As blood volume decreases, blood pressure . If greater than of the blood volume is lost from the blood vessels, the division of the autonomic nervous system (ANS) is activated, bringing about vasoconstriction of blood vessels, increased heart rate, and force of heart contraction in an attempt to maintain blood . These processes are effective in maintaining blood pressure until approximately of the blood is lost. Blood loss greater than this amount results in insufficient blood within the blood vessels, and blood pressure decreases to levels unable to support life.