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The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as the cardiac output (CO). Blood being pumped out of the heart first enters the aorta, the largest artery of the body.
The peripheral vascular system is the part of the circulatory system that consists of the veins and arteries not in the chest or abdomen (i.e. in the arms, hands, legs and feet). [1] [2] The peripheral arteries supply oxygenated blood to the body, and the peripheral veins lead deoxygenated blood from the capillaries in the extremities back to ...
The pressure on the wound constricts the blood vessels manually, helping to stem blood flow. When applying pressure, the type and direction of the wound may have an effect, for instance, a cut lengthways on the hand would be opened up by closing the hand into a fist, whilst a cut across the hand would be sealed by making a fist.
Pericytes also project finger-like extensions that wrap around the capillary wall, allowing the cells to regulate capillary blood flow. [6] Both pericytes and endothelial cells share a basement membrane where a variety of intercellular connections are made.
In the modified Allen test, one hand is examined at a time: [2] The patient is asked to clench their fist for about 30 seconds. Pressure is applied over the ulnar and the radial arteries so as to occlude both of them. Still elevated, the hand is then opened. It should appear blanched (pallor may be observed at the finger nails).
The sphincter can open and close the entrance to the capillary, by which contraction causes blood flow in a capillary to change as vasomotion occurs. [4] [unreliable source?] In some tissues, the entire capillary bed may be bypassed by blood flow through arteriovenous anastomoses or through preferential flow through metarterioles. [2]
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.
Vascular resistance occurs when the vessels away from the heart oppose the flow of blood. Resistance is an accumulation of three different factors: blood viscosity, blood vessel length and vessel radius. [30] Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood.