Bleeding
This is your second highest priority when assessing your casualty.
Any blood leaking from the body means there is a loss of blood from the circulatory system.
The circulatory system is made up of the heart, blood and blood vessels, it is a compact system that needs to be in tact or pressure will fall and oxygenated blood will not be able to circulate around your body effectively.
Think of a balloon filled with water and with pipes coming out of it and then heading back into the balloon, if there is a puncture in one of the pipes and water leaking out then after a while