Why do eyes turn red when you cry?

Answer 1

tears contain salt.

First, let's determine how eyes turn red. The tissues of the eye need oxygen and nutrients, so they have a blood supply through small vessels (all except the clear cornea over the pupil, which gets oxygen from the air). When our eyes are their usual white, the vessels are not dilated and are almost invisible. When our eyes get red, the vessels are dilated, and we see the blood in them against the white background. This means that whatever makes the blood vessels in the eye dilate will make them red. What makes blood vessels dilate? This is actually a wonderfully complex subject. However, to keep it simple, let's say that whenever a need for an increase in blood supply occurs, the vessels dilate in response to a variety of signals, some through the autonomic nervous system, others from substances released into the blood or surrounding tissues. An increase in blood supply can be needed for more oxygen or for other needs. OK, now, why when we are crying? We cry as the result of increased secretion from the lacrimal glands that keep our eyes moist. The secretion rate of these glands is controlled by the autonomic nervous system and by other signals in the blood. (See where we are going here?) When you cry, the fluid making up the tears has to come from somewhere; it is derived from the blood supply to the glands. Therefore, when you cry, the blood vessels to the glands have to dilate to provide the fluid for the tears. This is accomplished by the autonomic nervous system. If the crying is intense enough, the blood vessels on the surface of the eye also dilate.