▁closer ▁to , ▁longer ▁fib ers ▁further ▁from ▁the ▁gray ▁The ▁tract ▁is ▁one ▁of ▁three ▁propri osp inal ▁tract s ▁in ▁which ▁most ▁path ways ▁intr insic ▁to ▁the ▁sp inal ▁cord ▁are ▁located . ▁The ▁others ▁are ▁the ▁vent ral ▁propri osp inal ▁tract ▁and ▁the ▁d ors al ▁propri osp inal ▁tract . ▁ ▁References ▁ ▁External ▁links ▁▁ ▁More ▁information ▁at ▁Bra in Info ▁ ▁Category : Sp inal ▁cord ▁tract s <0x0A> </s> ▁The ▁elim ination ▁rate ▁constant ▁K ▁or ▁Ke ▁is ▁a ▁value ▁used ▁in ▁ph arm ac ok inet ics ▁to ▁describe ▁the ▁rate ▁at ▁which ▁a ▁drug ▁is ▁removed ▁from ▁the ▁system . ▁ ▁It ▁is ▁often ▁abbre vi ated ▁K ▁or ▁Ke . ▁It ▁is ▁equivalent ▁to ▁the ▁fraction ▁of ▁a ▁subst ance ▁that ▁is ▁removed ▁per ▁unit ▁time ▁measured ▁at ▁any ▁particular ▁instant ▁and ▁has ▁units ▁of ▁T − 1 . ▁This ▁can ▁be ▁expressed ▁mathemat ically ▁with ▁the ▁differential ▁equation ▁, ▁ ▁where ▁ ▁is ▁the ▁blood ▁pl asma ▁concentration ▁of ▁drug ▁in ▁the ▁system ▁at ▁a ▁given ▁point ▁in ▁time ▁, ▁ ▁is ▁an ▁infinitely ▁small ▁change ▁in ▁time , ▁and ▁ ▁is ▁the ▁concentration ▁of ▁drug ▁in ▁the ▁system ▁after ▁the ▁infinitely ▁small ▁change ▁in ▁time . ▁ ▁The ▁solution ▁of ▁this ▁differential ▁equation ▁is ▁useful ▁in ▁calculating ▁the ▁concentration ▁after ▁the ▁administration ▁of ▁a ▁single ▁do se ▁of ▁drug ▁via ▁IV ▁bol us ▁injection : ▁ ▁C t ▁is ▁concentration ▁after ▁time ▁t ▁C 0 ▁is ▁the ▁initial ▁concentration