b. Mechanisms of venous return
• Contractions of the heart pump blood through
the entire cardiovascular system, including the
veins.
• Contractions of skeletal muscles squeeze
veins and move blood along, thereby acting as
a
skeletal muscle pump
(shown here).
• Much like the skeletal muscle pump, pressure
changes in the thoracic and abdominal cavities
during breathing squeeze the abdominal veins
and move blood through them to create a
respiratory pump
.
Proximal
valve
Distal
valve
1. In relaxed muscle,
both proximal and
distal valves are
open and blood
flows upward.
2. When muscle
contracts, the vein
gets compressed.
• The proximal
valve opens and
blood gets
squeezed upward.
• The distal valve
closes and
pressure builds
behind it.
3. When muscle
relaxes again, the
vein gets
compressed.
• Pressure behind
the proximal valve
drops and the
valve closes.
• Increased
pressure behind
the distal valve
opens it and blood
flows back into it.
and blood flows upward toward the heart. However,
the pressure is barely enough to overcome the force of
gravity pushing the blood back down.
2.
Contraction of leg muscles, such as when you take
a step, compresses the vein, which pushes the blood
through the valve closer to the heart. At the same time,
the valve farther from the heart in the uncompressed
segment of the vein closes, as some blood is pushed
against it. People who are immobilized through injury
or disease lack these contractions of leg muscles. As
a result, their venous return is slower and they may
develop circulation problems.
3.
Finally, just after muscle relaxation, pressure falls in
the previously compressed section of vein, which causes
the valve closer to the heart to close. The valve farther
from the heart now opens because blood pressure in
the foot is higher than in the leg, and the vein fills with
blood from the foot.
The respiratory pump is also based on alternating
compression and decompression of veins. During inha-
lation (breathing in), the diaphragm moves downward,
which decreases the pressure in the thoracic cavity and
increases the pressure in the abdominal cavity. As a
result, abdominal veins are compressed, and a greater
volume of blood moves from the compressed abdomi-
nal veins into the thoracic veins and then into the right
atrium. When the pressure reverses during exhalation
(breathing out), the valves in the veins prevent back-
flow of blood from the thoracic veins into the abdominal
veins.
Veins may be superficial or deep. (Arteries are all deep
below the skin.) You can see the superficial veins as they
are located just beneath the skin; these are important
as sites for withdrawing blood or giving injections. Deep
veins generally travel alongside arteries and usually bear
the same name.
Blood Vessels Are the Body's Plumbing 327
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