[thedoc (OP)]

Jie Dong"  asked the Naked Scientists:
   
What's the link between tear duct activation when one defaecates and/or urinates?

Thank you,

Jie "Katie" Dong
What do you think?

[chris]

If they are painful, or associated with significant straining, then this will often be associated with screwing up the eyes which has the effect of squeezing more tears from the lacrimal gland and also temporarily obstructing the punctum that drains tears into the nose via the nasolacrimal duct. Blocking the outflow pipe and temporarily increasing the rate of inflow causes tears to spill over.

An alternative explanation is that defaecation and urination are both heavily dominated by input from the autonomic nervous system. So is tear production. So it's possible that the same pathways that are facilitating the excretory processes might augment activity in secretory systems like tear production in some people.