This article examines the physiological mechanisms of the human body to provide a medical answer to the question: How many times is it necessary to shoot an assailant before he is incapacitated?
Newgard reviews the physiological mechanisms of gunshot wound trauma incapacitation:
"The only method of reliably stopping a human with a handgun is to decrease the functioning capability of the central nervous system (CNS) and specifically, the brain and cervical spinal cord. There are two ways to accomplish this goal: 1) direct trauma to the CNS tissue resulting in tissue destruction and 2) lack of oxygen to the brain caused by bleeding and loss of blood pressure."
Newgard discusses the body's blood loss sensory and compensatory mechanisms (venous constriction, increased cardiac output and vascular fluid transfer), and the degree in which these mechanisms respond to, and compensate for, hemorrhagic shock. He reviews clinical tests of human tolerance for blood loss, which "demonstrate that adequate blood pressure can be maintained with minimal symptoms until a 20% blood deficit was reached." Newgard provides the following example:
"For an average 70 kg (155 lb.)* male the cardiac output will be 5.5 liters (~1.4 gallons) per minute. His blood volume will be 60 ml per kg (0.92 fl. oz. per lb.) or 4200 ml (~1.1 gallons). Assuming his cardiac output can double under stress (as his heart beats faster and with greater force). his aortic blood flow can reach 11 liters (~2.8 gallons) per minute. If one assumes a wound that totally severs the thoracic aorta, then it would take 4.6 seconds to lose 20% of his blood volume from one point of injury. This is the minimum time in which a person could lose 20% of his blood volume.... This analysis does not account for oxygen contained in the blood already perfusing the brain, that will keep the brain functioning for an even longer period of time.
"Most wounds will not bleed at this rate because: 1) bullets usually do not transect (completely sever) blood vessels, 2) as blood pressure falls, the bleeding slows, 3) surrounding tissue acts as a barrier to blood loss, 4) the bullet may only penetrate smaller blood vessels, 5) bullets can disrupt tissue without hitting any major blood vessels resulting in a slow ooze rather than rapid bleeding, and 6) the above mentioned compensatory mechanisms."
Newgard investigates the survival times of persons who received fatal gunshot wounds to determine if the person who was shot had enough time to shoot back. He concludes:
"Instantaneous incapacitation is not possible with non central nervous system wounds and does not always occur with central nervous system wounds. The intrinsic physiologic compensatory mechanisms of humans makes it difficult to inhibit a determined, aggressive person's activities until he has lost enough blood to cause hemorrhagic shock. The body's compensatory mechanisms designed to save a person's life after sustaining a bleeding wound, allow a person to continue to be a threat after receiving an eventually fatal wound, thus necessitating more rounds being fired in order to incapacitate or stop the assailant."