Well, a simple answer would be: it doesn't.
A better answer would be: since blood clearly does flow to the legs,
the pressure there is LOWER, not higher. Incidently, this has nothing
to do with blood or the body, per se, but with any fluid system. For
fluid to flow from one area to another, there has to be a pressure
gradient.
The best answer would be: blood flows from the body to the legs AND
BACK. Therefore there have to be two pressure gradients. In the
body, the arterial system and the venous system are separate. Taking
the left side of the heart as the starting point (the point of maximum
pressure, at least during systole or contraction), blood flows from
the heart into the aorta (then main artery coming from the heart),
then to the rest of the body (head, arms, belly, and legs). The
arteries subdivide and get smaller until they turn into capillaries,
which are so small that they only allow blood cells to flow through in
single file. The capillaries convene into larger and larger vessels
of the venous system until they arrive in one of two large veins that
empty into the right side of the heart. At every point along the way,
the blood flows from an area of higher pressure to an area of lower
pressure.
This is the basic answer...it's a bit more complicated because of
things like valves in the veins that only allow flow in one direction,
and the pumping action of the muscles in the legs, not to mention the
variable tension in the arteries and veins, but you get the idea. |
