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Thick blood may also be caused by the presence of harmful pathogens such as fungi, viruses, bacteria, and parasites. These pathogens can actually activate a coagulation response in the body as a way to avoid being attacked by the bodyâs immune system.
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.