Thoracic Collapse as Affected by the Retia Thoracica in the Dolphin
Authors: Clifford A. Hui
DOI / Source: https://doi.org/10.1016/0034-5687(75)90051-1
Date: 18 June 1975
Reading level: Intermediate
Why This Matters for Freedivers
This paper helps explain one of the “secret weapons” of deep-diving mammals: their chest doesn’t collapse randomly—its shape is guided by anatomy and by blood-filled structures that act like internal padding. For freedivers, it’s a useful mental model for why “blood shift” and chest mechanics matter at depth, and why deep diving is about managing where things go inside the thorax, not just squeezing air.
Synopsis
When a marine mammal dives, pressure squeezes the air in the lungs smaller and smaller. That missing space has to be taken up by something—mainly the chest wall moving inward and the abdominal organs shifting. But cetaceans (dolphins and whales) have extra plumbing: dense networks of arteries called retia thoracica (thoracic “rete mirabile”) sitting along the dorsal (back) side of the chest near the spine. The big question is: what do these retia actually do during a dive?
This study tests a very specific mechanical idea: if the thoracic retia engorge with blood during a dive, they might stiffen the dorsal chest and direct the collapse toward the ventral side, reducing how much the abdominal organs get shoved forward and stressed.
Because you can’t exactly ask a dolphin to hold still while you measure its chest at 7 atmospheres, the author used a controlled hyperbaric experiment on a dolphin carcass. A subadult female Delphinus was placed on its back in a recompression chamber and subjected to simulated dives to about 69.7 m (7 atm). The airway was secured so air couldn’t escape, keeping the lungs “full” in the way dolphins typically start a dive. The team measured several chest dimensions (circumference, dorsal–ventral thickness, and widths at two positions across the thorax) before, during, and after pressure exposure.
Then the key comparison: they repeated the same “dive,” but injected 100 mL of water into each pleural cavity (200 mL total). This was a clever hack to simulate what an engorged rete would do: occupy space in the same region (dorsal, near the vertebrae) and resist collapse there.
The results showed that pressure alone caused obvious chest compression, but the pattern changed when the pleural cavity contained that extra fluid. With the simulated “rete volume” present, overall collapse was greater and—most importantly—the ventral region compressed exceptionally, producing a more exaggerated “egg shape” (broad dorsally, narrow ventrally). In other words: adding volume in the dorsal region didn’t just “fill space,” it seemed to change the geometry of collapse so the chest narrowed more ventrally under pressure.
The author ties this to a simple volume argument using Boyle’s law: at 7 atmospheres, lung gas volume would be compressed to a small fraction of its surface value, meaning a very large percentage of the original chest volume must be temporarily replaced by tissue displacement. If a blood-filled structure already occupies part of that volume in the dorsal thorax, there is less “empty space” that needs to be created by pushing organs around. So the rete may act like a built-in, pressure-dependent support and spacer—guiding collapse and reducing displacement stress on internal organs during deep dives.
In short, this is a mechanical paper with a neat conclusion: the thoracic rete isn’t just a weird anatomical curiosity—it may help shape how the dolphin chest collapses under pressure, making deep diving more physically manageable.
Abstract
A subadult female dolphin carcass was placed in a hyperbaric chamber and exposed to two simulated dives to the equivalent of 69.7 m. In one dive the thorax was unaltered; in the other, 100 mL of water was injected into each pleural cavity to simulate the space-occupying effect of an engorged thoracic rete. Multiple thoracic dimensions were measured before, during, and after each exposure. The results indicate that when fluid occupies the same dorsal location as the retia thoracica, the pattern of thoracic collapse under diving pressures is altered, with greater overall collapse and pronounced ventral compression. The engorged thoracic rete may therefore influence the degree and pattern of collapse and reduce displacement stress on abdominal organs during exposure to high hydrostatic pressures.