The pathway of a red blood cell from the right atrium and back. (1) right atrium -> (2) -> right AV valve -> (3) right ventricle -> (4) pulmonary valve -> (5) pulmonary trunk -> (6) pulmonary arteries -> (7) lungs, for exchange of gases (not shown) -> (8) pulmonary veins -> (9) left atrium -> (10) left AV valve -> (11) left ventricle -> (12) aortic valve -> (13) aorta -> (14) other systemic vessels -> (15) inferior and superior venae cavae -> (16) back to the right atrium. The pathway from 5 to 8 is the pulmonary circuit, and the pathway from 13 to 15 is the systemic circuit.