How did William Harvey’s blood-flow experiments predict the existence of capillaries?

William Harvey did not directly observe capillaries; rather, he was forced to hypothesize their existence to complete his theory of blood circulation.

Through his experiments, Harvey demonstrated that the heart pumps a volume of blood that far exceeds what the body could possibly produce or consume. By calculating the heart's output, he proved that the blood must be moving in a continuous, closed loop. Because he established that blood travels from the arteries to the veins, he logically concluded there must be a connecting link between these two systems to facilitate this flow.12

At the time of his 1628 publication, capillaries remained unknown because the microscope had not yet been sufficiently developed to visualize them. The physical existence of these minute vessels was only confirmed later in the 17th century by Marcello Malpighi, who used a microscope to observe the network connecting arteries and veins in a frog's lung.123