A B777 FO LEFT THE COCKPIT FOR A PHYSIOLOGICAL NEED AFTER THE CAPT RETURNED; THE SOLO CAPT MISSED A DSCNT CROSSING RESTRICTION AFTER THE FO BRIEFED IT.

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CLRED TO FL320 BY ZZZ CTR. I WAS WAITING FOR FLT ATTENDANT TO RELIEVE ME SO I COULD USE THE RESTROOM. A FEW MINS LATER IRO WAS GIVEN CLRNC TO CROSS 70 MI N OF ABC VOR AT FL320. THE CAPT CAME BACK IN THE SEAT BEFORE THE FLT ATTENDANT RELIEVED ME. WE WERE FLYING IN VERT SPD AND I INCREASED IT WHILE I WAS WAITING FOR THE SEAT SWAP. I INFORMED THE CAPT OF THE RESTR AND LEFT FOR THE RESTROOM. WHEN I RETURNED HE INFORMED ME WE MISSED THE RESTR BY ABOUT 2000 FT. I THOUGHT HE UNDERSTOOD WE NEEDED TO GET DOWN FASTER; BUT HE MAY HAVE BEEN DISTR BY HIS STATED CONCERN THAT THE RESTR WAS NOT IN FMS. AT THIS POINT MY PHYSIOLOGICAL NEEDS WERE CAUSING A LESS THAN OPTIMAL XFER OF CTL OF ACFT. I SHOULD HAVE GOTTEN AN ACKNOWLEDGEMENT ABOUT BEING BEHIND IN DSCNT AND GETTING REVISED CLRNC FROM ATC. ALSO IF ZZZ CTR DID NOT START US DOWN 100 MI EARLY AS ALWAYS WE WOULD HAVE BEEN DSNDING MORE EXPEDITIOUSLY. WE WERE ON TOP WITH A GOOD RIDE AT FL400 AND WX AND TURB WERE RPTED AT FL320. I DON'T NORMALLY USE VERT SPD FOR DSCNT INSTEAD OF VNAV; BUT I WAS ONLY IN THE SEAT FOR ABOUT 2 MINS AND DID NOT WANT TO MAKE MAJOR CHANGES OTHER THAN INCREASING VERT SPD RATE OF DSCNT. I HOPE THERE IS A LONG TERM PLAN TO RESTRUCTURE NE CORRIDOR AIRSPACE BECAUSE IT IS A MESS IN MY OPINION. PERHAPS SPREADING OUT ZBW; ZNY; ZDC; AND ZTL AIRSPACE A LITTLE MORE EVENLY. I HAVE BEEN FORCED INTO EARLY DSCNTS BY ZZZ CTR INTO TURB AND BAD WX MORE TIMES THAN I CARE TO REMEMBER IN THE LAST 10 YRS. THERE MUST BE A SAFER/BETTER WAY OF MANAGING THE AIRSPACE.