Why is my Dive so Much Easier on the Ascent?
I first came across this question from by a very organised, very talented level 2 student (German, naturally) who not only wanted to know exactly why he felt much better on the ascent, but also had a few theories of his own. This got me thinking. Before him, lots of my furry little beginners had commented that as soon as they turned at the bottom and started to ascend, almost all their urge to breathe immediately disappeared. I’d noticed it myself, too, as a beginner, and later as an instructor with more CO2 than oxygen in my blood on my 60th dive of the day with a 40 second surface interval. (Thanks Freedive Gili) What a blessed relief to turn for the surface then! To begin with I just dismissed it - like most of us do - and told myself -and them- that it was all in our heads. “Well it’s obvious isn’t it? You’re on the way up!” But is that the whole story?
Psychological Components of the Urge to Breathe
Clearly it’s true that, especially for those of us who aren’t part fish, descending into the abyss on a single breath of air can be daunting. During this phase of the dive there are all kinds of triggers which say “hold on, what are you trying to do here?”:
- Pressure increases
- Light levels decrease
- Equalisation becomes more difficult
- Breath-hold time becomes longer
- The distance from the surface increases
All these things, regardless of the actual levels of CO2 and O2 which “really” trigger the urge to breathe can result in us feeling a sense of impending doom, which can translate into an urge to breathe, complete with gut-splitting contractions. Conversely, on the way up, those stimuli are gradually removed. We get all kinds different signals that tell us we’re heading in the ‘right’ direction. Foolish mind.
Physical Components of the Urge to Breathe
Another thing that characterises the difference between the ascent and descent on a normal dive is that on the descent we’re usually in a head down position, and on the ascent the reverse is usually true. This does some funny things to our anatomy:
- The blood vessels in the head are congested by gravity and decreased cardiac output which goes along with the dive reflex. This means that the vessels in the neck and brainstem which contain the chemical receptors for CO2 are stuffed full of relatively old, relatively stagnant blood which may cause them to overestimate the CO2 content of the blood as a whole.
- The air in the lungs which is usually only slightly over 1atm of pressure is compressed to at least double that by a decrease in size of the lungs. This is presumably very stimulating for the pulmonary stretch receptors which also contribute to the urge to breathe. The body is quite sensitive to changes in physiological quantities, rather than the absolute value and so might be more sensitive to increasing pressure in the lungs, rather than simply a high value.
- Like a plastic bag full of air that you immerse in water, the air in the lungs will naturally want to rise because (despite the fact that its pressure is equal to the water pressure for most dives) its density is much less. (This is another illustration of Archimedes’ principle). When you’re inverted in the descent, the air is trying to press upward against the diaphragm which again, is presumably very stimulating.
These factors, I think, all go some way towards explaining that feeling of relief which comes from turning head up at the bottom of a dive. Try doing your warmup hang upside down. I bet that in this situation where the psychological and physiological factors remain pretty much the same, the head up position still gives you less of an urge to breathe.
*Remember, only do this with a trained and competent buddy.*
Physiological Components of the Urge to Breathe
This is the bit where things seem to come unstuck. So far we’ve seen how psychological and physical factors all contribute to this feeling of an easier ascent. Physiology never lies though, does it? We “know” that the urge to breathe “really” comes from increasing partial pressure of CO2 in the blood and, sometimes, from decreasing partial pressure of O2. These changes happen continuously, incrementally, throughout the dive and should therefore result in a greater urge to breathe at the end, right?
Here’s the thing: We tend to forget that increasing depth (and therefore increasing pressure, Boyle’s law etc), as well as affecting Oxygen and causing shallow water blackout, also affect the partial pressure of CO2. The urge to breathe is triggered at an arterial partial pressure of CO2 (PaCO2) of around 6kPa. After a breathe-up, we probably start at around 3 or 4kPa.
As we descend, the PaCO2 increases, not just because we’re not releasing any, and we’re generating more of it by using our muscles, but also because of the increasing ambient pressure. (If you’re struggling with this concept, have a look at our physiology bootcamp article on partial pressure). On the ascent, particularly on shallower dives where the incremental change in pressure is greatest, this phenomenon is reversed. Difficult to study but you can imagine that the PaCO2 during a dive to 20m might look something like this:
Food for thought or what?!
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