Yawning is an instinctive reaction of the human body. It is not controlled by human will like heartbeat and breath. It has a good protective effect on protecting brain cells, increasing oxygen supply of brain cells and improving the stress ability of human body.
Why people yawn and shed tears
When I'm sleepy, I always yawn. Yawn a dozen, there will always be tears in the eyes. Why do people cry when yawning?
Human tears are secreted all the time, flowing on the surface of the eyeball. But usually the secretion of tears is very little, and is tightly along the eye surface and eyelid inside the micro gap flow, so you will not feel the tears in the flow.
However, in the blink of an eye, the tears on the surface of the eyeball are sucked into the nasolacrimal duct, the sewer of tears. Don't underestimate this not too long thin tube, it is the only way that tears can reach the nasal cavity. Lacrimal gland secretes tears continuously, nasolacrimal duct excretes tears unceasingly, so usually people will not shed tears.
When you yawn, your mouth is wide open, and your ruler may let your breath out of the body in order to exhale carbon dioxide and inhale oxygen. With the action of yawning, a strong puff of air is exhaled from the mouth. The muscles of cheek, tongue and throat are tense and contract, and the pressure of oral cavity and nasal cavity increases. As a result of the increased nasal pressure, the nasolacrimal duct will temporarily block the work of excreting tears. As the sewer is blocked, tears cannot flow down, and more and more tears accumulate in the eyes. Will be two tears, and finally burst into tears, flow to the face.
In fact, not only do yawns cause tears, but also when people laugh, sneeze, cough and vomit, they also cause such tears. The reason is the same.