This was the topic of my "teach me a ten minute topic" on my CFI oral. The examiner had Zero clue I was an AME
"(1) You dehydrate like crazy at altitude. The air has Zero moisture at 17,000. It's about 300 ml per hour.
(2) As a result cardiac output goes down so the blood delivery train discharges at lower residual partial pressures as you have to unload the remaining blood more thorougly to get enough.
(3) The brain shuts down frontal flow (judegment areas) to maintian function, so HYDRATE LIKE CRAZY. When I did PSP-->DPA isn the 1993 AC Spruce, after 7:50 I got out, had two whole Diet Cokes and still didn't pee. I was totally dry, and had almost made some stupid mistakes (like descending into a barrel after I had a Vac failure).
(4) Gas effects- there is always some wisenheimer who eats beans the night before the chamber ride. BTDT twice and it's not pretty. It uncovers bad dental abscesses you weren't aware you had. Sinus block if bad enough can lead to a brain blood clot (Cavernous Sinus Thrombosis) or compromise of an eye (the bone punches into the nerve or muscle).
(5) If you push it (I once did) above 18,000 you will be hypoxic on cannulas. You need an occlusive mask. Don't thy this without a built in mic; when you move the mask to talk, you get hypoxic before the end of the sentence. BTW, you won't have enough molecules to say more than twelve words in a row. The air it THIN up there. Get the one with the Mic INSIDE the mask.
(6) At 24,000 you need an occlusive face sealing affair as there just aren't enough molecules even if EVERY ONE is an oxygen molecule.
(7) At 27,000 you need a slight pressure mask; see 6 for reason.
(8) Airline pull downs at 10,000 isn't to keep you sane. When they lose pressure at 37,000, it's to keep you dumb stupid but alive until they can get to 10,000.
(9) Bends. If you have a screamer Turbo Single or T37 you can get to altitude fast enough to get bent even if you haven't been diving. It's happened.
Oh, and BTW, inspector: When you are planning to fly a GA light single to 18,000 it's probably because your mission is to >75% of the capability of your aircraft. So now you have all the weather, ice, and fuel considerations and might be very dumb. Brief The PLAN AND FLY the plan, or abort. No other choices."
"Um....oh I see. We'll let's move along to another topic......"