I have recently encountered this question:
Waldenström's macroglobulinemia is a condition which causes increased blood viscosity due to high protein content in the blood. How would Waldenström's macroglobulinemia influence blood flow and pressure?
(A) Blood flow would be increased, blood pressure would increase
(B) Blood flow would be increased, blood pressure would decrease
(C) Blood flow would decrease, blood pressure would decrease
(D) Blood flow would decrease, blood pressure would increase
My immediate thought was to consider the relationship between viscosity and resistance. If the viscosity of the blood increases, the resistance (to flow) should also increase. According to the equation for vascular resistance:
$$R=\frac{\Delta P}{Q}$$ Resistance is directly proportional to blood pressure. This lead me to believe that the increased resistance should lead to an increase in blood pressure. But even without the use of equations, intuitively, I suspect that the heart should work harder (thus raise blood pressure) if there is resistance to blood flow.
However, the answer key indicates that choice (C) is correct, and that the pressure decreases. How is this possible? The explanation given in the key is:
Viscosity is directly proportional to resistance.
Blood flow is inversely proportional to resistance.
Blood pressure is directly proportional to flow.
If you have increase viscosity, you'd have increased resistance, resulting in less flow, and consequently lower pressure.