Harvey's work was limited because scientists had not yet discovered what tiny vessels?

• A. Arteries
• B. Capillaries
• C. Veins
• D. Nerves

Answer: (B)

The missing link in understanding how blood travels is the tiny vessels that connect arteries to veins, known as capillaries. Harvey showed that the heart pumps blood in a closed loop, but he couldn’t demonstrate the full path from arteries to veins because these capillaries are extremely small and unseen in his time. Their discovery, enabled by improved microscopes, showed the microcirculation and completed the picture of blood moving in a continuous circuit. Arteries and veins were already known to carry blood in and out of the heart, but the essential connection between them lay in capillaries. Nerves have no direct role in the circulation, so they don’t fit the mechanism Harvey was describing.