What vessels ends in the pulmonary circulation?

A person has two circles of blood circulation. This is a large and correspondingly small circle. A small circle is necessary to saturate the blood with oxygen. It travels through a large capillary system in the lungs where red blood cells receive oxygen and break bonds with carbon dioxide. This circle begins in the right ventricle with the pulmonary trunk. The small circle ends with the left atrium. Four pulmonary veins flow into it. So venous blood becomes arterial and spreads throughout the body, gradually becoming venous.