As fluid moves through a vessel, which factors determine the flow resistance? (select all that apply.)
- the length of the vessel
- the radius of the vessel
- the viscosity of the fluid
- the pressure exerted by the heart

- the length of the vessel

- the radius of the vessel

- the viscosity of the fluid

Explanation: A blood vessel can contract and relax so therefore the most important of these three factors is the radius of the blood vessel as very small changes in the radius of the blood vessel may lead to large changes in the resistance.  The smaller (more narrow) the blood vessel is the more difficult it will be for the blood to flow increasing blood pressure and therefore resistance and an increase in radius will reduce resistance.   The longer the blood vessel the more resistance.  Blood viscosity generally does not change that much but it can be changed by factors such as temperature, hematocrit (ratio of volume of red blood cells to total blood volume) and low flow states.  The more viscous the blood the higher the resistance.  At higher temperatures the blood would be less viscous and there will be reduced flow resistance.