sentences:  As a consequence of the input of these vessels, blood in the left heart is less oxygenated than the blood found at the pulmonary capillary beds, albeit to a very small degree.The "openings of smallest cardiac veins" are located in the endocardium.Here the smallest cardiac veins return blood into the heart chambers from the capillary bed in the muscular cardiac wall, enabling a form of collateral circulation unique to the heart.