By
the time blood reaches the capillaries, its speed-originally 15 inches a second
in the aorta-has slowed to about one fiftieth of an inch a second. The
capillaries are so tiny that red blood cells have to move through them in
single file-and the red cells are so small that you could cover four or five
dozen of them with a single period, such as the one right here. But small as
the capillaries are, the capillary system is so extensive- many thousands of
miles, all told-that if all the capillaries were open to the flow of blood at any
one time, they could hold the entire five-quart supply of the body. This is
where the adaptability of the circulatory system comes in. All the capillaries
are never open at one time.
They open and close, first in one area,
then in another, depending upon need. When, for example, you are exercising
vigorously, the muscles need more blood. The heart responds; from over a gallon
of blood per minute, it can pump as much as five and a half gallons a minute.
And much of this flow now goes to the muscles which need it, diverted from
other organs which do not have such pressing need at the moment. In times of
extreme exertion, even though the heart is pumping only five times as much
blood, the muscles may receive eighteen times as much, as the capillaries in
various organs shut down to allow diversion.
The digestive system, for example, will get
only one fifth of its normal supply during extreme exertion. Blood flow to the
muscles goes up from about two pints a minute to about forty. At rest, the
muscles ordinarily get about 20 percent of the heart's output; with extreme
exertion, they may get as much as 88 percent. It is only from the brain that
blood is never diverted this way. The brain gets its required one and a third
pints a minute whether the body is at rest or furiously active.
No comments:
Post a Comment