BYDR. DANIEL L.JOHNSON

How Oxygenation WorksLack of oxygen dulls the mind and

judgment, slows the reflexes, weakens themuscles, and takes away our higher facul-ties. The higher one goes, the more seriousare these effects. Too many people forget thisexactly at a time when they should be mostresponsive to the danger. (Houston CS.What price a summit? Wilderness En-viron Med 1996; 7:287-8.)

Long ago, during an after-landoutprocess, I discovered that the soaringworld is inhabited by two species: theAnalysts, who are fascinated by howthings work and are happy to spend in-definite time satisfying their curiosity;and the Consumers, who are here to fly,and seem to believe that the Analystsare being distracted from "What's Re-ally Important".

This column is aimed at the How-it-Works folks. You may read it, though, ifyou're not part of that club. Those of uswho are Consumers sometimes encoun-ter surprise endings, based on what I'veseen. However, surprises in aviation arenot always thrilling, so you might puton an Analyst hat while reading this;it's a good thing, and we won't insist youmake it a habit.

As you all know, we breathe. Prettyregularly. Continuously. This, I'll call"ventilation."Its function is to get rid ofcarbon dioxide, a product of energy cre-ation, and to inhale oxygen, the easy halfof energy (the more difficult half beingfuel).

We function - thinking, movement,growth, immune protection - using en-ergy extracted from fuel through burn-ing it in finely tuned, tightly regulatedlittle furnaces in each cell, the mito-chondria. The firewood, we eat; the oxy-gen, we breathe. These are married andconsumed in the mitochondria to let usfunction. This fire I'll call "respiration."If the fire goes out, we quickly wilt.

The standard fuel is glucose - grapesugar to the tongue - an orderly assem-blage of 6 carbon and 12 hydrogen at-oms, plus 3 oxygens to help its shape. Asyou learned in biochemistry, this is dis-assembled by the "Krebs cycle" and the"Electron Transport Chain" into waterand carbon dioxide, each glucose mol-ecule yielding, through the process, upto 39 little packets of energy, ATP.

In general, every metabolic process inour cells happens through a multi-stepassembly line that precisely shapes theoutput and permits fine control. "Res-piration" is a bucket brigade of complexproteins that pass electrons from glucoseto oxygen, forming water and carbondioxide. The carbon dioxide we exhale,the water, we recycle. ATP is produced,providing energy for thinking and ma-nipulating the aircraft controls in thegraceful and skilled movements we're soused to.

(It's fascinating that iron is central tothis whole process: it's part of the he-moglobin that carries oxygen to cells,and it's part of the cytochromes that arein the electron bucket brigade. Thus ifyou're anemic and iron deficient, you ac-tually lack energy, both from inadequateoxygen transport and from inadequatecellular "respiration.")

Now, the simple reason you have anoxygen bottle in that glider is that yourfriends and heirs are very interested inyour exercising good judgment and inhaving a successful flight, not to men-tion the worried people over whosehouses you fly and who write letters tothe NTSB when we do things that makethem scared.

To produce energy, the oxygen thatstarts in the ambient air must be deliv-ered to the enzymes in the mitochon-dria. That delivery depends on a se-quence of steps. If one or more of these

steps is disrupted or sub-optimal, thepilot may be compromised. Here is whatyou need to succeed:

• Adequate amount of oxygen, mea-sured by its atmospheric pressure("partial pressure") in the air you inhale.

• Effective ventilation, to exhale staleair and to bring high-oxygen air intoyour lungs.

• Unhindered diffusion of oxygen andcarbon dioxide across the lung's alveolarmembrane. This thin membrane keepsour blood from leaking out our trachea,yet must allow gas through easily.

• Uptake of oxygen by the hemoglobinin red blood cells.

• Blood flow past the air sacs in thelung (the "alveoli").

• Blood flow (adequate pump func-tion, adequate pressure, unrestrictedflow) to the smallest blood vessels, thecapillaries, where complementary gasexchange occurs.

• Hemoglobin must release oxygen atthe capillaries, so it can diffuse acrossthe capillaries'walls into the interstitialfluid between cells.

• Oxygen must diffuse in, carbon diox-ide must diffuse out from the interstitialfluid in which all our cells are bathed.

• Cells must take up oxygen and itmust enter the cellular mitochondria.

The partial pressure of oxygen natu-rally decreases at each of these steps,like a brook tumbling down a moun-tain, as shown in the first figure. Thishas been called "the oxygen cascade."A deficiency of any step in this cascadecan cause damage downstream. (Toyou, dear reader.)

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Soaring • January 2012 • www.ssa.org

PG, J Physiol, March 1, 2006 571: 2415-424)

This oxygen-cascade figure showsthat during exercise we can extractmore oxygen from our blood than atrest. That's important when we exer-cise, but we are not exercising in thecockpit: the point I want you to noticehere is that there's a dramatic drop ofoxygen partial pressure from lung (al-veoli) to cells (mitochondria). The lowcellular oxygen pressure decreases withaltitude, or with any compromise toour delivery system. You can 'be a man',and go without supplemental oxygen aslong as it's legal - or, if you don't careabout rules, as long as you can function.(Only, be aware that the rare glider pi-lot has had brain damage from the ef-fects of high altitude, when the "bends"have combined with hypoxia.)

The second figure shows the differ-ence between sea level and 8100 meters(about 24,000 ft) on this oxygen cas-cade. This figure is for people acclima-tized to high altitude, not for the gliderpilot who's just ridden a 10-knot waveuntil his nails turned violet.

Level of oxygen cascade

(Reference: Beall CM, PNAS May15,2007,104, Suppl 1, 8655-8660.)

The main things affecting this cas-cade are the partial pressure of oxygentaken in (read: bottle and mask or can-nula), and the ability of the hemoglobinto take up and release oxygen ("oxygendissociation.")

We talk often, in aviation, about oxy-gen supplementation, but we don't men-tion things within ourselves that canlimit its effectiveness. Here's a short list:

Site DefectAirway: ventilation

Airflow hindered by plugged nose, bigtonsils, obesity

Lungs: ventilationFluid, inflammation (asthma, bron-

chitis, pneumonia)Lungs: gas exchangeChronic lung disease, esp. from

smokingBlood lack (anemia)Decreased oxygen-carrying capacityPump failure (heart)

Poor delivery to tissues for severalreasons

BloodHemoglobin fails to accept or release

oxygen well.

This last point is an important one.Under some conditions, the blood doesnot release oxygen well. This is repre-sented by a famous graph, the oxygendissociation curve. In this figure, the

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letter 'a' indicates the normal status inthe arteries V'the normal status in veins.

0 20 40 60 80 100

Parlial press.ure of oxygen (rnmHg)

(Reference: Oxford Textbook ofMedicine, 5th ed. 2010, Oxford Univer-sity Press, Oxford, England)

The high flattish part of this curveillustrates the fact that when oxygenpartial pressure is high (in your lungs, forexample), hemoglobin turns sticky foroxygen; the ski-slope part of this curveportrays the fact that when the oxygenpartial pressure is low (in your brain, forexample), the hemoglobin tends to loseits grip, and oxygen is released. Cool, eh?Almost like it's meant to work that way.Therefore, normally hemoglobin takesup oxygen well in the lungs, and gets ridof it well in the tissues.

This curve also implies that as altitudeincreases (lower partial pressure); hemo-globin does not take up oxygen as well.

Several things affect the stickiness ofhemoglobin for oxygen; some reducethe ability of hemoglobin to releaseoxygen to the tissues. This greatly reduc-es the effectiveness of any supplemental

oxygen you might use.• Hyperventilation. This raises the pH

of blood, making hemoglobin stickiereven while raising the oxygen contentwithin the lung; the net effect is lessoxygen delivered to hungry cells, such asour brain.

• Carbon monoxide. This ties up afraction of the hemoglobin, decreasingthe amount of oxygen transported, andmakes hemoglobin stickier for oxygen,hindering its release to tissues. Car-bon monoxide exposure is mainly fromsmoking and from engines. This is arisk to tow pilots, who have extra ex-posure due to slow airspeed, high noseattitude, and open cowl flaps. CO levelsin my airplane are about four times ashigh during climb as during cruise. Thefact that CO binds irreversibly to he-moglobin means that its effect is longlived (red cells are recycled after aboutthree months.)

• Body temperature. Fever helps releaseoxygen to the tissues, hypothermia hin-ders its release. Getting cold reducesthe benefit you get from supplementaloxygen.

• Anemia. When we're anemic, theamount of oxygen carried is less, butit's released more readily (though notenough to make up for the anemia).

Here's the point: all your cells will behappier and will work better - especiallythose important ones in your brain - ifthey have lots of oxygen. Of course, youuse oxygen even when it's not requiredby the FARs, because it makes you somuch smarter. However, you shouldn'tgo to altitude if you have lung disease(asthma, emphysema, etc.) or heart dis-ease (angina, heart failure, or bad valves).

Smokers should put on oxygen on theground, and will be well served by wear-ing it above about 5000 ft. Yes, I knowit doesn't feel like the manly thing, butyou'll do fewer stupid, embarrassingthings in the aircraft. Count on it.

Similarly, tow pilots who go soaringafter flying tows should put on oxygenon the ground, as well.

If you have anemia, or any heart orlung disease, it would be a safe thing totalk specifically about your own altitudetolerance with a lung specialist, and de-vise with advice a safe personal maxi-mum altitude and rates of oxygen flowwith altitude.

For more information, Google anyof the key phrases that are in quotesin this article. A good site on altitudephysiology is www.altitude.org, focusedon mountain climbing - click on "HighAltitude." Another is Patrick Nelligan'stutorial for anesthesia, http://www.ccmtutorials.com/rs/oxygen/.

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