The Physiology of the Deep Diving Adaptations of Whales
Lisa Carlson, Amy Schuler, and Vanessa Smith
ABSTRACT
The following information pertains to the diving adaptations of cetaceans (porpoises, dolphins, and whales), focusing on Odontocetes (toothed whales) and Mysticetes (baleen whales). The topics emphasized include the physiology of blubber, musculature, circulation, respiration, and the spermaceti organ (found only in sperm whales).
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Spending their entire lives in the waters of the ocean, whales have evolved physiological characteristics unique from other mammals; this uniqueness enables them to accomplish seemingly unbelievable feats. The ability to remain submerged underwater for extended periods of time and dive to amazing depths are achievable through the cooperation of the variety of organ systems that have become specialized to deal with the obstacles that a marine lifestyle presents. Take an educational dive to explore this miraculous creature that inhabits the bottomless depths of the cool, blue ocean.
Cetacean skin is very thin and closely attached to the thick layers of blubber; together, they encase the entire body. The blubber is not firmly attached to the muscle layers which lie underneath, however, so the two masses can move over one another a great deal.
The smooth-appearing skin is misleading - underneath lies a system of dermal ridges (these can be compared to the ridges on a human finger which produce a fingerprint). The function of these ridges is to reduce drag by ensuring laminar flow.
One of the
first hurdles that cetaceans have to overcome with their marine lifestyle
is the maintenance of a normal mammalian temperature of about 37 degrees
Celsius while emerged in the frigid waters of the ocean, especially at depths
where the sun's rays aren't able to reach and warm its waves of icy liquid.
The use of blubber is its main line of defense against this imbalance in
temperature.
In order for thermoregulation to occur between the surrounding water and the whale's warm inner core, the blubber lay must present a great enough amount of thermal resistance. The difference in water and core temperatures is equal to the product of the thermal resistance of the blubber times the outward flow of metabolic heat produced in the core. Over long periods of an imbalance in the necessary core temperature presented by the blubber, the whale in polar waters develops a thicker layer of blubber.
This increased thermal barrier may become too much of a good thing when the whale is active. As a result, the whale has to change its circulation strategy by increasing the blood flow near the body's surface, particularly through the flippers and flukes. By thermally bypassing the insulating blubber, the core temperature can return to normal.
The blubber of the whale not only serves to maintain its core temperature, but also provides enough buoyancy to offset the negative buoyancy of the whale's muscle and skeleton. It also constitutes a large enough food store to take care of the metabolic needs on the animal's seasonal migrations.
The muscles that provide the tail movements are arranged into two masses. The powerful up-stroke is provided by the epaxial mass, which is located on the upper side of the backbone. The down-stroke is powered by the hypaxial mass (known by whalers as the "back muscle" and the "under fillet," respectfully). The epaxial muscle mass must be much larger than the hypaxial mass because it is responsible for propelling the whale forward. Yet, the muscles of the belly wall are also connected to the tail, and the combined weight of the belly mass muscles and the hypaxial mass are close to the weight of the epaxial muscle mass. This may mean the down-stroke may also provide some propulsion. When travelling slowly, the whale's fluke oscillations are large; but when travelling at fast speeds the oscillations are rapid and small.
The majority of marine animal residents obtain oxygen directly from the
water surrounding them, but all marine mammals must reach the surface to
recharge with oxygen. A number of marine mammals, such as Pinnepeds (seals
and walruses) and Sirens (manatees) have nostrils on the front of their faces,
similar to land animals. The Cetacean's nostril or
blowhole, however, has migrated to the top of the
head over evolutionary time. The arrangement of the breathing mechanism allows
the whale to remain almost completely submerged, by using an arc swimming
pattern to grab a quick breath of air, and descend back down without having
to lift the entire head from the water. The rostrum around the blowhole acts
as a splashguard to keep water from
entering the nostrils
during the dive at the surface of the sea. When
whale watching, it is
the whale's exhaled breath and atomized water left around the blowhole that
forms the projected splash of water seen spouting from the blowhole as the
whale reaches the surface.
Oxygen storage and exchange:
Whales have numerous internal adaptations to deep diving as well, to allow for the depth of the dive, the time length of the dive, and body temperature homeostasis as the body descends into the cold, dark sea. Contrary to what one would expect, proportionally, the lung volume of whales is about one half that of terrestrial mammals. The exchange of air in one breath, or tidal volume, plays a key role in that a whale can exchange up to 85-90% of the air, whereas a human exchanges only 15% of the air.
In addition to the high exchange rate of air, the red blood cells of whales are larger than in humans; also, there are more red blood cells per unit of blood. Due to these two factors, the rate of exchange of oxygen from the lungs to hemoglobin (oxygen-carrying pigment in blood) is accelerated, distributing oxygen to the entire body at a faster rate. The myoglobin (oxygen-storing pigment in muscle) content in a whale's blood is up to ten times higher than in terrestrial animals, so cetacean muscle is richer and darker in color due to the high oxygen content. During a dive, limited oxygen is shunted to vital organs, the heart, and the brain via the blood, but the muscles continue working using the oxygen supply stored in the myoglobin in the muscles. When the supply in the muscles runs out, a whale's muscles work anaerobically. The concept is similar to extremely hard exercise in humans, when lactic acid is produced and builds up in muscles, however, whales have a high tolerance for lactic acid therefore their muscles can function anaerobically for extended periods. Marine mammals can tolerate CO2 build up in blood also, essentially because they can not breathe underwater. Due to the tolerance of CO2, a whale is allowed to stay underwater while their oxygen is used up. On account of the high oxygen-holding capacity of the muscles and blood in whales, they are able to perform long dives, plunging to the deepest depths of the sea. For ease in interpretation, the following table shows a comparison summary of the total oxygen storage capacities in a human diver and a whale dive:
|
Human | Whale |
| Oxygen in lungs | 34% |
9% |
| Oxygen in blood | 41% |
41% |
| Oxygen in muscles | 13% |
41% |
| Oxygen in tissues | 12% |
9% |
The "Bends"
The "bends," a condition that threatens scuba divers, is characterized by cramping pain and paralysis induced by a rapid return to normal atmospheric pressure after a period in a compressed atmosphere. The "bends" is caused by bubbles of gas, specifically nitrogen, forming in the blood during decompression. When the human diver is under pressure, nitrogen gas dissolves into the blood and body fluids. During ascent, if the pressure is released too quickly, the nitrogen comes out of solution and forms bubbles, which may obstruct small blood vessels and interfere with functioning of the joints and blood flow to the brain. Whales have derived protection mechanisms, to aid in avoidance of the bends. In the breath of a whale, limited amounts of nitrogen are taken in at the surface, unlike the breath of a scuba diver, which is taken in under great amounts of pressure. As whales descend, the air in the lungs compresses to a very small volume, and is forced to nonabsorptive portions of the lungs, (bronchioles, bronchi, and trachea) as well as safe areas in the nasal passages where little gas exchange into the blood can take place. In ascent to the surface, the compressed air expands, refilling the lung, and blood flow and gas exchange resume.
Respiration and circulation interact extensively, as we all know, but
many points related to blood flow and heat exchange are worth mentioning.
First of all, whales undergo what is termed the mammalian dive reflex, which
causes the heartbeat to slow, peripheral arteries to constrict, and shunting
of oxygenated blood to vital organs.
During
a whale's dive, the metabolic rate drops, causing a reduction in heart rate,
or bradycardia. A bradycardia state in an animal allows the animal to restrict
movement of blood to only regions of the heart, brain, and lungs. This
redistribution of arterial blood and vasoconstriction keeps blood away from
sensitive tissues, which require less oxygen supply in cold water.
Another feature of the whale circulatory system that aids in protecting the vital organs from effects of water pressure are the "retia mirabilia," or "wonderful network" associated with the counter current heat exchange system. When a marine mammal needs to conserve heat, a counter current heat exchange system is activated, in which arteries and veins work together to maintain consistent blood temperature. Warm arterial blood moves outward from the heart and organs passing through the wonderful network of veins bringing cool blood back. The venous blood flow surrounds the arteries and act as insulators for the warm arterial blood. Heat that may otherwise be lost into the surrounding sea is transferred to help warm up the blood in the veins as it returns towards the heart. When a whale is very active, the retia mirabilia act in a reverse manner, so the arteries are not insulated by the veins and can lose heat from the extremities, such as the fluke.
The spermaceti organ is unique to the sperm whale and may contribute to its ability to submerge and remain at great depths. This large organ is positioned in the whale's snout, surrounded by various nasal passages, and is composed of oil-filled connective tissue and a complex mass of muscle. In a large male sperm whale this organ may hold about four tons of spermaceti oil.
The actual
use of the spermaceti organ has been somewhat controversial, although it
is generally agreed that it holds a significant purpose due to its large
size. The most accepted theory is that small changes in temperature produce
large changes in the density of the spermaceti oil and, hence, can alter
their buoyancy as an aid for deep diving. The temperature of the water and
the spermaceti oil is 33 degrees Celsius when the sperm whale rests at the
surface of the water. As its temperature drops below 31 degrees Celsius the
oil gradually begins to crystallize and congeal into a solid. Freezing, it
becomes denser and will occupy less volume. As a result, the oil will displace
less ambient seawater and therefore is less buoyant.
There are two ways by which the temperature of the spermaceti oil is altered: the circulation of blood and the whale's nasal passages. The principle means of adding/conveying heat to the oil is through the circulation of arterial blood. When the sperm whale is ready to return to the ocean's surface, the amount of blood supplied to the spermaceti organ is increased so that it is warmed by the blood heat.
The flow of the blood also plays a role in the loss of heat. The larger arteries and veins in the whale's snout lie next to each other. The exchange of heat between the warmer incoming arterial blood and the cooler outgoing venous blood assists in the cooling of the spermaceti tissue.
A second means of heat loss is the whale's nasal passages. The intake of water through the two nasal passages, particularly the right passage which passes through the core of the spermaceti organ and its two sacs cover the front and back of the organ, quickly cools the spermaceti oil so that its buoyancy is decreased for a deep dive. Seawater can be drawn into the right nasal passage through the contracting of the surrounding outer wall of muscle tissue that encloses the spermaceti organ. Their contraction would lift the upper half of the nasal passage by lifting the front end of the spermaceti case. The relaxation of these muscles would result in the expelling of the water.
FUN
FACTS ABOUT DEEP DIVING DYNAMOS
"For an
average size Blue whale, a dog could run along some of its blood vessels,
it weighs about as much as 6,000 small children and you could park a car
in its mouth." (According to 'Whales and Dolphins' by Eva Plagani,
1994)
Migrating
whales have been recorded travelling over 3,700 km distances with speeds
ranging from 9 to 17 km/hr.
Cetaceans
have lost most olfactory receptors when the nostrils migrated to the top
of the head into a blowhole. "Air smelling" is very limited.
Whales
can not go into a full, deep sleep, because they need to be conscious to
breathe. EEG studies have shown whales and dolphins let half of their
brain sleep at a time.
The longest
recorded dive: Sperm Whale dive of 1 hour, 52 minutes (as a comparison,
a common dolphin dive is less than three minutes long).
The deepest
dive recorded: Sperm Whale dive of 3 km in depth.
Guiness
Book of Records shows a Blue Whale's heart weighing in at 1540 lbs.
Cetacean
life spans range from 15 years (Harbor Porpoise) to 90 years (Fin Whale).
Keiko (Free
Willy) is 16 years old.
Whale Sounds
Flying Blubber Story AND...Pictures!
The Status of the Great Whales
Back to Vertebrate Physiology Home Page
Questions? Comments? Please E-Mail us at:
vanessa.j.smith@uwrf.edu
amy.t.schuler@uwrf.edu
lisa.j.carlson@uwrf.edu
SKIN
AND BLUBBER
MUSCULATURE
