Inductive definition of CW complexes. If the largest dimension of any of the cells is n, then the CW complex is said to have dimension n. If there is no bound to the cell dimensions then it is said to be infinite-dimensional. The n-skeleton of a CW complex is the union of the cells whose dimension is at most n.
Deoxygenated blood enters the right atrium through three major veins: the superior and inferior vena cava and the coronary sinus. The superior vena cava returns all of the blood to the heart from tissues superior to the heart in the head, neck, arms and upper thorax.
The right atrium receives de-oxygenated blood from systemic circulation through the superior vena cava and inferior venae cavae. The left atrium receives oxygenated blood from pulmonary circulation through the left and right pulmonary veins. Blood passively flows into the atria without passing through valves.