In the first article the mechanics of gas exchange and gas composition were examined. In this article the focus is upon the interdependence of the various adaptations that occur during a deep breath-hold dive together with the energetics of deep breath-hold diving.
Chest walls. During deep freediving, thoracic gas volume (VTG) is gradually reduced by compression as ambient pressure increases. Since we, as terrestrial mammals, have stiff chest walls, we can dive very deep without experiencing any significant deformation, and as VTG decreases, the outward recoil of the chest wall increases. The net effect of this increased pressure is for the pressure inside the chest to become progressively lower in relation to the pressure exerted on the rest of the body. This adaptation forces blood into the chest from the extrathoracic reservoirs at the same time impeding ejection of blood from the left side of the heart, the end result manifesting itself as ‘thoracic squeeze’ which may have serious consequences such as pulmonary edema and cardiac arrhythmia. While this condition is unlikely to present itself during dives to depths of 150ft or less, freedivers attempting deeper dives of 250ft of more may experience bleeding into the lungs and possibly even broken ribs. The way deep-diving freedivers overcome this potentially dangerous situation is to ensure that their chest walls are as compliant as possible by integrating specific stretching exercises into their training schedule. During a typical training week, elite freedivers will usually incorporate stretching every other day, a standard session involving three sets (10 packs followed by 20 and then 40 packs) of negative diaphragm packs followed by a series of reverse packs. Because of the nature of these exercises, which essentially involve trying to suck the stomach in as far as possible they are usually performed on an empty stomach!
"Armored" airways. Fortunately for freedivers, our airways consist of cartilage rings that prevent them from closing, thereby allowing the lung to empty completely. This feature of the airways being resistant to collapse serve the freediver in a number of ways, one being that their non-compliance ensures they maintain a volume large enough to accept gas compressed into them from other air-spaces at depth. Of equal importance is the function of these "armored" airways during the ascent phase of a very deep dive, their inherent stiffness permitting high expiratory flows to be achieved at very low lung volumes.
Energy expenditure The previous article examined gas exchange and composition during a deep breath-hold dive, both elements being of obvious importance to the freediver since the oxygen consumed and the carbon dioxide produced is important in determining the time of a breath-hold and the depth of a dive. In the elite, typical values of the overall amount of oxygen taken up by the lungs are between 300 and 400ml.min, a value that is only slightly higher than resting oxygen uptake. Because of the problems associated with measuring metabolic rates in freedivers, the data concerning the energetics of breath-hold diving are largely based on estimates derived from normal energy expenditure and assumptions based on tissue oxygen stores during exercise. What is known, based on these scientific ‘guesses’ is that energy expenditure is low during freediving and that, despite the oxygen storage capacity of diving mammals, there is insufficient oxygen to last the length of the dive, implying therefore that some energy is derived from anaerobic sources. As freedivers dive deeper, they become increasingly reliant upon this anaerobic energy source, an observation that provides indirect support for a mammalian diving reflex/response that results in energy being produced through other (glycolytic) pathway and redirection of blood. This ‘mammalian diving reflex’ will be the subject of the third article in this series.