Final answer:
Increasing the length of a blood vessel decreases blood flow, as it increases resistance. Decreasing radius, blood viscosity, or increasing pressure gradient would actually increase flow contrary to the other options.
Step-by-step explanation:
The manipulation that will decrease blood flow through a blood vessel is d. Increasing vessel length. This is because blood flow resistance increases with the length of the blood vessel. Other options such as decreasing blood viscosity, increasing vessel radius, or increasing the pressure gradient, will actually increase blood flow. An increase in vessel radius greatly decreases resistance, thus increasing flow, as described by Poiseuille's law, where flow rate is proportional to the radius to the fourth power (r^4). Similarly, an increase in pressure gradient would also increase flow, while decreased viscosity would reduce resistance to flow.
The statement that 'increased viscosity increases blood flow' is incorrect as increased viscosity actually increases resistance and decreases flow. Venoconstriction increases the return of blood to the heart by increasing pressure within the vein. Plaque deposits that decrease a vessel's radius can significantly reduce flow rate and if not compensated by the body, it can lead to compromised circulation.