CLOSE PROX OF ACRS ON SAME RWY; OPPOSITE DIRECTION. TCASII RA.

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OUR ACFT WAS AT 4000 FT ON A VECTOR TO A VISUAL APCH TO RWY 15R AT BWI. WE WERE CLRED TO 2500 FT; RPTED THE ARPT IN SIGHT; AND WERE CLRED FOR A VISUAL TO RWY 15R. AT THIS POINT; WE WERE ON THE RWY 15L EXTENDED CTRLINE; AND CORRECTING OVER TO LINE UP WITH RWY 15R. WE CONTINUED DSCNT FOR RWY 15R; BUT THEN OBSERVED ANOTHER ACFT DEPARTING ON RWY 33L (SAME RWY). AT THIS TIME OUR HDG WAS 170 DEGS; IN ORDER TO INTERCEPT THE RWY 15R CTRLINE. TCASII THEN ISSUED A TA FOR TFC AT 10 O'CLOCK; 3 MI. THIS WAS FOLLOWED BY AN RA TO MONITOR VERT SPD; FOLLOWED BY A DSND COMMAND. WE ACQUIRED THE TFC AT 10 O'CLOCK AND 1 1/2 MI; AT OUR ALT; AND CONTINUED DSCNT TO 1700 FT ON A 170 DEG HDG TO REMAIN CLR. ONCE CLR; WE ADVISED APCH CTL OF OUR ACTIONS; AND REASONS. WE WERE THEN GIVEN A 180 DEG HDG AND TOLD TO MAINTAIN 2000 FT. WE WERE THEN CLRED FOR A VISUAL TO RWY 33L; AND CALLED THE TWR TURNING BASE. THE LNDG WAS NORMAL. TRACON LATER SAID THAT THE TCASII ACFT HAD DEPARTED RWY 33R AND WAS GIVEN AN IMMEDIATE L TURN SO AS TO AVOID OUR PATH. THIS PUT HIM ON A NEAR COLLISION COURSE WITH US; BUT WE WERE NEVER ADVISED BY APCH CTL. I WOULD RECOMMEND CLOSER COORD BTWN TWR AND APCH CTL (BWI); AS WELL AS ADVISORIES TO INBOUND TFC CONCERNING TFC ABOUT TO DEPART OR ABOUT TO LAND ON THE ACTIVE RWYS.