Blood is a fluid medium for transporting nutrients and oxygen around the body. It appears dark red due to iron and oxygen content. When the skin of a human or other animal is broken, blood leaks out, called bleeding. Generally, the best way to stop bleeding is to apply pressure to the wound to assist coagulatiom, one of the basic premises of first aid.
Blood as Symbol
Use of blood as a symbol is very common. A goutte de sang is an heraldic depiction of a drop of blood.
Blood is a common referent in period. Monarchs ruled "by right of blood", that is, through family descent. Other monarchs ruled "by right of conquest", having been willing to spill blood to reach the throne.
Disputes automatically became more serious if blood was spilt. In much of the medieval period, an argument that came to blows could be laughed off, so long as no blood flowed. If there was blood, the disputing parties were honour-bound to see it escalate.
Norse warriors were obliged to pay weregild, a blood-price if they killed another Norseman in a dispute. The blood-price was said to appease the dead man's spirit; since it was paid to the victim's kinsmen, one must assume it appeased them, too.
Blood will stain unvarnished wood or leather an unpleasant (and unmistakable) shade of brown which no amount of scrubbing will ever get rid of.
It is important to note that blood can be a transmission vector for a number of serious diseases, such as HIV/AIDS. If you should come into contact with another person's blood, take preventative measures such as wearing latex gloves and wash immediately afterwards with soap and hot water.