Which change would increase friction in the bloodstream?

• A. Increased blood viscosity
• B. Shorter vessel length
• C. Increased luminal diameter
• D. Decreased heart rate

Answer: (A)

Friction (resistance) to blood flow is driven by how thick the blood is, how long the vessels are, and how wide the vessels are. According to the principles behind Poiseuille’s law, resistance increases with viscosity and with vessel length, and decreases dramatically with an increased radius (to the fourth power). So making the blood more viscous makes it thicker and harder to move, which directly raises the friction experienced by the flowing blood. In contrast, shortening the vessels would reduce friction, widening the lumen would reduce resistance, and a slower heart rate changes flow dynamics but doesn’t inherently increase the friction within the vessels. Therefore, increasing blood viscosity is the change that would raise friction in the bloodstream.