What is the relationship between pressure and gas volume?
and how does that affect freedivers?
The relationship between pressure and gas volume is described by Boyle's Law, which states that at constant temperature, the pressure of a given amount of gas is inversely proportional to its volume...
In mathematical terms, Boyle's Law can be expressed as:
P⋅V=constant
where:
- P is the pressure of the gas
- V is the volume of the gas
This means that if you have a sample of gas at a constant temperature, doubling the pressure will result in the volume being halved, and vice versa. In other words, when the volume of a gas decreases, its pressure increases, and when the volume increases, the pressure decreases, as long as the temperature remains constant.
It's important to note that Boyle's Law is applicable under the assumption of constant temperature. If the temperature changes, other gas laws like Charles' Law or the combined gas law may need to be considered.
Boyle's Law has implications for freedivers, who engage in breath-holding and underwater diving without the use of breathing apparatus. As freedivers descend underwater, the pressure increases, and this affects the volume of air in their lungs.
According to Boyle's Law, as the pressure increases, the volume of a gas (in this case, the air in the lungs) decreases. Conversely, as freedivers ascend to the surface, the pressure decreases, and the volume of air in their lungs expands.
Freedivers need to be aware of these changes in pressure because they can impact lung volume, buoyancy, and the risk of barotrauma. Barotrauma refers to physical damage that can occur when there is a pressure difference between the inside and outside of the body. In the context of freediving, barotrauma can affect the ears, sinuses, and lungs.
How do you equalize your ears while freediving?
Your ears have an air space in the middle ear between the ear drum
and the inner ear. As you descend, pressure compresses this air space,
causing discomfort as the ear drum bulges inward...
a. An air passage called the Eustachian tube connects your middle ear
to your throat. Equalize by pinching your nose and gently blowing
against your nostrils. This sends air from your throat into the middle
ear. You feel a popping sensation.
b. Begin equalizing immediately as you start to descend. Don’t wait
for discomfort. Equalize frequently and gently, when you feel small
pressure changes, before you feel discomfort.
c. If you have diffi culty equalizing, end the dive and evaluate your
ears, then try another dive if appropriate. It may take some practice
to equalize reliably. Never continue descending without equalizing.
Doing so is painful, and can cause a middle ear squeeze and/or
air eardrum rupture. Ear squeeze is probably the most common
freediving injury.
d. Ear plugs cause an airspace you can’t equalize, so do not use ear
plugs while freediving, unless they are special ear protectors made
specifi cally for diving.
e. Equalize gently. Never attempt a forceful and/or extended
equalization. A forceful, extended equalization can cause serious,
permanent injuries to your ears and hearing.
f. There are variations on equalizing your ears, which you’ll learn
about more in the PADI Advanced Freediver course.
Why shouldn’t you take a breath from scuba while freediving?
Lung overexpansion injury is primarily an issue for scuba divers. Scuba divers’ lungs are at full volume at depth. The air in them expands as they ascend. This isn’t a problem if they breathe normally. But, if they were to hold their breath and/or ascend very rapidly, expanding air can rupture the lungs, causing very serious, life-threatening injuries..., do not mix freediving and scuba Even if you’re a scuba diver, don’t take a breath from scuba at depth while freediving. Even if you remember to exhale, you’ll be ascending far faster than is appropriate for scuba divers. (Scuba procedures like the buoyant emergency ascent are also faster than normally appropriate, but they are for emergencies only), The only exception is if a scuba diver must help you in an emergency of some kind (this would be exceptionally rare). In that case, you should continue to share air and ascend together, making a proper scuba ascent breathing normally, Lung overexpansion injuries are very serious, but they are very easy to avoid. Many freedivers are also avid scuba divers.
What happens in your body when you hold your breath underwater?
When you hold your breath, your body responds, and it responds
differently in some respects when you’re in/underwater from when
you’re not..., During apnea, your body uses oxygen in your blood, muscles and lungs.
It also uses short term chemical processes that don’t require oxygen
(anaerobic). Carbon dioxide rises within your tissues, bloodstream and
lungs.
Eventually, you feel the urge to breathe, triggered primarily by
carbon dioxide. Typically, this causes:
• contractions of muscles involved with breathing.
• a burning or “bursting” sensation in lungs and throat.
• feeling like you need to swallow
If continued to the extreme, you begin to feel lightheaded or notice
visual disturbances (e.g. “graying out”), especially as you ascend
.If apnea continues, the diver loses consciousness. Underwater,
drowning may result unless rescued.
Mammalian diving reflex
The mammalian diving reflex is a set of physiological responses that occur in mammals, including humans, when they are submerged in water. This reflex is more pronounced in marine mammals, such as seals and whales, but it is also observed in humans, especially in infants. The primary purpose of the diving reflex is to conserve oxygen and prioritize the oxygen supply to vital organs when submerged...
How do I trigger the mammalian diving reflex (response)?
Water on your face and immersion coupled with apnea (holding your breath) triggers the mammalian diving refl ex. (Many physiologists call it the “mammalian diving response,” which is probably the more accurate term, but people say “reflex” more commonly.)
This response is common in all mammals to a larger or lesser degree. It is very strong in aquatic mammals, and much less so in humans. Colder temperatures seem to make the response faster and stronger. Some freedivers try to help prompt this response with relaxed no-mask snorkel breathing, eyes closed, for several minutes after entering the water. Once triggered, your heart rate slows, which helps conserve oxygen. With training, the heart rate may slow as much as 50 percent. Blood vessels in your extremities constrict. This reduces blood fl ow to them and causes blood to pool in the core.
• This conserves oxygen for your vital organs.
• Blood vessels around the lungs enlarge, which helps offset the
loss of gas volume, especially in deeper freedives.
• To some extent, this blood shift occurs with immersion even
when not holding your breath.
* Medical studies have found that the spleen releases red blood cells,
increasing breathhold capacity