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The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in controlling hemorrhage and reducing acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance.
Another method of achieving constriction of the supplying artery is a tourniquet - a band tied tightly around a limb to restrict blood flow. Tourniquets are routinely used to bring veins to the surface for cannulation, though their use in emergency medicine is more limited. Many armies carry a tourniquet as part of their personal first aid kit.
Cerebral blood flow (CBF) is the blood supply to the brain in a given period of time. [8] In an adult, CBF is typically 750 millilitres per minute or 15.8 ± 5.7% of the cardiac output . [ 9 ] This equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute.
Below are several examples of differing types of local blood flow regulation by specific organ type or organ system. In each case, there is a specific type of intrinsic regulation occurring in order to maintain or alter blood flow to that given organ alone, instead of creating a systemic change that would affect the entire body.
In vitro, high oxygen concentrations cause pericyte constriction, while high CO 2 concentrations cause relaxation. This suggests that pericytes may have the ability to dilate blood vessels when oxygen is in demand and constrict them when it is in surplus, modifying the rate of blood flow to tissues depending on their metabolic activity. [6]
Hyperventilation is still used if ICP is resistant to other methods of control, or there are signs of brain herniation, because the damage herniation can cause is so severe that it may be worthwhile to constrict blood vessels even if doing so reduces blood flow. ICP can also be lowered by raising the head of the bed, improving venous drainage.
Hemostasis occurs when blood is present outside of the body or blood vessels. It is the innate response for the body to stop bleeding and loss of blood. During hemostasis three steps occur in a rapid sequence. Vascular spasm is the first response as the blood vessels constrict to allow less blood to be lost.
A blood choke disrupts blood circulation to the brain, while an air choke disrupts breathing. Blood chokes can be applied to efficiently cause loss of consciousness, i.e. a choke-out, while air chokes do not usually cause loss of consciousness without prolonged application (though air chokes are used to cause discomfort).