Transportation Of Blood The
Cardiac Muscle Impluses Transportation
In Blood Plasma Red
White Blood Cell Platelets Blood Vessles ArteriesCapillariesVein Distribution Of Blood
from the capillary now carrying carbon dioxide and other cellular waste
drains into larger vessels called venues that empty into still larger veins.
Veins provide a low- resistance pathway by which blood can return to the
heart. The walls of veins are much thinner and more expandable than those
of the arteries, although both contain a layer of smooth muscle. Because
blood pressure in the veins is low, the contractions of skeletal muscle
during exercise and breathing must assist in the return of the blood to
the heart. These muscular movements squeeze the veins, forcing blood through
them. When veins are compressed, you might predict that blood would be
forced away from the heart as well as toward it. To prevent this, veins
are eqipped with one-way valves that allow blood flow only toward the heart.
When you sit or stand for long periods, the lack of muscle activity allows
blood to accumulate in the veins of the lower legs. This accumulation accounts
for swollen feet often experienced by airplane passengers. Long periods
of inactivity can also contribute to varicose veins, in which the valves
become stretched and weakened. If blood pressure should fall, for instance
after extensive bleeding, veins can help restore it. The sympathetic nervous
system automatically stimulates contraction of the smooth muscles in the
vein walls. This action decreases the internal volume of the veins and
raises blood pressure, speeding up the return of the blood to the heart.