What is Blood Viscosity?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 12 August 2018
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Blood viscosity is a measure of the thickness of blood. The thinner the blood, the less it resists flow, moving smoothly throughout the body. Some studies have linked moderate to high blood viscosity with cardiovascular problems and sometimes people can develop a medical condition known as hyperviscosity syndrome. In people with this condition, thickened blood leads to health problems ranging from visual anomalies to coma.

Several factors are involved in blood viscosity. The composition of the blood is one factor. The more fluid in the blood, the thinner it will be. High counts of red blood cells and particles lead to an increase in blood thickness. Fats that circulate in the blood can also play a role in making it thicker or thinner, with high concentrations of fats increasing viscosity.

Temperature is another contributing factor. As with many other fluids, in low temperatures, the blood becomes thicker and moves more sluggishly. This is a concern with frostbite, when chilling of the extremities can make the blood so viscous that it does not circulate and the tissue dies as a result of lack of oxygen and nutrients. Hypothermia can also lead to concerns about blood viscosity.


When the flow of blood is slow, cellular reactions that lead to adhesions can take place. Cells in the blood will start to stick together, forming clumps that thicken the blood. Viscosity also tends to increase in narrow blood vessels. People with conditions that lead to a slowing in flow rates or a narrowing of the vessels can be at risk for higher blood viscosity.

Unusually thick blood can potentially clot in the patient's veins, leading to health problems. High blood viscosity also forces the heart to work harder to pump the blood, increasing the risk that a patient's heart will give out.

If a patient has high blood viscosity, there are treatment options available. Medications can be prescribed to reduce the viscosity and break up any clots that may have formed. Patients with thickened blood due to exposure to extreme cold can be slowly warmed up to allow the blood to thin and bloodflow to normalize. It's important to be aware that the return of blood to areas with poor circulation can be painful. Although a patient may want to stop because of the unpleasant sensations, slow and steady warming should be continued until the blood flow normalizes and the patient is stable.


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Post 3

I'm not sure if this is something that pertains to this, but I have a hard time giving blood. My veins end up closing down and I was told to make sure I was properly hydrated. Even when fasting you can drink up to 8 ounces of water. This was told to me at a lab I go to. I am always cold too, with a diagnosis of hypothyroid.

Post 2

Are blood viscosity and dehydration related? I would think that if there is more fluid in your body, there would be more fluid in your blood also.

I always have a hard time drinking enough throughout the day. I just don't think about it for some reason. It never occurred to me that it could result in problems with my blood.

Post 1

If one blood viscosity factor is temperature, like when frostbite or hypothermia is a problem, does having the temperature of your home too low present a risk? And if this is true, would it be a problem especially for people who may have a higher blood viscosity to begin with?

I know it saves money to keep the heat in your house set low, but I feel like it can't be healthy to be cold all the time.

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