It is a function of the amount of blood returned to the heart and the pumping ability of the heart.
(It; is; a function of the amount of blood returned to the heart and the pumping ability of the heart) (a function of the amount of blood; returned; to the heart and the pumping ability of the heart)
It a function of the amount of blood
is returned
a function of the amount of blood returned to the heart and the pumping ability of the heart
to the heart and the pumping ability of the heart