A friend of mine and I are having a friendly disagreement and I'm
hoping you can settle it for me.  The question concerns the
circulation of blood through the human body.  One of us contests that
the blood travels through the left side of the body then to the right
side of the body then back to the heart.  The other believes the blood
travels to all parts of the body at the same time.  Please clear this
up for us.

You're both kinda wrong and kinda right!  Let's just say I hope no
money was staked on this argument.

The statement that blood "travels through the left side of the body
then to the right" assumes that the blood travels through one
continuous tube that travels down one side then hangs a right.  Not
the case, read on.

The statement that blood "travels to all parts of the body at the same
time" is also wrong because naturally, not every part of the body
receives new blood at the same time.

It is important to know that the heart can be divided into right and
left hearts functionally.  The blood received from the large veins
goes to the right side of the heart and lacks oxygen.  The right side
of the heart pumps it to the lungs to be oxygenated and it flows
throught he lungs to end up on the left side of the heart.  This is
called the pulmonary circulation.  The systemic circulation starts
once the oxygenated blood has been received in the left side of the
heart and is ready to be pumped to the aorta and the rest of the body.

The aorta (the largest artery in the body) arches backwards and to the
left before going straight down the body.  Along its course, the aorta
gives off branches that go to the different parts of the body (head,
arms, legs, kidneys etc).  These in turn give out sub-branches to
cover all the different areas of the target organ and so on.  Blood
coming back to the heart is collected at the different target areas in
the form of small veins, which are tributaries to larger veins which
all end up forming the inferior and superior vena cava (the largest
veins in the body) and ultimately end up in the heart (the right
side).

It is correct to assume that blood is pumped to all parts
simultaneously because that is the function of the heart in each beat.
 It is however, not correct to assume that blood reaches every part of
the body at exactly the same time, as it has to travel further to
reach the big toe than to reach the armpit for example.  If i had to
choose which statement is "more correct" it would probably be the
second one.

Regards.