At rest, approximately what percentage of blood is directed to muscles, kidneys, and brain?

• A. 83%
• B. 50%
• C. 70%
• D. 90%

Answer: (A)

At rest, blood flow is distributed to ensure the organs with constant needs receive an appropriate share, while skeletal muscle receives a smaller portion unless activity increases. The brain uses about 15% of resting cardiac output to meet its high metabolic demands, the kidneys about 20% for filtration and waste management, and skeletal muscle about 15% to 20% when not active. Adding these together gives roughly 50% of the resting cardiac output directed to these three tissues. The other, higher percentages would imply a far larger share to these organs than occurs at rest; during exercise, muscle blood flow rises dramatically, but at rest it remains around that 50% total for these areas.