Alix, I have read the essay. First impression, excellent work! Keep up studying, you are on the right track.
A few typos I noticed are:
Bracardia instead of Bradycardia
Yorgos Haggi Statti instead of Giorgos Hadjistathi (I suppose because I am Greek)
You wrote "freediving is practiced by more than 20,000 people globally ranging from snorkelling, spear-fishing and worldwide competitions". I would assume that this number is higher. Only in Cyprus and Greece I estimate the number of people who practice it is over that number!
It would also be nice to explain the TLC:RV ratio with regards to the theoretical depth limits of free diving. This figure gives the maximum ambient pressure one may dive to without risking lung collapse. Until recently it was believed to be around 4 ATA. Total Lung Capacity / Residual Volume = 6 Litres / 1,5 Litres = 4 ATA (30 meters). Until the 70s most diving records were compatible with this concept. However, there are new findings:
Blood shift and Lung Packing may increase this ratio. For example in Pipin Ferreiras the total lung capacity has been documented to be 9,6 L. Blood shift increases the blood by 1,5 L to 2 L in the thoracic area thus RV decreases by an other let's say 1,5 L. With the example of Pipin Ferreiras the theoretic depth would be 127 msw (9,6 / 0,7 = 13,7 ATA). Lung packing has been documented to increase TLC by as much as 50%. Pipin with such increase could easily reach the theoretic depth of 156 msw. It is commonly accepted among the physiologists that neither for the redistribution of blood into the thorax nor for the hyperinflation of the Lung with Lung packing the absolute upper limits are known.
I am not quite sure about the filling of the alveolar space with plasma. According to my sources it is the thoracic vessels that are holding the blood allowing further reduction of the alveolar space without risking a collapse. When this happens (fluid in the alveolar space) it is actually a complication. If a critical depth is exceeded the vessels may rupture from overdistension or plasma will escape into the alveolar space causing intra-alveolar edema (either not good cases as they interfere with normal gas exchnage and may lead to Respiratory Insufficiency). BTW sperm whales can dive to 3000 msw with maximum dive times of approximately 120 minutes. They do not have a sternum which is main difference with humans. The lungs of these animals can collapse completely.
You may also like to add that competitive apnea divers train the elasticity of their chest wall and their diaphragm with special exercises to achieve an optimum TLC:RV ratio. Chest wall elasticity may allow further reduction of RV at deep expiration.
Nice work, again!
