Central arteries are really arterioles that drain into a capillary that may open freely into the red pulp cords (open circulation) or directly enter into sinuses (closed circulation). The sinuses/sinusoids (HA26, s) are the discontinuous capillaries in the red pulp, whereas the red pulp cords are the compact areas between the sinuses. Sinuses are drained by venules. These are drained by pulp veins (HA26, v).