Depth Limits Of Breath Hold Diving (An Example Of Fennology)
Authors: Albert B. Craig, Jr.
DOI / Source: https://doi.org/10.1016/0034-5687(68)90073-x
Date: June 1968
Reading level: Intermediate
Why This Matters for Freedivers
This paper is one of the classic scientific arguments for blood shift as a protective mechanism that helps humans dive deeper than simple RV/TLC math would predict. It also hints at a practical safety lesson: the deeper you go, the more you rely on complex fluid shifts and lung mechanics — which is why depth progression should be gradual, technique-driven, and respectful of squeeze risk rather than based on “theoretical limits.”
Synopsis
For a long time, physiology textbooks said there was a hard depth limit for breath-hold diving: once your lungs are compressed down to your residual volume (RV), you shouldn’t be able to go deeper without creating a dangerous pressure difference across the chest — the classic “thoracic squeeze.” The problem is that real divers kept breaking those predicted limits. This 1968 paper is a great example of scientists going back to basics and asking: what mechanism are we missing? 
Craig explains the standard theory (RV compared to total lung capacity, plus other “non-compressible” air spaces like mask and sinuses) and points out an embarrassment: by the 1960s, record dives implied lung volumes far below what should be “safe” if the chest simply hit RV and stopped compressing (he lists records down to the mid-60 meter range, where lung gas would be around ~13–14% of surface volume). 
To test the idea directly, he did a clever experiment: start the dive after a maximal exhale (so you’re already at RV) and see what happens to chest pressure during descent. He measured esophageal pressure (a proxy for internal chest pressure) and compared it to ambient pressure using an external balloon sensor. The key observation was that as the subject descended to about 4.75 m starting at RV, the pressure difference between inside and outside the chest did not increase with depth (it stayed essentially “balanced”). That should be impossible if the lung gas couldn’t compress further. The math implied the lung gas volume had compressed from ~2.0 L to ~1.4 L — about 600 mL below RV. 
So what compressed it? Craig’s main proposal is the now-famous idea of blood shift: as you go deeper, blood is pulled from the periphery toward the chest, “filling space” and allowing the lungs to compress further without catastrophic pressure gradients. He extends the logic to deeper depths and even discusses diving mammals, suggesting a mobile blood reservoir could help explain their extreme dives too. In short: the depth limit isn’t just lung volume ratios — it’s also the body’s ability to move fluid to protect and support the chest at depth.
Abstract
It is generally accepted that the depth to which a breath hold diver can descend is determined by the ratio of the RV to the TLC. If the diver descends farther, it is predicted that he would develop a “thoracic squeeze” as the intrathoracic pressure became less than the ambient pressure. The fact that divers have gone to depths at which the lung volume must have been less than 20 % of the surface volume suggests that some mechanism other than a decrease of the thoracic cage to RV must occur. The subject in the present study started each dive after expiring maximally. He was able to go as deep as 4.75 m without the development of a significant difference between the esophageal pressure and the ambient pressure at depth, both measured and recorded directly. These experiments indicated that the gas volume must have been compressed from the subject’s RV of 2.0 litres to 1.4 litre. It is suggested that this change of 600 cc could be due to a shift of blood from the peripheral to the cen- tral circulation. This additional mechanism of gas compression would help explain man’s demon- strated ability to dive to 65m.