Definition: Reynolds Number is used to predict the type of blood flow in a blood vessel. There are two types of blood flow:

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  • Laminar Flow: The laminar flow is described as the flow of fluid which is travelling in a calm, layered fashion. The layer of fluid flowing in the center most region of the blood vessel is said to have the highest velocity. Moving peripherally, the velocity of the layers decrease and the outer most layer, which is running along the vessel wall, is said to be travelling with the lowest velocity. This is due to friction which results in a backward drag produced by the wall on the layer adjacent to it. It is important for a blood vessel to exhibit laminar flow in order to maintain its physical integrity and carry out various cardiovascular functions. Laminar flow allows margination to occur efficiently among other functions. Margination is the process of adherence of blood cells to the vessel wall and their subsequent exit through the wall of the blood vessel to areas of need. This process gets largely disrupted if blood flow is not laminar.
  • Turbulent Flow:Blood flowing viciously and in a haphazard fashion produces turbulence within a blood vessel. The aorta normally contains blood with turbulent flow. This type of blood flow results in the production of Eddy currents within a blood vessel. These currents cause the blood to hit against the vessel wall with considerable amount of force. Repeated impaction of high pressure, turbulent blood on the vessel wall damages the elastic fibers within the tunica media. As a consequence, the elastic fibers break into smaller fragments, rendering the fibers non functional.

A small Reynolds number signifies that the blood is flowing in a smooth and laminar fashion. If the Reynolds number is high, the blood flowing through the blood vessel would be turbulent.

Murmurs and bruits point to the presence of turbulence in blood flow. These can be appreciated upon auscultation at the relevant areas. Murmurs are sounds of blood splattering on the rest of the blood due to turbulence. These disturbances result in vibrations that resonate through the walls of blood vessels or the heart itself. These are picked up as distinct sounds upon auscultation.

  • Reynolds Number=density x velocity x diameterviscosity

The formula says that the value of Reynolds number is directly proportional to density of the blood, the blood’s velocity and the diameter of the blood vessel. Increasing the magnitude of any of these parameters will result in an increase in Reynolds number, and add turbulence to the blood flow. Therefore, the aorta having the largest diameter compared to the rest of blood vessels, and the highest rate of blood flow, will give a large Reynolds number. Hence, the flow is turbulent.

The viscosity of blood has an inverse relationship with the Reynolds number. Increased viscosity of a fluid results in its flow becoming more laminar. Increased viscosity will also decrease the velocity of the blood flow. Similarly, decreasing the viscosity of the fluid will result in more viscous and turbulent flow. In terms of hemodynamics of the blood flow, changing the heamtaocrit value results in a change in the viscosity of blood. Anemia causes a decrease in the viscosity of blood and hence is attributable to generation of Eddy currents due to high turbulent blood flow.


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