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Jadyn CraneCody Takabuki
Respiratory System Model for Obdurodon tharalkooschild
Abstract:
Late in the Miocene era in the eastern waterways of Australia, roamed a toothed mammalian
platypus species, Obdurodon tharalkooschild (Pian et al., 2013). Their carnivorous appetite and
unique hunting patterns, requires increased amounts of oxygen to provide the species with higher
amounts of energy. In order to sustain the organisms high level of oxygen during resting and
maximal metabolic activity, increased intake of oxygen and release of carbon dioxide is involved
in energy production. This transfer of oxygen and carbon dioxide in and out of the body when
they breathe is described as respiration. The respiratory system for this ancient species of
platypus must have adapted to their active lifestyle and unique feeding habits. Here we show
higher oxygen extraction efficiency and altered breathing patterns required by their respiratory
system. The platypus is a mammal and does not have the ability to breathe underwater, so they
frequently expose themselves above the surface of the water during their hunting sessions to get
oxygen from the air. During maximum metabolic rate, Ob. tharalkooschild experiences an
increased breathing rate compared to resting metabolic rate to provide the species with more
oxygen after exertion. The lungs anatomy model provides an example of oxygen delivery
between the lungs and blood that may have occurred in the species. The respiratory system is
vital for all living organisms because this release of energy provides fuel for growth. The
mechanistic function of the lungs moves fresh oxygen into the body, while removing carbon
dioxide and other waste gases out. During inhalation and exhalation, specific muscle movements
are required to expand the lungs and allow oxygen to diffuse across the surface, while relaxation
of muscle allows the lungs to decrease in volume and expel carbon dioxide. This process that
takes place within the platypus is not unique but also occurs within all other mammals.
Introduction:
The extinct toothed mammalian platypus, Obdurodon tharalkooschild, lived in the
Miocene era and primarily habitized the freshwater ponds and rivers along the eastern coast of
Australia (Pian et al., 2013). The modern species, Ornithorhynchus anatinus, share many similar
bodily functions with its ancient ancestor and it is possible that they processed air through
respiration the same. Respiration is needed to supply the body with needed oxygen to the cells at
a continuous rate able to sustain the species metabolic rate. Resting metabolic rate (RMR in
kJ/hr) is the required amount of energy needed to sustain body temperature during minimal
activity and resting. Maximum metabolic rate (MMR in kJ/hr) is energy expended by the
organism after high activity such as running and swimming. To provide the organism with the
required amount of energy needed for these activities, it needs respiration to provide oxygen in
the conversion and release of stored energy inside the organism.
The platypus is a semi aquatic mammal and often spends half of its day foraging in the
water (Grant et al, 1978). Due to aerobic metabolism, the platypus can only spend an average of
30 seconds underwater and must rise to the water surface to breathe. Due to its unique diving
pattern, its breathing pattern comprises a high inspiration and inspiratory pause before
submersion. These diving practices caused the modern platypus to have a higher rate of oxygen
consumption compared to other similar sized monotremes (Frappell, 2003).
Respiration is the process of moving oxygen in and through the body, while releasing
gaseous waste like carbon dioxide to produce energy needed to maintain the animals active
lifestyle. The respiratory system is a group of organs and tissues needed by an organism to help
in the movements of breathing. The platypus uses its lungs, the tissue that surrounds the lungs,
and diaphragm to efficiently move air in and out of the body. The lungs are responsible for the
gas exchange between the blood and lungs, while diaphragm contractions aid in air inhalation
and exhalation. The lungs of platypus are similar to humans, with two lungs on either side of the
sternum and multiple lobes making up each lung. Human lungs possess three lobes on the right
side and two on the left, while the platypus lung has two lobes on the right side and one lobe to
the left (Grant, 1989). The left side has one less lobe to provide space for the heart. The lungs
consist of a trachea with two main bronchi that travel into the lungs where they branch into
bronchial tubes and alveoli attach to the ends. The alveoli is responsible for the exchange of
oxygen and carbon dioxide in the lungs when air is being passed through the respiratory system.
The diaphragm is a thin skeletal muscle located at the base of the rib cage and separates the lungs
from the stomach and intestines. Its contractual movements create a vacuum-like pressure to pull
in air and is released when the diaphragm relaxes.
Because Ob. tharalkooschild needed to maintain a high oxygen level to be able to dive
during hunting, either oxygen extraction efficiency will be high or breathing patterns, like high
inspiration with an inspiratory pause, will be enforced. Ob. tharalkooschild is also an active
animal, meaning breathing rate during exercise such as running on land will need to be high to
accommodate oxygen demand.
Figure 1. Sketch of Obdurodon tharalkooschildΚ»s respiratory system with labeled majorcomponents.
Methods:
A respiratory model was first developed for Obdurodon tharalkooschild. Using this
species' calculated mass, 4,259 grams (Crane & Takabuki, 2021), different lung volumes (total
lung volume, tidal volume, and dead space volume) were calculated using allometric variables
and formula noted in Table 1 (Stahl, 1967). Alveolar ventilation volume was calculated by
subtracting the dead space volume from the tidal volume ( = - ).ππ΄
ππ‘
ππ·
Table 1. Allometric formulas and variables to calculate different lung volume types. x = lungvolume (mL), a = standard 1-kg mammal variable, M = mass of animal in kg, b = slope onlog-log graph (Stahl, 1967).
Lung Volume Types Allometric Formula with Variables( )π₯ = πππ
Lung Volume ( )ππ 53. 5 * π1.06
Tidal Volume ( )ππ‘ 7. 69 * π1.04
Dead Space Volume ( )ππ· 2. 76 * π0.96
The expired partial pressure of oxygen was calculated using the tidal volume, dead space
volume, alveolar ventilation volume, partial pressure of oxygen in fresh air, and the partial
pressure of oxygen in alveolar air. The value used for partial pressure of oxygen in fresh air is the
standard 21.1 kPa for atmospheric pressure at sea level (Withers, 1992). The value used for the
partial pressure of oxygen in alveolar air is 13.8 kPa, a standard alveolar air pressure for most
tidal lungs (Withers, 1992). Values were input into the equation below to compute :ππ2ππ₯π
ππ2ππ₯π
= (π
π·
ππ‘
)ππ2 ππππ β πππ
+ (π
π΄
ππ‘
)πππ2
Oxygen extraction efficiency was calculated using the following equation, where ππ2πππ
was 21.1 kPa for atmospheric pressure of oxygen at sea level and was 101 kPa forππππππππ‘πππ
total atmospheric pressure:
πΈ = 100 Γ ππ
2πππ β ππ
2ππ₯π
ππππππππ‘πππ
Oxygen consumption ( ) was first calculated at resting metabolic rate (RMR). For Ob.ππ2
tharalkooschild, RMR is 35.349 kJ/hr (Crane & Takabuki, 2021). The RMR was divided by the
conversion factor 20 kJ/L and again divided by a conversion factor of 60 min to obtain aπ2
ππ2
value in the units L /min. , or the volume of air flowing into the lungs, was calculated byπ2
ππΈ
rearranging the equation to be equal to , with having a value of 101 kPa inππ2
ππΈ
ππππππππ‘πππ
correspondence to total atmospheric pressure:
βππ2
=π
πΈ(ππ
2πππ βππ
2ππ₯π)
ππππππππ‘πππ
ππΈ
=ππ
2*π
πππππππ‘πππ
(ππ2πππ
βππ2ππ₯π
)
was then converted to mL air/min in preparation to calculate breathing rate (BR).ππΈ
ππΈ
was divided by tidal volume to get the final breathing rate value. These calculations were
repeated for and for maximum metabolic rate (MMR), which for Ob. tharalkooschild isππ2
ππΈ
117.83 kJ/hr (Crane & Takabuki, 2021).
Diffusing capacity of oxygen ( ) was computed first by using variables given inπ·πΏπ2
Stahl (1967) and Ob. tharalkooschild mass in an format. This value was divided byπ₯ = πππ
0.133322 kPa and 4.259 kg to convert to mL/min kPa kg. From this, was calculatedπ·πΏπ2
ππ2
for an oxygen flux model using the below equation, where is 2.7 kPa which is theπππ2
β πππ2
standard value for most vertebrates:
ππ2
= π·πΏπ2(πππ
2β πππ
2)
Results:
Mass specific lung volumes can be seen in Table 2. Based on Stahlβs (1967) allometric
equations, dead space volume for Ob. tharalkooschild is less than alveolar ventilation volume by
about 12 mL.
Table 2. Lung volume values for total lung volume, tidal volume, dead space volume, andalveolar ventilation volume in milliliters.
Lung Volume Types Volumes (mL)
Lung Volume ( )ππ
248.6
Tidal Volume ( )ππ‘
34.71
Dead Space Volume ( )ππ·
11.09
Alveolar Ventilation Volume ( )ππ΄
23.62
Expired partial pressure of oxygen was 16.13 kPa, leading to an oxygen extraction
efficiency of around 4.92%. This value is lower than a typical humanβs oxygen extraction
efficiency of about 5.6% (Altman & Katz, 1971), which is reasonable because of smaller lung
volumes and less alveoli volume to absorb oxygen.
At resting metabolic rate, Ob. tharalkooschild had a breathing rate of 17.27 breaths per
minute, white at maximum metabolic rate, breathing rate was 57.51 breaths per minute. There is
a 40 breath difference between RMR and MMR, meaning that with increasing exercise, Ob.
tharalkooschild increases breath rate and therefore increases air and oxygen intake. This
relationship is further highlighted in Figure 2.
Figure 2. Positive linear trend shown between metabolic rate (kJ/hr) and breathing rate (BPM).
The for the oxygen flux model was computed to give a value of 17.9 mL/min, whichππ2
is less than the required needed for Ob. tharalkooschild. This means that the lung anatomyππ2
will not supply the resting metabolic rate for this animal.
Discussion:
The respiratory system for Ob. tharalkooschild, is adaptive and changed to best maintain
their resting and active metabolic rates. The resting metabolic rate is the required energy needed
to maintain your body at near complete rest. During hunting, feeding, and walking, an organism
expends an increased amount of energy to move and during a high intensity activity, the maximal
rate of oxygen transport can be achieved. Maximal metabolic rate occurs in the animal when the
maximum rate of aerobic metabolism is reached and oxygen moved from environment to tissue
has reached their limit. To cope with this increased muscle movement and metabolic rate, an
increase in breathing rate is expected to improve oxygen intake and carbon dioxide expulsion.
The increased intake of oxygen helps with both oxygen depletion and regulating body
temperature. The modern platypus shows signs of respiratory adaptations by changes in heart
rate and increased oxygen carrying capacity (Johansen et al., 2008) during diving sessions. They
saw consistent arterial blood pressure during submission and increased heart rate and higher
oxygen carrying capacity after resurfacing from diving. To recover from a strenuous activity, the
animal had to increase its breathing pattern and hold more oxygen to cope with the loss.
The movement of air through the trachea and bronchi to the lungs is done through
ventilation. The air moves through these passageways due to pressure gradients created by the
contraction of the animals diaphragm. The inhalation movement of the diaphragm is done
through active muscle contraction, while the exhalation and relaxation of the lungs are passive
due to their elastic properties. The lung structure of the adult modern platypus was described as
βprimitiveβ mammalian lungs by Engel (1962), with acinar structure. The structure found further
suggests that the ancient species had a similar lung structure and anatomy to the modern
platypus.
The oxygen consumption of the ancient species is represented by the amount of oxygen
intake and absorption to sustain the body during exercise. Optimized aerobic respiration requires
improved Vo2 usage to supply the animals resting metabolic rate. The current lung anatomy
model for Ob. tharalkooschild would not have supplied for the resting metabolic rate. mayπ·πΏπ2
have needed to be increased to accommodate the active lifestyle of the ancient platypus. Since
Ob. tharalkooschild was about twice the size of the modern platypus (Crane & Takabuki, 2021),
there could have also been differences in lung anatomy such as additional lobes to improve
oxygen flux.
The ancient platypus, Obdurodon tharalkooschild, is an all-around unique animal. Apart
from fully aquatic mammals and seal and walrus phylogeny, the platypus is one of very few
mammalian species to have adapted their respiratory systems for diving strategies. Because of
the current speciesβ small size, they are not able to dive to deep depths, however, because
Obdurodon tharalkooschild was larger, perhaps they were able to propel themselves further
under the surface of the water using their larger lungs to store more oxygen while submerged.
Their specialized breathing patterns could have also been different, inhaling a higher volume of
air in preparation for diving.
Author Contributions:
Jadyn is the primary author and Cody is the secondary author. Jadyn provided the majority of the
calculations, methods, and results. Cody provided the majority of the abstract, introduction, and
discussion. Together, changes were made to each section to improve the quality of writing.
Appendix:
Mass = 4,259g = 4.259kgRMR = 35.349 kJ/hrMMR = 117.83 kJ/hr
Lung Volumes
Lung Volume (VT) = = 248.6 mL53. 5 π₯ 4. 259ππ1.06
Tidal Volume (Vt) = = 34.71 mL7. 69 π₯ 4. 259ππ1.04
Dead Space Volume (Vd) = = 11.09 mL2. 76 π₯ 4. 259ππ0.96
Alveolar Ventilation Volume (VA) β Vt = VA + VdVA = 34.71mL - 11.09mL = 23.62 mL
Expired Partial Pressure of Oxygen= 16.13kPaππ
2ππ₯π= ( 11.09ππΏ
34.71ππΏ )21. 1πππ + ( 23.62ππΏ34.71ππΏ )13. 8πππ
Oxygen Extraction Efficiency= 4.92%πΈ = 100 Γ 21.1 πππ β 16.13 πππ
101 πππ
AT RMR...= = 1.768 literO2/hr = 0.0295 LO2/minππ
235.349 ππ½/βπ20 ππ½/πππ‘πππ
2
β βππΈ
ππ2
=π
πΈ(ππ
2πππ βππ
2ππ₯π)
ππππππππ‘πππ
0. 0295πΏπ2/πππ =
ππΈ
(21.1πππβ16.13πππ)
101πππ
= = 0.5996 liters air/min = 599.6 mL air/minππΈ
0.0295πΏπ2/πππ
0.0492
Breathing Rate = = 17.27 breath/min599.6ππΏπ
2/πππ
34.71 ππΏ
AT MMRβ¦
= = 5.892 literO2/hr = 0.0982 LO2/minππ2
117.83 ππ½/βπ20 ππ½/πππ‘πππ
2
β βππΈ
ππ2
=π
πΈ(ππ
2πππ βππ
2ππ₯π)
ππππππππ‘πππ
0. 0982πΏπ2/πππ =
ππΈ
(21.1πππβ16.13πππ)
101πππ
= = 1.996 liters air/min = 1996 mL air/minππΈ
0.0982πΏπ2/πππ
0.0492
Breathing Rate = = 57.51 breath/min1996ππΏπ
2/πππ
34.71 ππΏ
Oxygen Flux
= 0.8845 mL/min mmHg x (1 mmHg / 0.133322 kPa) =π·πΏπ2
= 0. 16 π₯ 4. 2591.18
6.634 mL/min kPa / 4.259 kg = 1.558 mL/min kPa kg= (1.558 mL/min kPa kg)(4.259kg)(2.7kPa) = 17.9 mL/minππ
2= π·πΏπ
2(πππ
2β πππ
2)
17.9 mL/min < 29.5 mL/min
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