People can black out from giving or loosing blood. This is termed, in the medical field, a vasovagal reaction. The reason I know this is because it happened to me once in the military. The nurse took blood from me when I was standing up. Next she told me to get in line to receive my hepatitis B vaccine. I remember feeling dizzy, so I went to sit down at the closest chair. The next thing I know, I wake up laying on the drill hall floor with fellow soldiers all around me. One person was holding my head. I went to get up and they all yelled for me to remain laying down, and not to move. The reason they were holding my head was because I passed out head-first on the cement drill hall floor, from a standing position. I was ok but it was definitely a strange experience.
You can also pass out from standing in one place to long. If you do not slightly bend your knees, it will cut off the blood circulation to your feet, causing you to black out. I also learned this in the Army. We had to learn to stand at position of attention for long stretches without moving or shifting or footing.