\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,positioning,calc}
\usetikzlibrary{narrow}

% The double arrow comes from
% http://www.texample.net/tikz/examples/double-arrows/
% with modification in 
% http://tex.stackexchange.com/questions/169061/tikz-increase-arrow-head-size
\usetikzlibrary{arrows,decorations.markings}

\tikzset{every picture/.style={auto, line width=1pt, >=narrow,
    x=1mm, y=1mm, font=\small}}
\tikzset{block/.style={draw, rectangle, line width=2pt, 
    fill=blue!10, minimum height=3em, minimum width=3em,
     outer sep=0pt}}
\tikzset{sumcircle/.style={draw, circle, outer sep=0pt,
      label=center:{{\Large$\Sigma$}}, minimum width=2em}}
\tikzset{vecArrow/.style={
  line width=1pt,
  decoration={
    markings,mark=at position 1 with {
       \arrow[scale=2.5,line width=0.4pt] {open triangle 60}
    }
  },
  double distance=4pt, shorten >= 13pt,
  preaction = {decorate},
  postaction = {draw,line width=4pt, white,shorten >= 10pt}
}}
\tikzset{innerWhite/.style={semithick, white, line width=1.4pt,
     shorten >= 4.5pt}}


\begin{document}
\begin{tikzpicture}
\node[sumcircle](sumA){};
\node[block, right=25mm of sumA](Adam){Adam};
\node[block,below=8mm of sumA,xshift=15mm](Bertil){Bertil};
\node[block,left=10mm of sumA](Cesar){Cesar};
\draw[->](Cesar.west)++(-15,0)--node[xshift=-2mm]{$AA$}(Cesar);
\draw[->](Cesar)--(sumA);
\draw[->](sumA)--node{$BB$}(Adam);
\draw[->](Adam.east)--node{$CC$}+(15,0);
\draw[vecArrow](Adam)|-(Bertil);
\draw[->](Bertil)-|(sumA);

\end{tikzpicture}
\end{document}
