Invasive testing begins... and ends ;)
Today was the lone test for the Lunar Analog Feasibility Study, the purpose being to understand the effects of lunar gravity on physiological changes in the human body while astronauts are on the moon. I haven't "gone to the moon" just yet, but this is to establish a "baseline" of my plasma volume, and it will be repeated later to determine any changes.
A nurse poked the vein in my inner left elbow, and left a catheter in my arm. Once that was inside, I just had to remain recumbent for a 20-minute period. They turned down the lights, and advised me to stay as still as possible... no crossing the legs or moving the arms. Being flat on my back through all this, sometimes it's difficult not to doze off! Chatted quietly with the two folks from the JSC Vascular Labs (low talking is acceptable).
After 20 minutes, they take an initial blood sample from the open arm vein, and are then ready to test plasma volume -- which basically means measuring the amount of blood circulating in all vessels. This is accomplished with a technique called "carbon monoxide rebreathing."
Most divers are familiar with basic rebreathers, a "closed circuit" set that contains oxygen but also recycles exhaled gases. NASA's is a bit different since it's not used for survival underwater so much as measuring what's absorbed and expelled after being directed through a cylinder of sand lime (to filter carbon dioxide).
In the first stage, a snorkel-ish mouthpiece attached to a hose was placed over my face, and I breathed 100% oxygen for two minutes. In the second stage, a tiny amount of carbon monoxide was released into the breathing apparatus... nothing dangerous (about the equivalent of two cigarettes), but the hose stays in, and I had to inhale and exhale normally for 10 minutes.
At certain intervals, they asked me to breathe ALL the way out, or ALL the way in, then resume normal respiration. All my communication had to be through waves or "thumbs up" since the mouthpiece cannot be removed.
After the 10-minute cycle, a second blood sample was taken from the arm catheter for analysis. From these samples they deduce total red blood cell count, hemoglobin concentration, and the proportion of blood volume that is occupied by red blood cells... also called the hematocrit (Ht or HCT) or "packed cell volume" (PCV).
What they should do is test your brain for the capacity to memorize masses of acronyms before and after doing a NASA study. My brain is full. But anyway.
In micro-gravity and lunar gravity, plasma volume is one of the very first things to change internally, and red blood cell mass decreases in both astronauts and analogs, so that's why they will repeat it after the bedrest phase.