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The flow of blood around the circulatory system is modulated by numerous interacting factors. The science dedicated to understanding this flow is called hemodynamics.
Velocity vs. Flow
It's important to understand the different between velocity and flow. Velocity refers to the distance an object moves over time; for example, in blood this measurement is often given as cm/sec. Flow refers to the movement of a volume of a liquid or gas over time; for example, in blood this measurement is often given as mL/sec
At its simplest, imagine a perfect, rigid tube with no resistance and with a homogeneous liquid flowing through in a perpendicular manner. Flow can be calculated using the following formula:
$F=v \cdot a$
Where F = flow, v = velocity and a = cross-sectional area.
While the above example is a simple calculation, in reality there are numerous factors that influence velocity and flow.
Velocity and Pressure
Movement of blood throughout the circulatory system is created by differences in pressure generated by the pumping of the heart. Pressure is greatest immediately after exiting the heart and drops as it circulates around the body, particularly through the arterioles and capillary networks. A greater difference in pressure results in a greater velocity assuming all else remains equal, so when increased blood flow is required the heart can pump more quickly and also in larger volume.
Velocity and Resistance
Resistance is the force that must be overcome by pressure in order for flow to occur, and is a factor of vessel length, diameter, surface composition, and the viscosity of the liquid flowing through. As resistance increases the difference in pressure which influences velocity decreases, which in turn reduces flow. For this reason, the narrow arterioles rapidly reduce local blood pressure and slow the flow of blood through the capillaries, a beneficial effect allowing for efficient transfer of chemicals and nutrients. However, pathological changes in blood vessels that result in narrowing or an increase in surface resistance can lead to a reduction in pressure, velocity, and thus flow, which can in turn lead to tissue damage.
Velocity and Viscosity
Blood is a complex liquid formed from plasma and containing numerous cell types. As such, its viscosity is changeable depending on osmotic balance and cell load. Increases in viscosity such as reduced water content lead to increases in resistance and thus reduction in flow.
Blood vessels are capable of vasodilation and vasoconstriction to alter their diameter. Assuming all else remains equal, a reduction in diameter results in a reduction in flow, whereas an increase in vessel diameter results in an increase in flow.
These individual elements are tightly regulated by the body to maintain sufficient flow to the body's organs and tissues
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