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 pumps blood through the right atrioventricular (tricuspid) valve to the right ventricle. From the right ventricle, blood flows through the pulmonary valve to the lungs via the pulmonary arteries (deoxygenated blood). In the lungs the blood becomes more red in color as it is oxygenated. Blood returns to the heart via the pulmonary veins (oxygenated blood) to the left atrium.