The pericardial sinuses are impressions in the pericardial sac formed between the points where great vessels enter it.
Pericardial sinus
Posterior wall of the pericardial sac, showing the lines of reflection of the serous pericardium on the great vessels. (Transverse sinus labeled at center. Oblique sinus not labeled, but visible inferior to transverse sinus between the right and left pulmonary veins)
The transverse sinus is the tunnel-shaped passage posterior to the aorta and pulmonary trunk , and anterior to the superior vena cava.[1][2] This sinus is clinically important because passing one end of clamp through the sinus, and the other end anterior to the aorta/pulmonary trunk will allow complete blockage of blood output. This is performed during some heart surgeries.[citation needed]