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 coagulation, one of the basic premises of first aid.
Blood as Symbol
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 in Classical Humoural Theory
Blood, one of the Four Humours is considered warm and moist. It is related to the element air, and the season of Spring.
The temperament of a person whose nature is dominated by blood is described as sanguine. A person who is sanguine is generally light hearted, fun loving, a people person, loves to entertain, spontaneous and confident. However they can be arrogant, indulgent, and capricious. Sanguine people were believed to be inclined towards feverish, inflammatory illnesses. An excess of blood may be treated through bleeding or the application of leeches.
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.