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1.1 Blood
Blood is a tissue that circulates in a closed circuit known as the circulatory system. A person that weighs 70 kg has 5 to 6 liters of blood which represents about 10% of the total body weight.
Blood is made of:
• A liquid:
o blood plasma
• Solids:
o red and white blood cells
o platelets

The red blood cells are shaped like biconcave disks and number between 4 to 5 million per cubic millimeter. A red blood cell contains 70% water and 30% hemoglobin. The key property of hemoglobin is to bind with gas into an unstable combination. Oxygen (O) is taken at the level of the lungs through a process called oxyhemoglobin and later released in the different tissues of the body. At this point, the hemoglobin charges itself with carbon dioxide (CO2) through a process called carboxyhemoglobin. The carboxyhemoglobin will go to the lungs and release the carbon dioxide in the process of re-oxygenation.
The blood is a lighter shade of red if it contains more oxyhemoglobin and is a darker shade of red if it contains more carboxyhemoglobin.
The hemoglobin can also combine with other gases such as carbon monoxide (CO) to create a very stable combination which can lead to asphyxia since the hemoglobin can no longer carry oxygen. This is why it is critical to protect the air intake for the compressor from exhaust gases that come from cars, boats, and the compressor itself.

1.2 Boyle-Mariotte Law
1.2.1 Experimental test

At 1 bar of pressure, the volume under the piston is 1 liter
At 2 bars of pressure, the volume under the piston is 0.5 liters
At 3 bars of pressure, the volume under the piston is 0.33 liters
At 4 bars of pressure, the volume under the piston is 0.25 liters
1.2.2 Law
At constant temperature, the variation of the volume, V, of a gas is inversely proportional to the absolute pressure
p1 x V1 = p2 x V2
where either p1 x V1 or p2 x V2 is held constant
1.2.3 Application to diving

A balloon which has a volume of 8 liters at the surface will have a volume:
4 liters at -10 meters since absolute pressure is 2 bars
2 liters at -30 meters since absolute pressure is 4 bars
1 liter at -70 meters since absolute pressure is now 8 bars

3 important applications are worth mentioning.

1.2.3.1 Lungs Volume
The lungs contain 6 liters of air at the surface. The diver receives air pressurized to the surrounding pressure from the regulator i.e. the absolute pressure at which the diver finds him/herself. At 30 meters the pressure is 4 bars. At 30 meters the volume of gas contained in the lungs is 4 x 6 = 24 liters, which is the volume the air would take at the surface. When the diver goes back up to the surface, the pressure becomes smaller and consequently, the volume of air will increase. Therefore it is important to breathe normally and exhale to let the air contained in the lungs out and avoid submitting the lungs to an expansion from the large internal pressure versus the external pressure.

1.2.3.2 Diving time
A diver takes a 12 liter tank inflated at 200 bars with a reserve calibrated at 30 bars. Knowing that the diver breathes at a rate of 16 breaths per minute at the surface and inhales 1 liter per breath, how long can the diver stay at a depth of 20 meters and 50 meters?

Starting point
Consumption at the surface is 16 x 1 = 16 liters per minute
Number of liters available in the tank is 200 x 12 = 2,400 liters
Number of liters earmarked is 30 x 12 = 360 liters
When the diver reaches the reserve air, the diver must go back to the surface or risk running out of air
The available amount of air for diving is equal to the total number of liters minus the reserve liters
2,400 – 360 = 2,040 liters of air available for diving

At 20 meters
The absolute pressure is 3 bars.
The consumption is 16 x 3 = 48 liters per minute because the diver consumes 3 liter at every breath (Mariotte Law)
Diving time = 2,040 / 48 = 42 minutes

At 50 meters
The absolute pressure is 6 bars.
The consumption is 16 x 6 = 96 liters per minute because the diver consumes 6 liters at every breath (Mariotte Law)
Diving time = 2,040 / 96 = 21 minutes

“The greater the depth, the shorter the dive”

1.2.3.3 Floatability
The diver uses lead weights to fight the buoyancy of his/her diving suit and BC (Buoyancy Compensator). When the diver goes back up, the pressure reduces and the volume of the air in the BC increases which increases the Archimedes force and increases the speed at which the diver goes up. A potentially dangerous situation (see decompression below)
As a rule of thumb the diver must always remember to go as slow as the slowest bubbles.