Appunti di Human Factor

Atmosphere

The atmosphere is a tenuous envelope which surrounds Earth and protects life. This protection regards radiation, thermal and meteors, and the atmosphere is made of some gases which support life, such as H₂O, O₂ and CO₂, but even of:

• nitrogen 78% which doesn’t reach in our lungs;
• oxygen 21% we inhale;
• other gases 1%, such as argon or CO₂.

At sea level the weight of the atmosphere (or pressure) is exactly the same of a column of mercury of the height of 760 mm (mmHg), and the pressure mostly decreases from sea level (s.l.) to 5000 feet, so the altitude can be measured in many ways:

• true altitude, with respect to the s.l. no matter where we are;
• absolute altitude, with respect to the ground, so it depends from the place;
• pressure altitude, very used in aviation and such that it depends from the temperature the place and the altitude because it is measured with respect to the standard: 760 mmHg. This means that planes flying at the same true altitude but in different places will have different pressure altitude.

In general the relation between pressure and altitude is the one of pressure, which is very different from the one between temperature and altitude.

The atmosphere is so divided in:

• troposphere where all the climatic phenomena happen;
• stratosphere → ionosphere → exosphere → space

In particular, ionosphere is such that it goes from s.l. to 60000 ft and the temperature rate of drop is 2°C per 1000 ft and in this layer wind, rain etc. are very important.

Another important division is the one of 10000 ft, which is the altitude over which we don’t have enough oxygen for staying there for a very long time such as 1 hour. Over 14000 ft we need an extra source of oxygen because the pressure is so low that we don’t have enough air to breathe.

here we need to pressurize both the cabin and a pressure suit to let workers breathe because of the very, very low pressure.

Generally up to 63500 ft, the armstrong line is reached and here no life is possible. The physiological division of the atmosphere is such that:

• physiological zone: from 0 ft to 10000 ft (523 mmHg) and we don’t need any oxygen from extra sources;
• physiologically deficient zone: from 10K to 50K ft (87 mmHg) where we need artificial help to have the right pressure and extra sources of oxygen;
• space equivalent zone: where life can not be sustained instead of creating an artificial atmosphere.

The definition of the Armstrong line is related to the fact that at that altitude the pressure is 47 mmHg, and at that pressure the boiling temperature of body fluids is ~37°, which is exactly the body temperature.

Let’s now see some gas laws to understand how gasses affect our body:

• Boyle’s law: which is related to a barotrauma which means a trauma caused by a rapid change of pressure. In fact, at constant temperature, gas is such that P₁/P₂ = V₂/V₁, so varying the altitude cabins must be pressurized. Moreover, we must take into account that in our body only wet gasses are present and due to the fact that liquids are not compressible this^{(less oxygen than needed)} law is not properly exactly;
• Dalton’s law: is instead related to hypoxia, and it is such that the pressure of a mixture of gas is P_tot = P₁ + P₂ + P_n, where n is the number of different species. This means that:
• at 0 ft, P_tot = 760 mmHg and the atmosphere is N₂ at 79% and O₂ at 21%, so P_tot = P_N2 + P_O2 = no
• P_O2 = 0.21 P_tot = 160 mmHg, P_N2 = 0.79 P_tot = 600 mmHg
• while at 18K ft we will have P_O2 = 80 mmHg and P_N2 = 300 mmHg because P_tot = 380 mmHg. This means that the amount of oxygen needed is insufficient
• Henry’s law: which is related to the problem of mixing gas and liquids at a certain partial pressure because some of the gas won't chemically react with the liquid. In fact P₁/P₂ = A₁/A₂ where A* is the concentration of the gasses and P_A is the partial pressure. This law is related to the fact that changes in pressure could lead to formations of bubbles that could stop the flow of blood or other problems;
• Fick’s law: says that a gas goes from zones with higher concentrations to zones with lower concentrations. If those zones are divided from a wall, it affects the average which is exactly the law of proportionality and mixing. In fact, if the exchange of CO₂ and O₂ (P_O2 700 mmHg without flight and P_O2 = no

It marks the two valves (tricuspid on the right and mitral on the left) atrium and ventricles are separated when they are closed. Once atrium are fill, valves open and there is an atriale distole and diastole ventricularis (diastole is enlargement of the chamber while sistole when fluid is pressed to make it exit) ones ventricles are fill, valves close and there is a ventricles sistole (and atrium diastole) to make blood going to general circulation (arterial blood and to pulmonay circulation venous blood) then the cycle start again.

More in detail, the right side takes venous blood and makes it flow to the lungs who then become real of O₂ and CO₂ exit the left side takes instead arteral blood from vena pulmonares even if vens in this case are full of arterial blood ones reach to general circulation. It must be noted that as general circulation is in need of high quantity of blood the left side of the heart must have thick walls to give an higher boost of blood and make it arrive everywhere.

By calling cardiac output the amount of blood ejected by ventricles in one min must = 5 litre/min so it can be expressed even as

cordiac output = stroke volume x heart rate

• blood passing thry ~70 beat/min get ventricles both left and right in a beat, ~70 ml
• this volume is given by the difference between the blood entering the chamber and the one exiting after the sistole (infact ventricles can obviously be compressed to zero volume) and parasited and this difference is such that a 60% of blood exit then 40% remaining is a sort of cardiac resereve that is used in case of necrosity. Related to that the frank starling law explains weel the fact that heart adapts its volume and its abilities to respond to changes made for cardiac output (most sec.) which demand for example from the oxigen request by muscles, composed by fibers that contracting a muscle or relaxing (i.e. for examp. the arm to move a muscle is contract and the other relaxed) and to do that mor blood in nec.

11/03/22

As we said, from a certain altitude it is necessary in case of depressurization of the cabin, to put the oxygen regulator in the pressure breathing O₂ mode. This can in the worst lead to hyperventilation which is theoretically an increase in rate and depth of ventilation that protect the level of alveolar PO₂ from the decrease of the inspired PO₂ in case of hypoxia.

By the way the metabolic mech of O₂ has a limit related to the acid-base pH of the blood and hyperventilation can lead to some problems:

• hypocapnia which is a low CO₂ level;
• alkalosis the presence of less H⁺ than required and so too many bases.

But remembering how ventilation is controlled from the brain and chemoreceptors in order to controll PO₂, PCO₂ and pH normally if PO₂ is lower then needed then chemoreceptors tells the brain that more O₂ is required and so brain stimulates an hyperventilation process so that in the moment that PO₂ comes back to normal levels it stops. The problem comes when the hyperventilation becomes to much, for exampl: due to panic and it can lead to:

• cerbro vasoconstriction so that less blood goes to the brain (it leads to a left shift of the "saturation curve") so we are reducing O₂ going to the brain and this is exectly the opposite of what we want, so the brain will remain into a side of hypoxia with the risk of loosing consciousness. Moreover alkalosis leads to spasms and these are exactly the same symptoms of hypoxia and so the treatment to overcome this problem must be the same:
• put the oxygen regulator on 100% oxygen;
• check oxygen equipment;
• control breathing;
• descend below 10.000 ft -> if oxygen equipment doesn't work
• communicate.

After having seen hypoxia (related to PO₂) and hyperventilation let's see the baro-trauma which is related to the variation of pressure in terms of overall pressure not partial pressure, because any gas into our body will change its volume due to Boyle's law. This type of trauma is one of the most common problem in aviation.

As said our body has trapped gas into ears, sinuses, gastro intestinal truct, meson in teeth and lungs but these gases are wet by the presence of water vapor and the variation of density with pressure is slower them in the ideal gases in dry, almost non compressible.