Blood circulation in the human body is a very
complicated process that occurs within an
intricate system. To compare to other creatures,
insects and spiders have open circulatory
systems in which blood is pumped forward by the
heart and then flows through the whole body
cavity, bathing all the internal organs in
blood. Vertebrates, such as humans, have closed
circulatory systems in which the blood remains
within the circulatory network as it circulates.
The human system that circulates blood is called
the cardiovascular system and it is made up of
the heart plus our arteries and veins. The
distinction between arteries and veins is made
by the direction of blood flow. Veins carry
blood towards the heart from all over the body.
Arteries carry blood away from the heart to
reach every point in our body. This means blood
is flowing in opposite directions at every point
in our body: from your hands to you heart and
your heart to your hands; from your heart to
your brain and your brain to your heart; from
your heart to your feet and your feet to your
heart, etc. That´s a lot of different
To make matters more complicated, we actually
have a system of double circulation: we have a
completely separate `pulmonary circuit´ to carry
blood without oxygen to the lungs to get oxygen
and then to take that oxygenated blood back to
the heart to get pumped to the rest of the body,
and a separate `systemic circuit´ that takes
blood to and from every point in the body.
The typical adult body contains 5 liters of
blood which can complete an entire circuit
through the body in about one minute: the blood
recirculates roughly 1500 times each day!
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