Pulmonary and Circulatory Adjustments Determining the Limits of Depths in Breathhold Diving
Authors: Karl E. Schaefer, Robert D. Allison, James H. Dougherty Jr., Charles R. Carey, Roger Walker, Frank Yost, Donald Parker
DOI / Source: https://doi.org/10.1126/science.162.3857.1020
Date: 29 November 1968
Reading level: Intermediate
Why This Matters for Freedivers
This paper is one of the classic “proof points” that deep freediving is not limited by simple lung-volume math. It shows direct evidence that, as pressure increases, blood is pushed into the chest (“blood shift”), effectively protecting the lungs and letting divers go deeper than you’d predict from total lung capacity and residual volume alone.
Synopsis
In the 1960s, record breath-hold dives were already passing 200 ft (60 m), and that created a big problem for the standard textbook prediction: that depth limit should happen when your total lung volume compresses down to your residual volume (the air you can’t blow out). If that were the whole story, the deepest divers shouldn’t be able to exceed the depth predicted by their total lung capacity / residual volume ratio.
The authors studied Robert Croft, a U.S. Navy diver and record-holder, in tank dives and open-sea dives down to around 217–225 ft. Croft had unusually large lungs and a small residual volume, which already gave him a deeper “predicted limit” than average — but he still went deeper than his prediction. That’s where the key mechanism comes in: at depth, the body doesn’t just compress air; it also re-arranges blood.
To test this, the researchers used an impedance plethysmograph (a method that detects changes in electrical resistance across the chest). Because blood conducts electricity better than air, changes in resistance can be used to estimate changes in “conductive volume” in the thorax. By calibrating how resistance changes with known lung volumes, they separated the effect of air compression from changes due to blood moving into the chest.
They found substantial shifts of blood into the thorax during breath-hold dives, especially at deeper depths. At around 90 ft they estimated roughly about a liter of blood moved into the chest, and at 130 ft they still saw a very large shift. This supports the idea that blood can “replace” some of the compressed lung volume, reducing the effective residual volume and allowing deeper dives than simple Boyle’s-law lung-volume predictions.
They also collected end-of-dive alveolar gas samples after rapid ascents from different depths. In these dives, Croft’s end-dive alveolar oxygen and carbon dioxide levels were surprisingly consistent across depths and did not look like a classic “CO₂ limit” or a catastrophic “O₂ crash” limit under those specific conditions. The authors interpret this to mean that, for Croft in these dives, neither hypoxia nor hypercapnia alone explained the depth boundary — and that mechanical/circulatory adjustments (including blood shift) are central to understanding extreme depth capability.
Overall, this is an early cornerstone paper for modern deep freediving physiology: it connects real-world record diving to measurable changes inside the chest, and it shows why the body at depth becomes a moving target rather than a simple balloon being squeezed.
Abstract
Data from controlled tank dives and open-sea breath-hold dives in an elite diver were used to examine what limits depth. End-of-dive alveolar oxygen and carbon dioxide tensions were measured after dives to progressively greater depths, and thoracic conductive-volume changes were tracked using impedance plethysmography at multiple depths. The results showed substantial movement of blood into the thorax during deeper dives (“blood shift”), helping explain how divers can exceed depth limits predicted by lung-volume compression alone, while end-dive alveolar gas tensions did not indicate hypoxia or hypercapnia as the primary limiting factors under the conditions studied.