**3. The anatomy of the pulmonary veins**

The pulmonary veins, in contrast to systemic veins that collect deoxygenated blood from all organs except lungs, deliver oxygen-rich blood from the lungs to the left atrium. Generally, there are four tributaries of pulmonary vein that would form four ostia on the left atrial wall, two from the right cranial and caudal pulmonary vein and the other two from the left cranial and caudal pulmonary vein (**Figure 2**).

#### **Figure 2.**

*Normal anatomy of pulmonary veins. The blue (deoxygenated) marks pulmonary arteries, and the red (oxygenated) marks the pulmonary veins.*

The right cranial pulmonary vein collects blood from the right cranial and middle lung lobe, and the right caudal pulmonary vein receives blood from the right caudal and accessory lung lobe. The rest pulmonary veins serve for the corresponded lung lobs that they are named after [9].

In atypical but not rare situations in human, pulmonary veins that both originate from right (4%) or left (17.8%) may fuse into a common trunk before entering the left atrium [10]. Additional pulmonary veins derive from individual lung lobes can also happen. Generally, these variations of the number of pulmonary veins are not always problematic, but it may interfere with clinical decisions especially in surgical procedures.
