Yawning and Blood Pressure.
Frank Russo - Jan 12, 2011.
In my many thousands of blood pressure measurements that I've carried out on myself, I've actually measured it a number of times whilst yawning! The result has usually been about 30 units lower than my usual 150 reading for the systolic.
This is very interesting data, because it implies that the yawning reflex may be a simple way of introducing a lot more fresh air, to the lungs and pulmonary circulation. No wonder it often happens when one is sleep deprived, for it would allow the individual to remain more alert and stop him from getting too drowsy, whilst attempting to still concentrate on something! (In a similar way to how opening the window on long drives allows one to stop being drowsy!)
Perhaps the reflex mechanism is as simple as just stretching the rib cage out, and lowering the thoracic pressure to allow more air to go in, perhaps even attempting to exchange some of the air in the "residual" spaces.
Science is all about observations, so I'm just posting this interesting fact... hopefully somebody may find it useful.