Aves vs. Reptilia Lungs in Sauropoda

["Mike Milbocker" <mmilbocker@psdllc.com>]

The possible ratios between volumetric dead space and tidal volume in large
Sauropod respiratory systems is investigated. Conservative anatomical
estimates of Sauropod esophageal size, lung capacity, and reserve volume
yield a ratio of volumetric dead space to tidal volume of approximately 2. A
value over 1 for bi-directional respiration (reptilian) implies oxygen must
reach the lungs by diffusion since fresh air is not directly inspired into
the lungs. A uni-directional (aves) respiratory system by definition
possesses no dead space.

Respiratory Systems

The familiar reptilian/mammalian respiratory system relies on a single
pathway, the esophagus, to draw oxygen into the lungs and expel carbon
dioxide. During inspiration the volume of the esophagus as well as the finer
pathways leading to the alveoli fill with fresh air. During expiration this
air is expired without participation in blood oxygenation. The volume of the
air expired that does not participate in direct oxygenation of blood is
called dead space. Therefore, the inspired volume must exceed the dead space
in order to directly oxygenate blood.

The avian lung, in contrast, passes air continuously through the lung in one
direction. The entry and exit orifices are distinct and the special air sacs
permit only uni-directional flow.

The calculations that follow assume a reptilian/mammalian respiratory
system.

Definitions

The reptilian lung is neither fully collapsed during exspiration, nor fully
inflated during inspiration, thus the lung capacity LC is given by

LC = RV + IRV + Vt

Where IRV is the maximal inspired volume from end-tidal inspiration
(Inspiratory Reserve Volume), Vt is the volume inspired and expired with
each normal breath (Tidal Volume), and RV is the volume remaining in the
lungs after maximum expiration (Expiratory Residual Volume).

The maximum volume displaced by the lung in the chest (LV) is given by

LV = LC + D + T

Where D is the dead space excluding the volume of the esophagus and V is the
volume of tissue forming the passageways and outer structure of the lungs.

For the purposes of discussion here, we set LV = LC, which is a conservative
estimate on the required chest volume for housing the lungs. This puts a
lower bound on the ratio of dead space to tidal volume.

The canonical large Sauropod is given a 10 m long neck (nostrils to lung)
and 50,000 kg weight.

The Chest Volume Occupied by the Sauropod Lung

For high metabolism individuals the following apply:

Vt = 3-7 ml/kg, Vt/RV = 0.28, RV/LC = 0.34

Then it follows for a 50,000 kg Sauropod,

Vt = 250 L using a conversion factor of 5 ml/kg
RV = 893 L
LC = 2626 L

Assuming a spherical geometry for the lungs, and excluding the volume
occupied by lung tissue and dead space the minimum radius of the spherical
lung is 85 cm (34 inch) or a diameter of 5?7?.

The Sauropod Respiratory Dead Space

>From Brachiosaurus reconstructions the ratio of the length of the neck to
the diameter of the neck is 6. Using this and Rneck/r = 5, where Rneck is
the radius of the neck and r is the radius of the esophageal lumen, then

Rneck = 83.4 cm, r = 16.7 cm.

The esophageal volume DS is

DS = ½ pi r^^2 L

Where L is the neck length (10m).

Neglecting the dead space of the pathways leading from the esophagus to the
aveoli, the dead space (=DS) is 437 L.

Result and Conclusion

Note that the conservative dead space is 437 L and the tital volume (given
above) is 250 L. The maximum additional inspired volume (IRV) is 1483 L.

Normal breathing is ineffective since the dead space is greater than the
tital volume by nearly a factor of 2. At maximum inspiration the ratio is
437/1733 or 0.25. The ratio for humans is 0.3 for normal breathing.

This treatment suggests either the Sauropod respiratory system was
avian-like or deep breathing. More thorough study of the atypical
positioning of the nostrils may shed light on this subject.