Asked by: Luke Azzopardi, Malta
Either for emotional reasons, or to wash away irritants such as dust, grit, insects and ‘lachrymatory agents’ – chemicals that make you cry. When an onion is cut, its enzymes mix with sulphoxides and sulphenic acids to produce a gas called propanethiol S-oxide, which reacts with tears to form sulphuric acid. This irritation alerts brain systems that then tell the lacrimal glands to stimulate tears to wash it away. These are called ‘reflex tears’ and are also provoked by coughing and yawning.
Emotional tears are different, containing more prolactin, adrenocorticotropic hormone and encephalin (a natural painkiller). Part of the brain’s limbic system, including the hypothalamus, control emotional responses including fear, anger and grief, and can signal the lacrimal glands to produce tears.
The really difficult question is why emotional humans cry at all. The reason may be social. Blurred vision and sobbing provide a social signal of weakness and neediness, and crying can bring groups together – for example, when a family is in a state of grief.