage between these two systems in tetrapods, which implies homology.
   Earless seals sleep bihemispherically like most mammals, under water, hanging at the water surface or on land. They hold their breath while sleeping under water, and wake up regularly to surface and breathe. They can also hang with their nostrils above water and in that position have REM sleep, but they do not have REM sleep underwater.
   In fish, the long bony cover for the gill (the operculum) can be used for pushing water. Some fish pump water using the operculum. Without an operculum, other methods, such as ram ventilation, are required. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.
   Two other marine vertebrates, the Steller sea lion and the loggerhead turtle, display a rather low number of pseudogenes with respect to cetaceans. The preservation of their OR genes is correlated with their semi-adaptation to an aquatic habitat, since these organisms still perform many functions on land (e.g. mating, basking). Cetaceans do not depend on terrestrial environments, and have thusly lost a large fraction of their OR repertoire. Echolocation was presumable evolved to compensate for this loss.
  
  Section::::Psychophysical Implications.
   Senses of taste and smell are not very important to the diver in the water but more important to the saturation diver while in accommodation chambers. There is evidence of a slight decrease in threshold for taste and smell after extended periods under pressure.
  
  Section::::Adaptation in other animals.
  
  Aquatic mammals such as seals and whales dive after full exhalation, which would reduce the amount of nitrogen available to saturate the tissues by 80 to 90%.
  
  Aquatic mammals are also less sensitive to low alveolar oxygen concentrations and high carbon dioxide concentrations than purely terrestrial mammals.
   In physiology, dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it remains in the conducting airways, or reaches alveoli that are not perfused or poorly perfused. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inspiration which remains in the conducting airways where no gas exchange can occur. In humans, about a third of every resting breath has no change in O and CO levels.
   BULLET::::- Dolphin kick is a powerful stroke where both feet are kept together and moved up and down. This is the only stroke possible with monofins, and can be very effective for the skilled practitioner. It is not good for precise maneuvering.
   Indirect evidence for endothermy is provided by the body shape of derived ichthyosaurs, which with its short tail and vertical tail fin seems optimised for a high cruising speed that can only be sustained by a high metabolism: all extant animals swimming this way are either fully warm-blooded or, like sharks and tuna, maintain a high temperature in their body core. This argument does not cover basal forms with a more eel-like body and undulating swimming movement. In 1996, Richard Cowen, while accepting endothermy for the group, presumed that ichthyosaurs would have been subject to "Carrier's constraint", a limitation to reptilian respiration pointed out in 1987 by David Carrier: their undulated locomotion forces the air out of the lungs and thus prevents them from taking breath while moving. Cowen hypothesised that ichthyosaurs would have overcome this problem by porpoising: constantly jumping out of the water would have allowed them to take a gulp of fresh air during each jump. Other researchers have tended to assume that for at least derived ichthyosaurs Carrier's constraint did not apply, because of their stiff bodies, which seems to be confirmed by their good diving capacity, implying an effective respiration and oxygen storage system. For these species porpoising was not a necessity. Nevertheless, ichthyosaurs would have often surfaced to breathe, probably tilting their heads slightly to take in air, because of the lower position of the nostrils compared to that of dolphins.
   Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters, including breathing tubes as in mollusc siphons. Fish have gills instead of lungs, although some species of fish, such as the lungfish, have both. Marine mammals, such as dolphins, whales, otters, and seals need to surface periodically to breathe air.
   Although air-breathing fish can absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the ability of lungs to discharge CO came about gradually, and was not fully attained until the evolution of amniotes. The same limitation applies to gut air breathing (GUT), i.e., breathing with the lining of the digestive tract. Tetrapod skin would have been effective for both absorbing oxygen and discharging CO, but only up to a point. For this reason, early tetrapods may have experienced chronic hypercapnia (high levels of blood CO). This is not uncommon in fish that inhabit waters high in CO.
   Section::::Benefits.:Surfacing for air and pod cohesion.
  
  Unihemispheric slow-wave sleep seems to allow the simultaneous sleeping and surfacing to breathe of aquatic mammals including both dolphins and seals. Bottlenose dolphins are one specific species of cetaceans that have been proven experimentally to use USWS in order to maintain both swimming patterns and the surfacing for air while sleeping.
   Respiration in newts can take place through the skin, the lungs, or the buccal cavity, but newts mainly respire through the skin due to the fact that the buccal cavity and lungs are poorly vascularized. However, when "T. carnifex" are placed in poorly oxygenized water, they switch their primary respiratory surface from skin to lungs. Through further tests it was shown that "T. carnifex" can stay under water for long periods of time by using their skin to respire, but when they become active they must switch and usetilize their lungs or buccal cavity. Eddy and McDonald (1977) also discovered that many nsetilize their lungs to help regulate their buoyancy, similar to fish that use a swim bladder to aid in swimming.
   Other animals, such as cephalopods, use jet propulsion to travel fast, taking in water then squirting it back out in an explosive burst. Other swimming animals may rely predominantly on their limbs, much as humans do when swimming. Though life on land originated from the seas, terrestrial animals have returned to an aquatic lifestyle on several occasions, such as the fully aquatic cetaceans, now very distinct from their terrestrial ancestors.
  
  Dolphins sometimes ride on the bow waves created by boats or surf on naturally breaking waves.
  
  Section::::Aquatic.:Benthic.
  