I’ve often heard the expression “blood is thicker than water” and been somewhat bewildered that people would so commonly comment on the relative viscosity of two arbitrary selected fluids during a seemingly unrelated and often trivial conversation.
In order to better understand this I thought first to confirm that it was true.
At 40°C the Dynamic viscosity of blood is typically 2.55 mPa.s compared to the unvarying value for water which is 0.6527. So indeed the expression is correct.
However, in common parlance “thicker” is often mistakenly assigned to density rather than viscosity. But even here blood density is typically 1060 kg/m3 at 40°C depending on many factors such as age, gender, and even posture and sampling point. But it is always higher than water which has a density of 992.2 kg/m3 at the same temperature.
So indeed the expression is correct both in strict and informal interpretation.
It seems then that this message is a comfort statement of known fact whose purpose is simply to foster an unconfrontational atmosphere for the purpose of social cohesion. So if someone uses this expression during a conversation then I would suggest the socially required response would be to do likewise. For example with “an apple never falls far from the tree” which is double comforting as it incorporates aspects of both Newton's law of universal gravitation and his first law of motion.