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 tharalkooschildʻs 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 ( = - ).𝑉𝐴

𝑉𝑡

𝑉𝐷

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( )𝑥 = 𝑎𝑀𝑏

Lung Volume ( )𝑉𝑇 53. 5 * 𝑀1.06

Tidal Volume ( )𝑉𝑡 7. 69 * 𝑀1.04

Dead Space Volume ( )𝑉𝐷 2. 76 * 𝑀0.96

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 :𝑝𝑂2𝑒𝑥𝑝

𝑝𝑂2𝑒𝑥𝑝

= (𝑉

𝐷

𝑉𝑡

)𝑝𝑂2 𝑓𝑟𝑒𝑠ℎ 𝑎𝑖𝑟

+ (𝑉

𝐴

𝑉𝑡

)𝑝𝑎𝑂2

Oxygen extraction efficiency was calculated using the following equation, where 𝑝𝑂2𝑖𝑛𝑠

was 21.1 kPa for atmospheric pressure of oxygen at sea level and was 101 kPa for𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

total atmospheric pressure:

𝐸 = 100 × 𝑝𝑂

2𝑖𝑛𝑠 − 𝑝𝑂

2𝑒𝑥𝑝

𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

Oxygen consumption ( ) was first calculated at resting metabolic rate (RMR). For Ob.𝑉𝑂2

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 a𝑂2

𝑉𝑂2

value in the units L /min. , or the volume of air flowing into the lungs, was calculated by𝑂2

𝑉𝐸

rearranging the equation to be equal to , with having a value of 101 kPa in𝑉𝑂2

𝑉𝐸

𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

correspondence to total atmospheric pressure:

→𝑉𝑂2

=𝑉

𝐸(𝑝𝑂

2𝑖𝑛𝑠−𝑝𝑂

2𝑒𝑥𝑝)

𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

𝑉𝐸

=𝑉𝑂

2*𝑃

𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

(𝑝𝑂2𝑖𝑛𝑠

−𝑝𝑂2𝑒𝑥𝑝

)

was then converted to mL air/min in preparation to calculate breathing rate (BR).𝑉𝐸

𝑉𝐸

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 is𝑉𝑂2

𝑉𝐸

117.83 kJ/hr (Crane & Takabuki, 2021).

Diffusing capacity of oxygen ( ) was computed first by using variables given in𝐷𝐿𝑂2

Stahl (1967) and Ob. tharalkooschild mass in an format. This value was divided by𝑥 = 𝑎𝑀𝑏

0.133322 kPa and 4.259 kg to convert to mL/min kPa kg. From this, was calculated𝐷𝐿𝑂2

𝑉𝑂2

for an oxygen flux model using the below equation, where is 2.7 kPa which is the𝑝𝑎𝑂2

− 𝑝𝑐𝑂2

standard value for most vertebrates:

𝑉𝑂2

= 𝐷𝐿𝑂2(𝑝𝑎𝑂

2− 𝑝𝑐𝑂

2)

Results:

Mass specific lung volumes can be seen in Table 2. Based on Stahl’s (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 ( )𝑉𝑇

248.6

Tidal Volume ( )𝑉𝑡

34.71

Dead Space Volume ( )𝑉𝐷

11.09

Alveolar Ventilation Volume ( )𝑉𝐴

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 human’s 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, which𝑉𝑂2

is less than the required needed for Ob. tharalkooschild. This means that the lung anatomy𝑉𝑂2

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

“primitive” 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. may𝐷𝐿𝑂2

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 species’ 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 𝑥 4. 259𝑘𝑔1.06

Tidal Volume (Vt) = = 34.71 mL7. 69 𝑥 4. 259𝑘𝑔1.04

Dead Space Volume (Vd) = = 11.09 mL2. 76 𝑥 4. 259𝑘𝑔0.96

Alveolar Ventilation Volume (VA) → Vt = VA + VdVA = 34.71mL - 11.09mL = 23.62 mL

Expired Partial Pressure of Oxygen= 16.13kPa𝑝𝑂

2𝑒𝑥𝑝= ( 11.09𝑚𝐿

34.71𝑚𝐿 )21. 1𝑘𝑃𝑎 + ( 23.62𝑚𝐿34.71𝑚𝐿 )13. 8𝑘𝑃𝑎

Oxygen Extraction Efficiency= 4.92%𝐸 = 100 × 21.1 𝑘𝑃𝑎 − 16.13 𝑘𝑃𝑎

101 𝑘𝑃𝑎

AT RMR...= = 1.768 literO2/hr = 0.0295 LO2/min𝑉𝑂

235.349 𝑘𝐽/ℎ𝑟20 𝑘𝐽/𝑙𝑖𝑡𝑒𝑟𝑂

2

→ →𝑉𝐸

𝑉𝑂2

=𝑉

𝐸(𝑝𝑂

2𝑖𝑛𝑠−𝑝𝑂

2𝑒𝑥𝑝)

𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

0. 0295𝐿𝑂2/𝑚𝑖𝑛 =

𝑉𝐸

(21.1𝑘𝑃𝑎−16.13𝑘𝑃𝑎)

101𝑘𝑃𝑎

= = 0.5996 liters air/min = 599.6 mL air/min𝑉𝐸

0.0295𝐿𝑂2/𝑚𝑖𝑛

0.0492

Breathing Rate = = 17.27 breath/min599.6𝑚𝐿𝑂

2/𝑚𝑖𝑛

34.71 𝑚𝐿

AT MMR…

= = 5.892 literO2/hr = 0.0982 LO2/min𝑉𝑂2

117.83 𝑘𝐽/ℎ𝑟20 𝑘𝐽/𝑙𝑖𝑡𝑒𝑟𝑂

2

→ →𝑉𝐸

𝑉𝑂2

=𝑉

𝐸(𝑝𝑂

2𝑖𝑛𝑠−𝑝𝑂

2𝑒𝑥𝑝)

𝑃𝑏𝑎𝑟𝑜𝑚𝑒𝑡𝑟𝑖𝑐

0. 0982𝐿𝑂2/𝑚𝑖𝑛 =

𝑉𝐸

(21.1𝑘𝑃𝑎−16.13𝑘𝑃𝑎)

101𝑘𝑃𝑎

= = 1.996 liters air/min = 1996 mL air/min𝑉𝐸

0.0982𝐿𝑂2/𝑚𝑖𝑛

0.0492

Breathing Rate = = 57.51 breath/min1996𝑚𝐿𝑂

2/𝑚𝑖𝑛

34.71 𝑚𝐿

Oxygen Flux

= 0.8845 mL/min mmHg x (1 mmHg / 0.133322 kPa) =𝐷𝐿𝑂2

= 0. 16 𝑥 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/min𝑉𝑂

2= 𝐷𝐿𝑂

2(𝑝𝑎𝑂

2− 𝑝𝑐𝑂

2)

17.9 mL/min < 29.5 mL/min

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