Fysiology: Bloodshift, the redistribution of the blood.

Written by Rik Rösken

Scope of the article:

Bloodshift is one of the most important factors that enables breath-hold divers to dive deeper. This article explores this phenomenon.

Introduction

One of the first questions that people ask when confronted with the deep dives of breath-hold divers is how it is possible. How is it it possible that the lungs do not collaps? And could the lungs not be damaged by the increasing pressure due to the increasing depth?

Redistribution of the blood

One of the key elements of the compensation in the lungs is that the bloodvessels in the lungs swell in size. This is quite small on a local scale, but all the bloodvessels taken together can displace over one liter volume of air or more. This makes a lot of differences, especially below 40 to 50 meters where the effect is starting to make itself known.

The bloodshift is not induced at once, but is facilitated by several different factors. One of them is known by all swimmers, the immersionreflex, where blood normally pooled in the lower extremities is pushed upward. Together with the factor of cold, it is this effect that make the swimmer want to pee after being in the water for some time. It also accounts for this effect in divers on compressed air or air mixtures.

A second, but equally important factor is the mammal dive reflex. This reflex, caused by the cold tempurature of the face when exposed to water, facilitates not only a slower heartbeat, but also vasoconstriction in arms and legs. This also leads to an increasing ammount of displaced blood.

A decreasing presure in the lungs, might also assist in the pull of the blood to the lungs, however there is not yet a clear answer to give if this is a important factor in the shift itself.

Blood in the lungs

One of the questions that often is asked, is if the bloodshift also causes bloodplasma and bloodcells to leak in the lungs themselves. Normally the capilary walls of the bloodvessels in the lungs wouldn't allow this. Coughing up blood after a dive is not a sign of bloodshift, it is a sign of a possible lungsqueeze or lung damage, and should taken seriously.

Due of the nature of the bloodshift and the risk on lungtrauma due to contractions, too fast increasing depth, or empty lung diving, it should be stated that diving on blood thinning medication, is not recommended.

Bloodshift as an factor in breath-holding

While the bloodshift does facilitates deeper diving as previously thought, people should be aware that it doesn't make people imune to lung underpressure trauma. The effect has its limits and a slow increase of depth, together with learning new equilisation techniques, seems to be the path to succes.

Used sources:

H.J. Corriol, La Plongée en apnée : Physiologie et médecine, 3e édition, uitgeverij Masson 2002.

P. Radermacher, C-M, Muth, Apnoetauchen - Physiology und Pathophysiologie, Deutche Zeitschrift Für Sportmedizin, Jahrgang 53, Nr. 6 (2002).