It is difficult to argue with the statement that it is nature that has the most vivid imagination. Each of the representatives of flora and fauna has its own unique, and sometimes even strange features that often do not fit in our heads. Take, for example, the same mantis shrimp. This predatory creature is capable of attacking a victim or offender with its powerful claws at a speed of 83 km / h, and their visual system is one of the most complex ever studied by man. Mantis shrimp, although fierce, are not particularly large - up to 35 cm in length. The largest inhabitant of the seas and oceans, as well as the planet in general, is the blue whale. The length of this mammal can reach more than 30 meters, and the weight is 150 tons. Despite their impressive size, blue whales can hardly be called formidable hunters, because. they prefer plankton.
The anatomy of blue whales has always been of interest to scientists who want to better understand how such a huge organism and organs work in it. Despite the fact that we have known about the existence of blue whales for several hundred years (since 1694, to be more precise), these giants have not revealed all their secrets. Today, we take a look at a study in which a group of scientists from Stanford University developed a device that captured the first recordings of a blue whale's heartbeat. How does the heart of the ruler of the seas work, what discoveries have scientists made, and why can't an organism larger than a blue whale exist? We learn about this from the report of the research group. Go.
Exploration hero
The blue whale is the largest mammal, the largest inhabitant of the seas and oceans, the largest animal, the largest whale. What can I say, the blue whale is really the best in terms of dimensions - a length of 33 meters and a weight of 150 tons. The figures are approximate, but no less impressive.
Even the head of this giant deserves a separate line in the Guinness Book of Records, since it occupies about 27% of the total body length. At the same time, the eyes of blue whales are quite small, no larger than a grapefruit. If it will be difficult for you to see the eyes of a whale, then you will notice the mouth right away. The mouth of a blue whale can hold up to 100 people (a creepy example, but blue whales do not eat people, at least not intentionally). The large size of the mouth is due to gastronomic preferences: whales eat plankton, swallowing huge volumes of water, which they then release through a sieve, filtering out food. Under fairly favorable circumstances, the blue whale absorbs about 6 tons of plankton per day.
Another important feature of blue whales is their lungs. They are able to hold their breath for 1 hour and dive to a depth of 100 m. But, like other marine mammals, blue whales periodically emerge to the surface of the water to breathe. Rising to the surface of the water, the whales use the blowhole - a breathing hole from two large holes (nostrils) on the back of the head. The exhalation of a whale through a blowhole is often accompanied by a vertical fountain of water up to 10 m high. Given the characteristics of the habitat of whales, their lungs work much more efficiently than ours - whale lungs absorb 80-90% of oxygen, and ours only about 15%. The volume of the lungs is about 3 thousand liters, in humans, this figure varies in the region of 3-6 liters.
Model of a blue whale's heart in a museum in New Bedford (USA).
The circulatory system of the blue whale is also full of record parameters. For example, their vessels are simply huge, the diameter of the aorta alone is about 40 cm. The heart of blue whales is considered the largest heart in the world and weighs about a ton. With such a big heart, a whale has a lot of blood - more than 8000 liters in an adult.
And so we smoothly approached the essence of the study itself. The heart of the blue whale is large, as we have already understood, but it beats rather slowly. Previously, it was believed that the pulse is about 5-10 beats per minute, in rare cases up to 20. But no one has made accurate measurements, until now.
Scientists from Stanford University say that the scale in biology is of great importance, especially when it comes to determining the functional characteristics of the organs of living beings. The study of various creatures, from mice to whales, allows you to determine the size limits that a living organism cannot exceed. And the heart and the cardiovascular system as a whole are important attributes of such studies.
In marine mammals, whose physiology has fully adapted to their way of life, diving and breath-holding adaptations play an important role. It has been found that in many such creatures, during a dive, the heart rate drops to levels below the resting state. And when you rise to the surface, the heart rate becomes more rapid.
Decreased heart rate during diving is necessary to reduce the rate of oxygen delivery to tissues and cells, thereby slowing down the process of depleting oxygen reserves in the blood and reducing oxygen consumption by the heart itself.
There is a hypothesis that exercise (i.e. increased physical activity) modulates the dive response and increases heart rate during a dive. This hypothesis is especially important for the study of blue whales, because due to the special method of feeding (suddenly to swallow water), the metabolic rate, in theory, should exceed the base values ββ(resting state) by 50 times. It is hypothesized that such lunges accelerate the depletion of oxygen, hence reducing the duration of the dive.
The increased heart rate and increased oxygen transfer from the blood to the muscles during a lunge may play an important role due to the metabolic cost of such physical activity. In addition, it is worth considering the low concentration myoglobin* (Mb) in blue whales (5-10 times lower than in other marine mammals: 0.8 g Mb per 100 g-1 muscle in blue whales and 1.8-10 g Mb in other marine mammals.
Myoglobin* - oxygen-binding protein of skeletal muscle and heart muscle.
As a conclusion, physical activity, diving depth and volitional control change the heart rate during diving through the autonomic nervous system.
An additional factor in reducing the heart rate may be compression / expansion of the lungs during a dive / ascent.
Thus, the heart rate during the dive and during the stay on the surface is directly related to the patterns of arterial hemodynamics.
Finwal
An earlier study of the biomechanical properties and dimensions of the aortic walls in fin whales (Balaenoptera physalus) showed that during diving at a heart rate β€10 beats/min, the aortic arch realizes a reservoir effect (Windkessel effect), which maintains blood flow for long diastolic periods* between heartbeats and reduces the pulsation of blood flow into the rigid distal aorta.
Diastole* (diastolic period) - the period of relaxation of the heart between contractions.
All the hypotheses, theories and conclusions described above must have material evidence, that is, be confirmed or refuted in practice. But for this you need to conduct an electrocardiography of a freely moving blue whale. Simple methods will not work here, because scientists have created their own device for electrocardiography.
A video in which the researchers briefly talk about their work.
The whale's ECG was recorded using a custom-made ECG recorder built into a special capsule with 4 suction cups. Surface ECG electrodes were embedded in two of the suction cups. The researchers went by boat to Monterey Bay (Pacific Ocean, near California). When scientists finally met a blue whale that floated to the surface of the water, they attached an ECG recorder to its body (next to the left fin). According to previously collected data, this whale is a male at the age of 15 years. It is important to note that this device is non-invasive, that is, it does not require the introduction of any sensors or electrodes into the skin of the animal. That is, for the whale, this procedure is completely painless and with minimal stress from human contact, which is also extremely important, given that heartbeat readings are taken that could be distorted due to stress. The result was an 8.5-hour ECG recording from which scientists were able to build a heart rate profile (image below).
Image #1: Blue whale heart rate profile.
The ECG waveform was similar to that recorded from small whales in captivity using the same device. The whale's foraging behavior was quite normal for its species: diving for 16.5 minutes to a depth of 184 m and surface intervals of 1 to 4 minutes.
The heart rate profile, consistent with the cardiovascular response to the dive, showed that a heart rate of 4 to 8 beats per minute predominated in the lower phase of foraging dives, regardless of dive duration or maximum depth. Dive heart rate (calculated over the entire duration of the dive) and minimum instantaneous heart rate during the dive decreased with dive duration, while the maximum post-dive surface heart rate increased with dive duration. That is, the longer the whale was under water, the slower the heart beat during the dive and the faster after the ascent.
In turn, allometric equations for mammals state that a whale weighing 70000 kg has a heart weighing 319 kg, and its stroke volume (the volume of blood ejected per beat) is about 80 liters, therefore, the heart rate at rest should be 15 beats / min.
During the lower phases of dives, the instantaneous heart rate was 1/3 to 1/2 of the predicted resting heart rate. However, the heart rate increased during the ascent stage. At surface intervals, the heart rate was about twice the predicted resting heart rate and predominantly ranged from 30 to 37 beats per minute after deep dives (>125 meters) and from 20 to 30 beats per minute after shallower dives.
This observation may indicate that the acceleration of heart rate is necessary to achieve the desired respiratory gas exchange and reperfusion (restoration of blood flow) of tissues between deep dives.
Shallow, brief night dives have been associated with rest and are therefore more characteristic of a less active state. The typical heart rates seen with a 5-minute night dive (8 beats per minute) and the accompanying 2-minute surface interval (25 beats per minute) can collectively result in a heart rate on the order of 13 beats per minute. This figure, as we can see, is surprisingly close to the calculated predictions of allometric models.
The scientists then profiled the heart rate, depth, and relative lung capacity of 4 separate dives to explore the potential impact of physical activity and depth on heart rate regulation.
Image #2: Heart rate, depth and relative lung volume profiles from 4 separate dives.
While eating food at great depths, the whale performs a certain lunge maneuver - it sharply opens its mouth to swallow water with plankton, and then filters out the food. It was noted that the heart rate at the time of swallowing water is 2.5 times higher than at the time of filtration. This directly indicates the dependence of the heart rate on physical activity.
As for the lungs, their effect on heart rate is highly unlikely, as no significant changes in relative lung volume were observed during the dives in question.
At the same time, in the lower phases of shallow dives, a short-term increase in heart rate was associated precisely with changes in the relative volume of the lungs and could be caused by activation of the lung stretch receptor.
Summarizing the above observations, the scientists came to the conclusion that during feeding at great depths, there is a short increase in heart rate by 2.5 times. However, the average peak heart rate during lunges at the time of feeding was still only half the predicted value at rest. These data are consistent with the hypothesis that the flexible aortic arches of large whales realize a reservoir effect during slow heart rates during dives. In addition, the range of higher heart rates during the post-dive period supported the hypothesis that aortic impedance and cardiac workload decrease during the surface interval due to the destructive interference of outgoing and reflected pressure waves in the aorta.
The extreme bradycardia observed by the researchers is an unexpected result of the study, given the colossal effort required by the whale to lunge when swallowing water with plankton. However, the metabolic cost of this maneuver may not match heart rate or convective oxygen transport due in part to the short duration of feeding and the possible recruitment of glycolytic, fast twitch muscle fibers.
During a lunge, blue whales accelerate to high speeds and absorb a volume of water that can be larger than their own body. Scientists suggest that the high resistance and energy required to maneuver quickly deplete the body's total oxygen supply, which limits the dive time. The mechanical force required to absorb large volumes of water is likely to far exceed the aerobic metabolic force. That is why during the performance of such maneuvers, the heart rate, although it increased, but for a very short time.
For a more detailed acquaintance with the nuances of the study, I recommend looking at .
Finale
One of the most important findings is that blue whales require near-maximal heart rates for gas exchange and reperfusion during short surface intervals, regardless of the pattern of oxygen depletion in blood and muscle during dives. Given that larger blue whales have to put in more labor in a shorter period of time to obtain food (according to the hypotheses of allometry), they inevitably face several physiological limitations both during the dive and during the surface interval. And this means that evolutionarily the size of their body is limited, since if it were larger, the process of obtaining food would be very costly and would not be compensated by the food received. The researchers themselves believe that the heart of the blue whale is working at the limit of its capabilities.
In the future, scientists plan to expand the capabilities of their device, including adding an accelerometer to better understand the impact of various physical activities on heart rate. They also plan to use their ECG sensor on other marine life.
As this study has shown, being the biggest creature with the biggest heart is not so easy. However, no matter how big sea creatures are, no matter what diet they follow, we need to understand that the water column, which is used by humans for fishing, mining and transportation, remains their home. We are only guests, and therefore we must behave accordingly.
Friday off-top:
Rare footage of a blue whale demonstrating the capacity of its mouth.
Another giant of the seas is the sperm whale. In this video, scientists using a remote-controlled ROV Hercules filmed a curious sperm whale at a depth of 598 meters.
Thanks for watching, stay curious and have a great weekend everyone! π
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Source: habr.com