Condition check to launch Trigger

Question

I have this table:

CREATE TABLE public.tbtramite
(
  idtramite integer NOT NULL DEFAULT nextval('tbtramite_idtramite_seq'::regclass),
  dsdescricao character varying(255),
  dttramite timestamp without time zone,
  stnotificacao character varying(1),
  stretornada character varying(1),
  sttramitepublico character varying(1),
  idencaminhamento integer NOT NULL,
  idunidadeenvio integer NOT NULL,
  idusuarioemissor integer,
  idusuarioreceptor integer,
  CONSTRAINT tbtramite_pkey PRIMARY KEY (idtramite),
  CONSTRAINT fk_1pybfodhvhni89th78wu28av3 FOREIGN KEY (idunidadeenvio)
      REFERENCES public.tbunidade (idunidade) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT fk_7njc4ihx8teld84auv7b83cht FOREIGN KEY (idusuarioemissor)
      REFERENCES public.tbusuario (idusuario) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT fk_986qxqfo4nx3e6chji0q7mymg FOREIGN KEY (idusuarioreceptor)
      REFERENCES public.tbusuario (idusuario) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT fk_iqha0kxcqwwfweibygg4p91xu FOREIGN KEY (idencaminhamento)
      REFERENCES public.tbencaminhamento (idencaminhamento) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
  OIDS=FALSE
);
ALTER TABLE public.tbtramite
  OWNER TO postgres;

And I have a Rigger who executes every time a piece of data is entered in this table.

However, I would like Trigger to be executed only when inserting the new column data idusuarioreceptor were NULL and the field of idunidadeenvio were !=(different) from 1.

Is it possible to perform this check when launching Trigger? Or make a "light" process that checks and is then call the heavier process?

To Trigger:

CREATE TRIGGER acerto_de_caminho_trigger
AFTER INSERT ON tbtramite
FOR EACH ROW
EXECUTE PROCEDURE acerto_de_caminho();