Were there many or few vessels serving as conduits between the lungs and the heart? why is this important?

True, there are many vessels serving as a conduit between the lungs and the heart.

The number of the vessel is many because all the carbon dioxide from body need to be released at the lungs. More vessel means more surface area and blood flow rate to do the diffusion of the gas which means increased diffusion rate. This will allow the lung to transfer oxygen and dump carbon dioxide faster.