Q:

Separately annotate parts of a fraction

I'm trying to create the following image in TikZ as part of a handout explaining blood pressure to patients in a clinic. 
I am unsure how to annotate a fraction in tikz. I know of the example using Beamer arrows to explain terms. The example for Carl's students doesn't apply because it only colors parts of the terms of the equation. 

I've come to TikZ after using a simple array (code below), which won't look as nice. 
$\begin{array}{cc}
  \underline{120} & \textrm{La cifra superior mide la fuerza de la sangre cuando se contrae el corazon}\\
  80 & \textrm{La cifra inferior mide la fuerza de la sangre cuando el corazon       relaje}
   \end{array}$

A:

Something like this?

\documentclass[tikz]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usetikzlibrary{arrows,positioning}

\begin{document}
  \begin{tikzpicture}[> = stealth']
  \node[font=\Large] (alta) {120}; 
  \node[font=\Large, below = 3mm of alta] (baja) {80};
  \draw[thick, <-] (alta) --++(30:1cm) node[right, text width=5cm, align=left]{La cifra superior mide la fuerza de la sangre cuando se contrae el corazón};
  \draw[thick, <-] (baja) --++(-30:1cm) node[right, text width=5cm, align=left]{La cifra inferior mide la fuerza de la sangre cuando el corazón relaje};
  \draw[thick] ([yshift=-1.5mm]alta.south west) --  ([yshift=-1.5mm]alta.south east);
 \end{tikzpicture}
\end{document}

