Leonardo's investigations of the heart and circulation began in the 1490s; this anatomical depiction was produced around 1510 while he was based in Milan. Many of da Vinci's heart drawings were made from studies of the organs of oxen and pigs. It was only later in his life that he had access to human organs. Leonardo made a number of advances in the understanding of blood flow:
- showed the heart is indeed a muscle and that it does not warm the blood
- found it has four chambers and connected the pulse in the wrist with contraction of the left ventricle
- deduced that eddy currents in the blood flow - created by structures in the main aorta artery - help heart valves to close
- suggested that arteries fur up over a lifetime, creating a health risk
He didn't understand that the blood was in a circulation system. He went along with the belief (commonly held at the time) that blood was made in the liver, cooled in the lungs, pumped by the heart and consumed in the muscles.
The sketch is one of the Windsor Folios, part of the Royal Collection, held at Windsor. | 
Renaissance medicine