Blood for the tonic?

[Serena]

I should have taken the blood from Transcept but now the door that enters the room is destroyed (I've entered the Chancel and get hit by the 3 zombies). All I need is the blood, what could I do?

When you return to the Nave after being captured, there is a room to the viewer's left of Agrippa that has a grating on the floor you can use to collect the blood.