THE PULMONARY CIRCULATION
The Pulmonary Arteries
The pulmonary trunk arises from the pulmonary orifice of the right ventricle on the craniosinistral aspect of the heart. It is slightly expanded at its origin where it presents a small sinus above each cusp of the pulmonary valve.
The trunk (Figure 7-9, A/5) passes between the two auricles then bends caudally over the base of the heart, where it is joined on its right face by the ligamentum arteriosum, the fibrosed remnant of the ductus arteriosus (p. 256). After penetrating the pericardium, it divides into right and left pulmonary arteries, each directed to the hilus of the corresponding lung in company with the principal bronchus and pulmonary veins (Figure 7-9/10,10). The course of the right artery carries it ventral to the trachea.The pulmonary arteries make their initial branching before entering the lung (Figure 4-23); their further ramifications have already been briefly noted (p. 164).
The Pulmonary Veins
The pulmonary veins open variously into the roof of the left atrium. They form two clusters in the dog: one for the veins draining each lung. In some other species the veins draining the caudal lobes of both lungs form a separate third cluster. Valves are absent from these veins.
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