"Shock" is a medical term for vascular collapse, which is so complicated I had a hundred page book just on the one topic.

The trouble is, it's not a well-defined phenomenon.

In the ICU it refers to something like "drop in blood pressure accompanied by impaired flow of blood to the peripheral and even the vital organs with resulting multi-system failure; death ensues unless the process can be interrupted and reversed," which is more a description than an explanation.

This means we can't say much about exactly what caused it in any given case (if indeed there is "a" cause), or precisely where the lesion is. We do recognize it when we see it, and we know a bunch of things to try, and then we see if the patient responds or not.

I don't imagine it's too different in veterinary medicine.