When adapting to high elevation, does your blood get thicker? I hear a lot of commentary on altitude related heart attacks, strokes, and frostbite (from expeditions in the Himilayas) being caused by poor circulation due to thicker blood, is this true?
Yes, blood viscosity changes with altitude. This is a study in rats where the control group was kept at normal altitude and the test group at simulated 5500 meters.
Besides, RBC counts, hematocrit, whole blood viscosity, erythrocyte aggregation index of hypoxia model rats were all notably higher than those of normal control rats respectively.
Changes in the rheologic properties of blood after a high altitude expedition The effects you are describing is called "high altitude deterioration" (HADT) and in patients with this, blood viscosity is one of the symptoms.
but other indicators including the blood viscosity, hematocrit, hemoglobin concentration and heart rate were higher in patients with HADT
This study was only in old women, but nevertheless I will include it here:
It is found that the correlation between geographical factors and the normal reference value of whole blood viscosity (230s(-1)) of old women is quite significant (F=30.137, P=0.000).
Most relevant to your question, this was a small study on mountaineers to Mount Everest:
In blood samples collected immediately after return, alterations of some haemorheologic parameters were observed in comparison with baseline values, and namely an increase in relative blood viscosity, fibrinogen and erythrocyte filtration time along with a fall in platelet count
Making the connection to vascular problems:
Increased viscosity is associated with aging, obesity, carotid intima-media thickness, metabolic syndrome, hypertension, diabetes, ischemic heart disease, and stroke