\documentclass[10pt,border=0pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage{DejaVuSans}
\usepackage{amsmath}
\usetikzlibrary{positioning, calc}
\renewcommand*{\familydefault}{\sfdefault}
 
\begin{document}
\begin{tikzpicture}[
 >=latex,
 arrow/.style={->,thick},
 cat2/.style={color=blue,font=\footnotesize, minimum width=1.5cm},
 cat/.style={cat2, font=\itshape\footnotesize},
 every node/.style={minimum width=1cm, align=center}
]
\node (d1) {Diacylglycerol or phospholipid};
\node[below=1.5cm of d1,font=\large] (d2) {Arachidonic acid};
\node[right=3cm of d2] (d22) {HPETE (hydroperoxy-\\eicosatetraenoic acid)};
\node[below=2cm of d2,font=\large] (d3) {Prostaglandin H\({}_\text{2}\) (PGH\({}_\text {2}\))};
\node[below=1.25cm of d22] (d32) {Leukotriene A\(_\text4\)};
\node[left=1.7cm of d32] (d33) {LTB\(_4\)};
\node[below=1.25cm of d32] (d42) {Leukotriene C\(_\text4\)};
\node[below=1cm of d42] (d52) {Leukotriene D\(_\text4\)};
\node[below=1cm of d52] (d62) {Leukotriene E\(_\text4\)};
\node[below=4.5cm of d3,anchor=65] (d501) {Prostacyclin\\(PGI\({}_\text2\))};
\node[below right=4.5cm and -1.5cm of d3,anchor=155] (d502) {Thromboxane\\(TXA\({}_\text2\))};
\draw[arrow] (d1) -- 
 node[pos=.5,left,cat] {Phospho-\\lipase C} 
 node[pos=.5,right,cat] {Phospho-\\lipase A\({}_{\textit2}\)} 
 (d2);
\draw[arrow] (d2) -- 
 node[pos=.5,below,cat] {Lipooxygenase\\(FLAP, Alox5)} 
 (d22);
\draw[arrow] (d2) -- 
 node[pos=.5,left,cat,align=right] {PGH\({}_{\textit2}\) synthase\\ (cox--1 or cox--2 and\\ peroxidase)}
 (d3);
\draw [arrow] (d22) -- (d32);
\draw[arrow]
 (d32) ++(-1.75,.75) node (h2o) {H\(_\text2\)O} 
 (h2o) to [out=-90,in=0] (d33)
 (d32) -- (d33)
 ;
\draw[arrow]
 (d32) -- node [pos=.5,above left=.15cm and .5cm] (glut1) {Glutathione}
 node[pos=.5,right,cat] {Glutathione-\\S-transferase}
 (d42)
 (glut1) to [out=0,in=90] (d42)
 ;
\draw[arrow]
 (d42) -- node [pos=.5,below left=.1cm and .75cm] (glut2) {Glutatmic acid}
 (d52)
 ;
\draw [arrow] (d42) to [out=-90,in=0] (glut2);
\draw [arrow] (d52) -- (d62);
\draw [arrow] (d3) to [out=180,in=90] node[pos=.5,above left,cat2] {PGD\\synthase} ++(-3.25,-.75) 
 node [anchor=north] {PGD\(_\text2\)}
 ;
\draw [arrow] (d3.south) ++(-1.75,0) 
 -- node[pos=.5,right,cat2,align=left] {PGE\\synthase} 
 ++(0,-2) node[anchor=north] (d41) {PGE\(_\text2\)};
\draw [arrow] (d41) to [out=-90,in=0] ++(-1,-1) node [anchor=east] {PGF\(_\text2\)};
\draw [arrow] (d3) -- node[pos=.5,rotate=90,cat2,below] {Prostacyclin synthase}
 (d501.north -| d3);
\draw [arrow] (d3.south -| d502) -- node[pos=.5,rotate=90,cat2,below] {Thromboxane synthase}
 (d502);
\draw [arrow] (d501) -- node[pos=.5,below,cat2] {endothelium}
 ++(-2.8,0)
 node[anchor=east] {6-keto-\\PGF\(_{\text1\mathrm\alpha}\)};
\draw [arrow] (d502) -- node[pos=.5,below,cat2] {platelets}
 ++(2.8,0)
 node[anchor=west] {Thromboxane\\(TXA\(_\text2\))};
\end{tikzpicture}
\end{document}