Biomedical Engineering Reference
In-Depth Information
Although there is a common structure between arteries and veins, there are some
important differences to discuss. Generally, the walls of arteries are thicker than those of
veins. This difference is primarily manifested in the tunica media, for arteries are com-
posed of significantly more muscle cells and more elastic fibers. Arteries are normally
cylindrical in shape, whereas veins do not have a typical structure because of the low
hydrostatic pressure throughout the system. Also, when under high pressure, it is more
beneficial for blood vessels to have a uniform (or regular) geometry, so that the pressure
forces are equally distributed at all locations and therefore one location does not experi-
ence an extremely high load. Otherwise, this would induce a weak spot within the vessel
wall that would be more likely to fail. Lastly, veins contain valves to prevent the backflow
of blood toward the capillary beds.
Venous circulation begins as blood passes out from the capillary beds and enters the
venules. These blood vessels act to collect and store blood, as well as to pass the blood
along to the large veins. Therefore, venous flow is convergent flow, meaning that two (or
more) blood vessels combine to form a large vessel. This continually occurs within the
venous system until blood enters the heart via the superior (blood from the head and
upper torso) and inferior (blood from the lower torso) vena cava.
Large veins have very little resistance to flow when they are fully open. However,
under normal conditions, the veins in the chest cavity are compressed to some extent.
Compression of veins occurs due to the low hydrostatic pressures within the venous sys-
tem, the higher pressure from other surrounding organs or the interstitial space acting on
the veins and veins bending over bones or other stiffer biological materials (e.g., the sub-
clavian vein bends over the first rib bone). This compression partially regulates venous
return into the heart by increasing flow resistance. As we discussed in Chapter 3, the
venous flow is also a function of the hydrostatic pressure within the fluid, because the
pressure head (driving force
P
) for flow is so low. An average standing adult experiences
an approximately 100 mmHg pressure difference between the heart and the toes. The forc-
ing pressure head (
r
P
) in the venous system cannot overcome this pressure difference
itself, and that is why if you stand still for an extended period of time, your lower limbs
can cramp and you experience the “pins and needles” feeling. Conversely, when standing,
the pressure in your head is approximately 10 mmHg lower than the pressure of the right
atrium. This aids in constant blood flow from the head back to the heart, and therefore,
there is little pooling of the blood in the cranium and neck. As you should know,
increased pressure on the brain, including blood pressure, can have dramatic effects that
can lead to death and/or improper brain function.
To help blood flow from the lower limbs back toward the heart, the venous system can
act as a pump. The venous pump uses the valves that were discussed briefly to prevent
the backflow of blood toward the capillary beds. In fact, without the valve system, it
would be nearly impossible for the blood to ever flow back into the heart when a person
is a standing or sitting position. The opening and closing of the venous valves help to reg-
ulate and induce blood flow through the majority of the venous system. This valve move-
ment is facilitated via skeletal muscle contraction, and the increase in local fluid pressure
induced by skeletal muscle contraction. Every time a skeletal muscle contracts, it com-
presses the veins that are in close proximity to the muscle, forcing the blood to flow
through the downstream valves. Similar to the heart valves, the venous valves are
designed in such a way that blood flows toward the heart and not toward the capillaries
r
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