\begin{tikzpicture}[
    mycircle/.style={
    circle,
    draw=black,
    fill=white,
    fill opacity = 0.3,
    text opacity=1,
    inner sep=0pt,
    minimum size=20pt,
    font=\small,
    align=center},
    myarrow/.style={-Stealth},
    node distance=0.6cm and 1.0cm
    ]

  \node[mycircle] (c1) {1\\(central)\\$V_1$};
  \node[mycircle,draw=white,above left=of c1] (c2) {IV bolus};
  \node[mycircle,draw=white,above right=0.5cm of c1] (c3) {sample};
  \node[mycircle,draw=white,below=of c1] (c4) {};
  \node[mycircle,right=of c1] (c5) {2\\(peripheral)\ };
  \node[mycircle,draw=white,below=of c5] (c6) {};

%loop to create standard edges
  \foreach \i/\j/\txt/\p/\r in {% start node/end node/text/position
    c1/c4/$K_{10}$/left/90,
    c5/c6/$K_{20}$/left/90,
    c1.350/c5.190/$K_{21}$/below/0
  }
  \draw [myarrow] (\i) -- node[sloped,font=\small,\p,rotate=\r] {\txt} (\j);

%draw special edges
  \draw [-Stealth,dashed] (c2) edge[in=130,out=-90](c1);
  \draw [*-] (c3) -- node[font=\small] {} (c1);
  \draw [myarrow] (c5.168) -- node[font=\small,above] {$K_{12}$} (c1.15);

% title
	\node[align=center,font=\bfseries, yshift=2em] (title) 
    at (current bounding box.north)
    {Unidentifiable Two-compartment Model};

% caption
	\node[align=center,font=\bfseries, yshift=-2em] (caption) 
    at (current bounding box.south)
    {The model parameters cannot be estimated uniquely \\ unless concentration data is  available from both compartments};

\end{tikzpicture}
