Answer:
It was difficult for him to understand how the heart worked because he wasn't fully aware of the pumping mechanism of the heart and how the circulation of blood goes inside the body. Not seeing the capillaries meant that the heart pumped blood towards the vessels and into a dead end. He couldn't understand where the blood that was being pumped goes to.
Step-by-step explanation:
Leonardo da Vinci is considered one of the earliest persons to study the heart. Although he made an exceptional observation of the different parts of the heart, he couldn't track the capillaries. He didn't know that these tiny structures connected the arteries and the veins. He believed that the heart was meant to transfer heat from the liver going to the muscles. He also believed that the lungs helped cool the blood.