What happens physiologically when you get the wind knocked out of you?

The answer to the other question is satisfactory in describing what usually happens when the "wind is knocked out of you." The treatment was described well on the Lake Forest High School website for coaches

(http://www.lfhs.org/athletics/athletic_trainers/docs/handbook.PDF):

1. If an athlete has "the wind knocked out" and the ATC (athletic trainer) is not present, do not move the athlete until his/her breathing is back under control.
a. Speak calmly and do not allow teammates to gather around. The
athlete's breathing will be harder to control if emotional.
b. After breathing is under control, ask if the athlete hurts anywhere.
c. If the athlete has no pain and is breathing normally, allow them to
rest on the sideline for a few minutes. They can compete when
fully recovered.

As a physician and former paramedic, I think it's important to also mention that not all hits to the abdomen/chest/"solar plexus" that result in someone falling to the ground seemingly unable to breathe are due to the "wind being knocked out." It is fortunately EXTREMELY rare but isolated cases of a young athlete's heart stopping have been reported (see the news story at

http://www.nctimes.com/articles/2005/08/11/backpage/8_10_0516_35_26.txt

for one). As a paramedic, I was once called to a baseball field where a batter was being tended to at home plate. He had been struck in the side of the chest by a pitched ball and collapsed. Some bystanders were doing CPR but the 10 year old boy was pulseless and could not be revived.

The mechanism is similar to the "chest thump" taught in CPR and first aid courses when a defibrillator is not available. If someone is struck in just the wrong part of the chest at the wrong moment, it can interfere with the normal heart rhythm, resulting in a fibrillation or erratic twitching of the heart, which is unable to sustain life. This is most likely what happened to the cheerleader in the story above.

If a first responder remembers the ABCs of first aid (Airway, Breathing, and Circulation), it is unlikely the two very different conditions would be confused.