Why is it that the plural of 'blood' is 'blood' in normal usage but 'bloods' (e.g. 'I'll be taking some bloods') is acceptable in a medical context? Are there any words with similar pluralisation anomalies?
This odd pluralisation may be confined to British English, here are some examples
http://www.babyandbump.com/trying-to-conceive/866899-have-bloods-tested-progesterone.html
http://www.mail-to.co.uk/Forum/topic.asp?TOPIC_ID=13134
http://www.imt.ie/opinion/2011/09/restraint-should-be-last-alternative-in-taking-bloods.html
UPDATE: I'm struggling to find online examples of this usage outside of midwifery as per the links above. Now I'm wondering if 'bloods' is actually a pluralisation that refers to both the mother and baby's blood. Has anyone an example of a medical usage outside of pregnancy?

