The structure indicated is the pulmonary trunk.

The pulmonary trunk is one of the great vessels – a term which refers to the major arteries and veins which bring blood to and from the heart. These include:

• Superior and inferior vena cavae
• Pulmonary arteries
• Pulmonary veins
• Aorta

The pulmonary trunk carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation, via the right and left pulmonary arteries. This is the only artery in the adult human body that carries deoxygenated blood. In fetal life, the umbilical arteries also carry deoxygenated blood – this artery extends into the umbilical cord and to the placenta for oxygenation.

The pulmonary trunk bifurcates to form the left and the right pulmonary arteries. The right pulmonary artery passes underneath the arch of the aorta towards the right lung, and the left pulmonary artery extends to the left lung.

Learn more about the great vessels in this tutorial