Systemic circulation refers to the path blood takes on its route around the body as it brings oxygen to the tissues, while pulmonary circulation is the smaller loop blood takes to the lungs. Each loop runs through a different half of the heart, according to class notes from East Tennessee State University.
Blood from the lungs enters the left side of the heart via the left atrium and drains into the left ventricle through the bicuspid valve, explains class notes from East Tennessee State University. During the phase of the cardiac cycle known as systole, the heart contracts and forces blood through the atrium toward the body. Blood in the arteries is under pressure as it enters the systemic circulation loop. The pressure gradually drops as it flows into progressively smaller blood vessels before passing through the capillary beds and exchanging oxygen for molecular waste with the cells.
From these beds, the exhausted blood flows into the low-pressure venous network for a return to the heart. At the heart, the blood passes from the right atrium to the right ventricle to be pumped to the lungs. This is the pulmonary cycle, and it is a much shorter, lower-pressure loop. Another difference is that the blood reverses the gas exchange that took place in the cell beds, taking up fresh oxygen for another trip through the body, according to class notes from East Tennessee State University.