Bradycardia, a reduction in the heart rate (of up to 50% in humans).
Peripheral Vasoconstriction, a decrease in blood flow to the extremities, in order to increase the supply of blood and oxygen to the vital organs, especially the brain.
Blood shift, the shifting of blood to the thoracic cavity, i.e. the chest between the diaphragm and the neck to avoid the collapse of the lungs under higher pressure during deeper dives.
Thus, both a conscious and an unconscious person can survive longer without oxygen underwater then in a comparable situation on dry land.
Pressure
The underwater environment has very different physical and chemical characteristics to the world in which we live above the surface. Water is denser then air and acts on the organism to produce modifications that are of great consequence to the freediver. The aspect that requires the most attention is pressure. Each dive exposes the body to variations of pressure proportional to depth. These variations in pressure require specific behavioral strategies. Physics teaches us the liquids are practically incompressible while gases are compressible. Water constitutes about 70% of our body mass. The remaining 30% is either solid (also incompressible) or spaces containing gas, which are subjected to the same pressure variations as those that affect us during our diving. This explains why when we immerse the 'empty' spaces of our body and equipment they are subject to a squeeze. Hence the ears, lungs and mask must all be compensated (see Equalization).
Bradycardia, a reduction in the heart rate (of up to 50% in humans).
Peripheral Vasoconstriction, a decrease in blood flow to the extremities, in order to increase the supply of blood and oxygen to the vital organs, especially the brain.
Blood shift, the shifting of blood to the thoracic cavity, i.e. the chest between the diaphragm and the neck to avoid the collapse of the lungs under higher pressure during deeper dives.
Thus, both a conscious and an unconscious person can survive longer without oxygen underwater then in a comparable situation on dry land.
Pressure
The underwater environment has very different physical and chemical characteristics to the world in which we live above the surface. Water is denser then air and acts on the organism to produce modifications that are of great consequence to the freediver. The aspect that requires the most attention is pressure. Each dive exposes the body to variations of pressure proportional to depth. These variations in pressure require specific behavioral strategies. Physics teaches us the liquids are practically incompressible while gases are compressible. Water constitutes about 70% of our body mass. The remaining 30% is either solid (also incompressible) or spaces containing gas, which are subjected to the same pressure variations as those that affect us during our diving. This explains why when we immerse the 'empty' spaces of our body and equipment they are subject to a squeeze. Hence the ears, lungs and mask must all be compensated (see Equalization).
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