Differential of Volume Spherical Coordinates

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\documentclass{article}
\usepackage{tikz}
\usepackage{tikz-3dplot}
\usepackage{ifthen}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{1pt}
%
% File name: differential-of-volume-spherical-coordinates.tex
% Description: 
% A geometric representation of the differential of volume 
% in spherical coordinates is shown.
% 
% Date of creation: November, 10th, 2021.
% Date of last modification: October, 9th, 2022.
% Author: Efraín Soto Apolinar.
% https://www.aprendematematicas.org.mx/author/efrain-soto-apolinar/instructing-courses/
% Source: page 122 of the 
% Glosario Ilustrado de Matem\'aticas Escolares.
% https://tinyurl.com/5udm2ufy
%
% Terms of use:
% According to TikZ.net
% https://creativecommons.org/licenses/by-nc-sa/4.0/
%
\begin{document}
%
\begin{center}
\tdplotsetmaincoords{70}{120}
%
\begin{tikzpicture}[tdplot_main_coords]
    % Coordinates of the location of the differential of volume
    \pgfmathsetmacro{\x}{0.75}
    \pgfmathsetmacro{\y}{1.5}
    \pgfmathsetmacro{\z}{2.25}
    \pgfmathsetmacro{\step}{0.025}
    % coordinates in spherical coordinates
    \pgfmathsetmacro{\radio}{sqrt(\x*\x+\y*\y+\z*\z)}
    \pgfmathsetmacro{\zf}{\radio+1.0} % To indicate the end point in the z axis
    \pgfmathsetmacro{\angulot}{atan(\y/\x)} % angle $\theta$
    \pgfmathsetmacro{\dominio}{\angulot*pi/180} % Convert $\theta$ into radians
 
 
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