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Thick blood is caused by a variety of conditions, such as heavy proteins in the blood, too much blood in circulation or a blood clot system imbalance, according to Southeastern Medical Oncology Center. Diseases such as cancer and lupus can also lead to thick blood. Continue Reading.
When high blood pressure causes the heart muscle to thicken, there is less space for the oxygen-rich blood to flow from the lungs into the chamber or ventricle that pumps it to the rest of the body. This can cause blood to back up into the lungs, causing shortness of breath.