Skip to main content
Medicine LibreTexts

18.7A: Velocity of Blood Flow

  • Page ID
    7869
  • [ "article:topic" ]

    Blood flow is a pulse wave that moves out from the aorta and through the arterial branches, then is reflected back to the heart.

     

    LEARNING OBJECTIVES

     

    Explain the velocity of blood flow

     

    KEY TAKEAWAYS

    Key Points

     

    • Flow is the movement of a liquid or gas over time.
    • Flow can be calculated by multiplying velocity, the distance moved by an object over time, with cross-sectional area.
    • Within the circulatory system, velocity can be altered by changes in blood pressure, vessel resistance, and blood viscosity.
    • Blood vessels can vasoconstrict and vasodilate, which alters cross-sectional area.
    • These various factors are under tight regulation to maintain sufficient blood flow to the body’s organs and tissues.

     

    Key Terms

     

    • flow: The movement of a volume of a liquid or gas over time, e.g. mL/sec.
    • velocity: The distance moved by an object over time, e.g. cm/sec.

    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⋅aF=v⋅a

    Where F = flow, v = velocity and a = cross-sectional area.

     

    Potential Complications

     

    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.

     

    Vessel Area

     

    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.

     

    Regulation

     

    These individual elements are tightly regulated by the body to maintain sufficient flow to the body’s organs and tissues