He swallowed blood whilst he was praying
Question

I used the siwaak before Zuhr prayer, and in the middle of the prayer I felt a small amount of blood coming out. I tried not to swallow any of it but I could not help it. After the prayer I spat out some of the saliva that was in my mouth and found out that it was in fact blood. What is the ruling and do I have to make up this day?.
Praise be to Allah.You should not have swallowed this blood; you should have taken out a tissue whilst praying and spat the blood into it. 
Because you swallowed it without meaning to, your fast is still valid, but if you swallowed it deliberately then you have to make up this day. 
Ibn Qudamah (may Allah have mercy on him) said: If blood flows in his mouth or he burps up something (reflux) from the stomach, and he swallows it, then he has broken his fast even if it was a small amount, because the basic principle is that anything that goes from the mouth to the stomach breaks the fast. 
The basic principle is that the fast is broken by everything that reaches the stomach from the mouth, with exception of saliva because it is not possible to avoid it. But this principle applies to everything else. 
Al-Mughni, 4/356 
And Allah knows best.